Engineering and Construction

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Estimate the depth of wood rafters on the basis of the horizontal (not slope) distance from the outside wall of the building to the ridge board in a gable or hip roof and the horizontal distance between supports in a shed roof. A 2 X 4 rafter spans approximately 7 feet (2.1 m), a 2 X 6 10 feet (3.0 m), a 2 8 14 feet (4.3 m), and a 2 X 10 17 feet (5.2 m).

The depth of wood light roof trusses is usually based on the desired roof pitch. A typical depth is one-quarter of the width of the building, which corresponds to a 6/ 12 pitch in a gable truss. Trusses are generally spaced 24 inches (600 mm) o.c. and can span up to approximately  65 feet (20 m).

Estimate the depth of wood floor joists as follows: 2 x 6 joists span up to 9 feet (2.7 m), 2 x 8 joists 11 feet (3.4 m), 2 x 10 joists 14 feet (4.3 m), and 2 x 12 joists 17 feet (5.2 m).

Estimate the depth of manufactured  wood I-joists as follows: 9½-inch (240-mm) joists span 16 feet (4.9
m), 117 /8 -inch (300-mm) joists 19 feet (5.8 m), 14-inch (360-mm) joists span 23 feet (7.0 m), and 16-inch (400-mm) joists span 25 feet (7.6 m).

Estimate the depth of  wood floor  trusses as 1/ 18 of their span. Typical depths of ß oor trusses range from 12 to 28 inches (305-710 mm) in 2-inch (51-mm) increments.

2 X 4 wood studs 24 inches (600 mm) o.c. can support attic and roof loads only. Either 2 X 4 studs 16 inches (400 mm) o.c. or 2 6 studs 24 inches o.c. can support one floor plus attic and roof. 2 X 6 studs 16 inches o.c. can support two ß oors plus attic and roof.

Framing members in light frame buildings are usually spaced either 16 or 24 inches (400 or 600 mm) o.c. For actual sizes of dimension lumber in both conventional and metric units, see Figure 3.22.

These approximations are valid only for purposes of preliminary building layout and must not be used  to select final member sizes. They apply to the normal range of building occupancies such as residential, office, commercial, and institutional buildings. For manufacturing and storage buildings, use somewhat larger  members.

Figure 3.22 The relationship between nominal and actual dimensions for the most common sizes of kiln-dried lumber is given in this simplified chart

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Roof trusses, and to a lesser extent floor trusses, are used in platform frame buildings because of their speed of erection, economy of material usage, and long spans. Though many floor  trusses are light enough to be lifted and installed by two carpenters, most truss assemblies are erected with the  aid of a small crane that often is attached to the truck on which the trusses are delivered (Figure 5.65).  Roof trusses are particularly slender in proportion, usually only 1½ inches (38 mm) thick and capable of spanning 24 to 32 feet (7.5 to 10 m). They must be temporarily braced during construction to prevent buckling or  the dominolike collapse of all the trusses until they are adequately secured permanently by the application of roof sheathing panels and interior finishes (Figure 5.66).

Figure 5.65 Roof trusses are typically lifted to the roof by a boom mounted on the delivery truck. This is one of a series of identical attic trusses, which will frame a habitable space under the roof.

Manufactured wall panels have been adopted more slowly than roof and floor trusses, and are used mostly  by large builders who build hundreds or thousands of houses per year. For  the smaller builder, wall framing can be done on site with the same amount of material as with panels and with  little or no additional overall expenditure of labor, especially when the building requires walls of varying heights and shapes.

Figure 5.66 A roof framed with prefabricated trusses. Approximately midway up the upper chords of the trusses, temporary strapping ties the trusses to one another for bracing. Other diagonal bracing, not visible in this photograph, ties the trusses to floor or ceiling framing to prevent the entire row from collectively tipping sideways.

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The 2 x 4 (38 x 89 mm) has been the standard-size wall stud since light framing was invented. In recent  years, however, pressures for heating fuel conservation have led to energy code requirements for more thermal insulation than can be inserted in the cavities of a wall framed with  members only 3½ inches (89 mm) deep. One solution is to frame walls with 2 x  6 (38 x 140 mm) studs, usually at a spacing of 24 inches (610 mm), creating an insulation cavity 5½ inches (140 mm) deep. Alternatively,  2 x 4-framed walls may be covered either inside or out with insulating plastic foam sheathing, thus reaching an insulation value about the  same as that of a conventionally insulated 2   6-framed wall. For even greater insulation performance, 2   6 studs and insulating sheathing may be used in tandem. Alternatively, in very cold climates, even deeper wall  assemblies that can achieve greater insulation values may be constructed of two separate layers of wall studs  or of vertical truss studs made up of pairs of ordinary studs joined at intervals by plywood plates. Some of these constructions methods are illustrated in Figures 7.17 through 7.21.

Figure 7.17 Insulation levels in walls of light frame buildings can be increased from the R13 to R-15 (RSI-90 to RSI-104) of a 2 x 4 stud wall to R19 (RSI-132) or more by using either 2 x 6 framing and thicker batt insulation (left) or 2 x 4 framing with plastic foam sheathing in combination with batt insulation (right). The foam sheathing insulates the wood framing members as well as the cavities between them but can complicate the process of installing some types of siding.

Figure 7.18 Window and door headers in 2 x 6 framing require special detailing. Two alternative header details are shown here in section view: (a) The header members are installed flush with the interior and exterior surfaces of the studs, with an insulated space between. This detail is thermally efficient but may not provide sufficient nailing for interior finish materials around the window. (b) A 3 x 2 spacer provides full nailing around the opening.

Figure 7.19 Two alternative corner post details for 2 x 6 stud walls, shown in plan view: (a) Each wall frame ends with a full 2 x 6 stud, and a 2 x 4 nailer is added to accept fasteners from the interior wall finish. (b) For maximum thermal efficiency, one wall frame ends with a 2 x 4 stud flush with the interior surface, which eliminates any thermal bridging through studs.

Figure 7.20 With the two framing methods shown here, walls can be insulated to any desired level of thermal resistance.

Figure 7.21 A raised-heel roof truss provides plenty of space for attic insulation at the eave.

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The generic roof shapes for wood light frame buildings are shown in Figure 5.50. These are often combined to make roofs that are suited to the covering of more complex plan shapes and building volumes.

Figure 5.50 Basic roof shapes for wood light frame buildings.

For structural stability, the rafters in gable and hip roofs must be securely tied together at the top of the supporting walls by well-nailed ceiling joists to make what is, in effect, a series of tri- angular trusses. If the designer wishes to eliminate the ceiling joists to expose  the sloping underside of the roof as  the finished ceiling surface, a beam or bearing wall must be inserted at the  ridge or a system of exposed horizontal  ties must be used in place of the ceiling joists. Sometimes a designer wishes  to raise the ceiling joists or exposed ties to a higher elevation than the tops of the wall plates. This greatly increases  the bending forces in the rafters and should be done only after consultation with a structural engineer.

Although a college graduate  ar  chitect or engineer would find it difficult  to lay out all the cuts of the rafters for a sloping roof using trigonometry, a  skilled carpenter, without resorting to  mathematics, has little problem making the layout if the  pitch (slope) is  specified as a ratio of rise to run. Rise is  the vertical dimension and run is the  horizontal. In the United States, pitch  is usually given on the architectÕs drawings as inches of rise per foot (12 inches) of run. A skilled carpenter uses  these two figures on the two edges of  a framing square to lay out the rafter as  shown in Figures 5.52 and 5.58. The  actual length of the rafter is never figured, nor does it need to be, because all the measurements are made as  horizontal and vertical distances with  the aid of the square. Today, many  carpenters prefer to do rafter layout  with the aid of tables that give actual  rafter lengths for various pitches and  horizontal distances; these tables are  stamped on the framing square itself  or printed in pocket-size booklets. Also available are hand-held calcula- tors that are specially programmed to find dimensions of rafters.

Hips and valleys introduce another level of trigonometric complexity  in rafter layout, but the experienced  carpenter has little difficulty  even  here: Again, he or she can use published tables for hip and valley rafters or do the layout the traditional way, as illustrated in Figure 5.56. The  head carpenter lays out only one rafter of each type by these procedures. This then becomes the  pattern rafter from which other carpenters trace  and cut the remainder of the rafters (Figure 5.59).

In areas that are subject to hurricanes, the building codes may require  that rafters be attached to their sup- porting walls with sheet metal rafter anchors. The type, size, and spacing of the nails that attach the roof sheathing to the rafters are also closely controlled.  The intent of both of these measures is to reduce the likelihood that the roof  will be blown off in high winds. Similar to requirements for bridging in floor joists, the International  Residential Code requires bridging  for ceiling joists and rafters where the  depth of these members exceeds their  thickness by a ratio of 6 to 1 based on   nominal dimensions (for conventional solid wood framing, members deeper  than 2 12 or 38 286 mm). Where  the undersides of the joists or rafters  remain unfinished, wood strapping  nailed perpendicular to the bottoms of the joists or rafters may be used instead of solid blocking or crossbracing installed between framing members.

Figure 5.52 Roof framing details and procedures: The lettered details are keyed to Figure 5.53. The remainder of the page shows how a framing square is used to lay out a pattern rafter, reading from the fi rst step at the lower end of the rafter to the last step at the top.

Figure 5.53 Step Seven: framing the attic fl oor and roof. The outer ends of attic fl oor joists are not usually headed off, but instead are face nailed to the rafter pairs that overlap them.

Figure 5.54 A summary of the major details for the structure shown in Figure 5.55, aligned in relationship to one another. The gable end studs are cut as shown in A and face nailed to the end rafter.

Figure 5.55 Step Eight: The sheathed frame is completed. The letters A, B, and C indicate portions of the framing that are detailed in Figure 5.54.

Figure 5.56 Framing for a hip roof. The diffi  cult geometric problem of laying out the diagonal hip rafter is solved easily by using the framing square in the manner shown. Toward the bottom of the rafter shown in the lower part of this fi  gure is a birdsmouth cut, an angled notch in the rafter that allows the rafter to seat securely on the top plate of the wall.

Figure 5.57 Framing for an overhanging rake.

Figure 5.58 A framing square being used to mark rafter cuts. The run of the roof, 12 inches, is aligned with the edge of the rafter on the blade (the wider leg) of a square, and the rise, 7 inches in this case, is aligned on the tongue (the narrower leg) of the square. A pencil line along the tongue will be perfectly vertical (a plumb cut) when the rafter is installed in the roof, and one along the blade will be horizontal (a level cut). True horizontal and vertical distances can be measured on the blade and tongue, respectively. The layout of these types of cuts can also be seen in Figures 5.52 and 5.56.

Figure 5.59 Tracing a pattern rafter to mark cuts for the rest of the rafters. The corner of the building behind the carpenters has let-in corner braces on both fl oors, and most of the rafters are already installed.

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Wall framing, like floor framing, is laid out in such a way that a framing member, in this case a stud, occurs under each vertical joint between sheathing panels (Figures 5.29-5.32). The  lead carpenter initiates wall framing by laying out the stud locations on the  top plate and sole plate of each wall. Other carpenters follow behind to cut the studs and headers and assemble the walls in a horizontal position  on the subfloor. As each wall frame is completed, it is tilted up and nailed into position, with temporary bracing  as needed (Figures 5.34-5.37).

Figure 5.29 Typical ground-floor wall framing details, keyed by letter to Figure 5.30.

Figure 5.30 Step Four in erecting a platform frame building: The ground-floor walls are framed. The letters A, B, and C indicate portions of the framing that are detailed in Figure 5.29.

Figure 5.31 Framing details for nonloadbearing interior partitions.

Figure 5.32 Steps in the framing of a typical wall and details at wall intersections and a window opening. In D, the short studs above the header and below the rough sill are called trimmer or cripple studs.

Figure 5.33 As the fi rst step in constructing a wall frame, the chief carpenter aligns the top plate and sole plate side by side on the floor platform and marks each stud location on both of them simultaneously with a pencil and framing square.

Figure 5.34 Nailing studs to a plate, using a pneu- matic nail gun. The triple studs are for a partition intersection.

Figure 5.35 Tilting an interior partition into position. The gap in the upper top plate will receive the projecting end of the upper top plate from another partition that intersects at this point.

Figure 5.36 Fastening a wall to the floor platform. Horizontal blocking between studs as seen toward the left in this photo is typically installed to provide a solid substrate for the later attachment of bathroom hardware, exterior panels, or any number of possible other items.

Figure 5.37 Lower-level wall framing is held up by temporary diagonal bracing until the floor joist framing is in place above and wall bracing or sheathing is complete, after which the frame becomes completely self-bracing. The outer walls of this building are framed with 2 x 6 (38 x 140 mm) studs to allow for a greater thickness of thermal insulation, whereas the interior partitions are made of 2 x 4 (38 x 89 mm) stock.

Figure 5.38 Prefabricated shear wall panels are especially useful in garage wall framing or other walls with large openings where only narrow, solid sections of wall remain to resist lateral forces. They may be made of all wood components or, where even greater strength is required, of wood and metal, as shown here. The corrugated galvanized steel in this panel is over 1/ 8  inch (3.5 mm) thick, and the panel’s capacity to resist lateral forces is several times greater than that of comparable all-wood prefabricated panels. The bottom of the panel will be anchored with bolts embedded 21 inches (533 mm) or more into the concrete foundation, and the sides and top will be screw-fastened to the surrounding wood framing. Holes in the panel can accommodate wiring runs within the wall.

Long studs for tall walls often must be larger than a 2 X 4 in order  to resist wind forces. Because long  pieces of sawn dimension lumber tend not to be straight and may be less  readily available, studs manufactured  of structural composite lumber may  be used instead. The International Residential Code requires horizontal  wood blocking to be installed between studs at midheight in wall frames that are taller than 10 feet (3 m). The purpose of this blocking is to stop off the cavities between studs to restrict the  spread of fire.

Headers over window and door openings must be sized in accordance  with building code criteria. Typically,  a wall opening header consists of two nominal 2-inch members standing on  edge, separated by a plywood spacer  that serves to make the header as  thick as the depth of the wall studs.  Headers for long spans and/or heavy loads are often made of laminated  veneer lumber or parallel strand lumber, either of which is stronger and stiffer than dimension lumber. Several manufacturers market prefabri- cated headers that include thermal insulation as a way of reducing the heat loss that occurs through these difficult-to-insulate assemblies. At either end, headers rest on shortened studs called trimmer or jack studs, which  themselves are nailed to full-height king studs. At the bottom of a window  opening, the  rough sill is supported  on cripple studs (Figure 5.32).

Each corner and partition intersection must furnish nailing surfaces for the edge of each plane of exterior  and interior finish materials. This requires a minimum of three studs  at each intersection, unless special  metal clips are used to reduce the number to two (Figure 5.32).

Sheathing was originally made of solid boards, usually 6 to 10 inches  (150 to 250 mm) wide. If applied  horizontally, these boards did little  to brace the building against wracking caused by the forces of wind or
earthquake. If applied on a diagonal, however, they produced a rigid frame. Today, walls are sheathed with  either plywood or OSB, which provide permanent, very stiff bracing, or with let-in bracing  as described below. Panels are applied as soon as possible after the wall  is framed, often while it is still lying on the floor platform.

In regions of strong winds or earthquakes, wall sheathing plays  a very important part in the lateral stability the structure. A properly sheathed wall acts as a  shear wall to  resist lateral forces. Both interior and  exterior walls can act as shear walls,  which must be provided in both east west and north-south orientations  and must be distributed more or less symmetrically in the floor plan. Stresses within shear walls are proportional  to their length, with shorter walls being subjected to higher stresses than those that are longer. Where walls have large openings for windows or doors, the remaining solid portions  that can contribute to lateral force resistance may become relatively short and therefore exposed to very  high stresses. In such cases, sheathing may have to be attached with  larger nails at very close spacings, the  horizontal edges of sheathing panels  may have to be supported by wood  blocking to keep them from buckling, and studs over which sheathing panels join may need to be larger in  size, such as 3 x or 4 x (64 mm or 89 mm) members, in order to hold  the required nails without splitting.

Where the needed strength cannot  be reliably achieved using site-con- struction methods, factory-fabricated  panels made of wood or steel com- ponents may be used (Figure 5.38).  Shear walls subject to high forces may also require hold-downs to prevent the  walls from pulling up off the foundation or floor platform (Figures 5.39-5.41). Consultation with a structural engineer is recommended (and often  legally required) when building in areas that are earthquake-prone or subject to very high wind forces.

Sheathing panels made from wood or paper fiber, plastic foam, and glass fiber are intended principally as  thermal insulation and to provide a base for water-resistant building paper or house wrap. Most panels of these types are nonstructural, so walls sheathed with such panels rely on  letin diagonal bracing or strategically located structural panels for lateral force  resistance. Let-in bracing may be made  of wood members, such as 1 x 4 (19 x 89 mm) boards, or light steel members that are recessed into the outer  face of the studs of the wall before it is sheathed (Figures 5.32 and 5.42).

Figure 5.39 Strap tie hold-downs are made of galvanized steel straps with hooked or deformed ends cast into the concrete foundation wall (top). After the wall is framed, the exposed length of strap may be nailed directly to framing or, as seen here, nailed through the sheathing into the studs or posts behind (bottom). The number, size, and spacing of nails used to fasten the strap depend on the magnitude of the loads that must be resisted and the capacity of the wood member to hold nails without splitting. In the top image, anchor bolts cast into the top of the foun- dation wall that will be used to secure the sill plate in place are also visible.

Figure 5.40 Hold-downs made from threaded rod and steel plate anchors can resist much greater forces than strap tie hold-downs. They may be used at each floor level to securely tie the full height of the building frame to its foundation. For the type shown here, the nuts at the ends of the threaded rods may require retightening after the fi rst heating season to compensate for wood shrinkage, which can mean that access holes must be provided through the interior wall surfaces. Other models rely on spring-loaded tapered shims or other mechanical compensation so as to be self-adjusting

Figure 5.41 A heavy-duty seismic hold-down similar to that illustrated in Figure 5.40. The anchor rod with one end cast into the concrete foundation wall protrudes through the preservative-treated wood sill where its threaded end is bolted to the anchor. The anchor in turn is bolted to a 4 4 (89 89 mm) post with fi ve bolts of substantial diameter. A conventional foundation anchor bolt with an oversized square washer is also partially visible to the right of the hold-down. Also note the thicker-than-normal, 3-inch nominal (64-mm) sill plate, a common feature in wood light framing designed for high seismic forces.

Figure 5.42 Applying a panel of insulating foam sheathing. Because this type of sheathing is too soft and weak to brace the frame, diagonal bracing is inserted into the outside faces of the studs at the corners of the building. Steel bracing, nailed at each stud, is used in this frame and is visible just to the right of the carpenter’s leg.

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Floor framing (Figure 5.19) is laid out in such a way that the ends of un-cut 4-foot by 8-foot (1.2-m by 2.4-m) subflooring panels will fall directly over joists; otherwise, many panels will have to be cut, wasting both materials and time. Standard joist spacings are 16 or 24 inches o.c. (406 or 610 mm o.c.; o.c. stands for on  center,  meaning that the spacing is measured from center to center of the joists). Though less common, a joist spacing of 19.2 inches (486 mm) can also be used. Any of these spacings automatically provides a joist to support each end of every 8-foot (2.4-m) panel (Figure 5.23).

Figure 5.19 Step Three in erecting a typical platform frame building: the ground- floor platform. Compare this drawing with the framing plan in Figure 5.17. Notice that the direction of the joists must be changed to construct the cantilevered bay on the end of the building. A cantilevered bay on a long side of the building could be framed by merely extending the  floor joists over the foundation.

Figure 5.23 Installing floor joists. Blocking will be inserted between the joists over the two interior beams to prevent overturning of the joists.

Wood composite I-joists or manufactured trusses are frequently used in place of solid wood joists. Their advantages include the ability to span greater distances, straightness  and uniformity, and reduced drying  shrinkage of the floor  platform. These joist types are most frequently  spaced at 19.2 or 24 inches o.c. (488  or 610 mm o.c.) I-joists, with their thin webs, often require reinforcing where concentrated loads occur,  such as at joist ends, at cantilevers, or where interior loadbearing partitions bear on the joists (Figures 5.44 and 5.45).

Figure 5.44 Alternative details to those shown in Figure 5.43 for a fl oor framed with I-joists rather than solid lumber. For clarity, the web of the I-joist is rendered light gray and blocking with a darker shade. The web stiffener shown in A is cut slightly shorter than the height of the web of the joist, and is installed so that a small space remains between the top of the blocking and the underside of the top fl ange. This prevents the blocking from prying the fl anges apart if the I-joist itself shrinks or is compressed slightly (see also Figure 5.45). The I-joist blocking panel in B-1 is exactly analogous to the solid blocking shown in Figure 5.45 B. A squash block is shown in B-2. This is a short section of 2x (38-mm) framing, installed vertically like a very short stud on either side of the I-joist. Squash blocks are used under points of concentrated load, such as under loadbearing posts or studs on either side of a large opening in a wall above. They are cut slightly longer than the full height of the joist to ensure that the loads are transmitted through the blocks and not the joist

Figure 5.45 Blocking where an I-joist frames into a glue-laminated wood beam, as illustrated in Figure 5.44A. This blocking acts as a web stiffener and also prevents the I-joist from rotating within the metal hanger, which is not tall enough to restrain the upper fl ange of the joist. The need for blocking depends on the size of the I-joist, the magnitude of the loads, and the type of hanger used.

After floor framing is complete, subflooring is fastened in place. To reduce squeaking in the finished floor and to increase floor  stiffness, adhesive may be used in conjunction with mechanical fasteners (Figures 5.26 and 5.27). Deformed shank nails or screws, either of which hold subflooring more securely than common nails, also contribute to the prevention of squeaks. Plywood and OSB panels must be laid with  the grain of their face layers perpendicular to the direction of the joists because these panels are considerably stiffer in this orientation (that is, a standard 4-foot by 8-foot (1.2-m by 2.4-m) panel should be laid with its  longer dimension perpendicular to the joists). Sheathing and subflooring panels are normally manufactured  1 /8  inch (3 mm) short in each face dimension so that they may be spaced slightly apart at all their edges  to prevent floor buckling during construction from the expansion of storm-wetted panels. Bridging, consisting of solid blocking or wood or metal crossbracing inserted between joists at  midspan or at intervals not exceeding  8 feet (2.4 m), is a traditional feature of floor framing (Figures 5.19, 5.28). (The term blocking refers, in general, to short lengths of lumber inserted between framing members for any  of a variety of purposes, such as bracing, reinforcing, or providing solid backing where fasteners are required for attachment of finish materials or equipment.) Its function is to hold the joists straight and to help them  share concentrated loads. The International Residential Code requires bridging only for joists deeper than 2 x 12 feet (38 x 286 mm). In better-quality construction, however, bridging is frequently used in all floors, regardless of depth, to improve stiffness and reduce vibration. Solid blocking is also required wherever floor joists span continuously across supporting beams to resist overturning of the joists (Figure 5.18B, D).

Where the ends of joists butt into supporting headers, as around stair openings and at changes of joist di- rection for projecting bays, end nails and toe nails cannot transfer the full  load between members, and sheet  metal joist hangers must be used. Each hanger provides a secure pocket  for the end of the joist and punched holes into which a number of special short nails are driven to make a safe connection.

Figure 5.26 Applying OSB subfl ooring. Note that the longer dimension of the panel runs perpendicular to the joist framing.

Figure 5.27 For a stiffer, quieter fl oor, subfl  ooring should be glued to the joists. The adhesive is a thick mastic that is squeezed from a sealant gun on the tops of the joists just before the subfl  ooring is laid in place.

Figure 5.28 Bridging between joists may be solid blocks of joist lumber, wood crossbridging, or, as seen here, steel crossbridging. During manufacture, the thick steel strip is folded across its width into a V- shape to increase its stiffness. Steel crossbridging with one end toothed requires only one nail per piece and no cutting to size, so it is the fastest to install.

Figure 5.18 Ground-fl oor framing details, keyed to the lettered circles in Figure 5.19. A compressible or resilient sill sealer strip should be installed between the sill and the top of the foundation to reduce air leakage but is not shown on these diagrams. Using accepted architectural drafting conventions, continuous pieces of lumber are drawn with an X inside and intermittent blocking with a single diagonal. The metal termite shield in detail C is used in areas where the risk of termite infestation is high. It prevents subterranean termites from traveling undetected from cracks in the concrete into the wood framing above.

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The erection of a typical platform  frame (referred to, imprecisely, as  rough carpentry in the architectÕs specifications) can best be understood by following this post s sequential  isometric diagrams, beginning with  Figure 5.19. Notice the basic simplici- ty of the building process: A platform  is built on top of the foundation.  Walls are assembled horizontally on the platform and tilted up into place.

Another platform or a roof is built on top of the walls. Most of the work is accomplished without the use of ladders or scaffolding, and temporary  bracing is needed only to support the walls until the next level of framing is  installed and sheathed.

The details of a platform frame  are not left to chance. While there  are countless regional and personal  variations in framing details and techniques, the sizes, spacings, and  connections of the members in a  platform frame are standardized  and closely regulated by building  codes, even down to the size and  number of nails for each connection (Figure 5.21) and the thicknesses and nailing patterns of the sheathing panels.

Figure 5.19 Step Three in erecting a typical platform frame building: the ground- fl oor platform. Compare this drawing with the framing plan in Figure 5.17. Notice that the direction of the joists must be changed to construct the cantilevered bay on the end of the building. A cantilevered bay on a long side of the building could be framed by merely extending the floor joists over the foundation.

Figure 5.20 Alternative ways of constructing an interior line of support for ground-fl oor joists. Space is left over the tops of the beams in B and D to allow for drying shrinkage in the joists.

Figure 5.21 Platform framing members are fastened according to this nailing schedule, which framing carpenters know by memory and which  is incorporated into most building codes and construction standards.

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An experienced carpenter can frame a simple building from the most minimal drawings, but the framing for a  larger or custom-designed structure may need to be planned as carefully as for a steel- or concrete-framed building (Figure 5.16). The architect or engineer determines an effcient layout and the appropriate sizes for joists and rafters, and communicates this information to the carpenters by means of  framing plans (Figures 5.17 and 5.51). For most purposes, member sizes can be determined using standardized structural tables that are part of residential building codes, or, for more complex framing or special conditions, custom engineering may be required. Larger-scale  section details, similar to those seen throughout this chapter, are prepared for major connections in the building system. The architectural floor plans serve to indicate the locations and dimensions of walls, partitions, and openings, and the exterior elevations show the outside faces of the building, with vertical dimensions or elevations indicated as required. For most buildings,  building sections are also drawn that cut completely through the building, showing the dimensional relation- ships of the various floor levels and roof planes and the slopes of the roof surfaces.  Interior elevations are often prepared for kitchens, bathrooms, and other rooms with elaborate interior features.

Figure 5.16 A floor plan and building section are two important components of the construction drawings for a simple house with wood light framing. The ground floor is a concrete slab on grade.

Figure 5.17 A framing plan for the ground-floor platform of the building

Figure 5.51

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Although complex in its details, the platform frame is simple in concept  (Figure 5.3). A floor  platform  is  built. Loadbearing walls are erected  upon it. A second-floor platform is  built upon these walls and a second  set of walls upon this platform. The  attic and roof are then built upon  the second set of walls. There are,  of course, many variations: A concrete slab that lies directly on the ground is sometimes substituted for  the ground-floor platform; a building may be one or three stories tall  instead of two; and several types of  roofs are frequently built that do not  incorporate attics. The essentials,  however, remain: A floor  platform  is completed at each level, and the  walls bear upon the platform rather  than directly upon the walls of the  story below.

Figure 5.3 The concept of platform framing, shown in cross section, reading from left to right: A foundation wall is constructed. A ground-fl oor platform is framed and sheathed. Ground-fl oor wall frames are assembled horizontally on the platform, then tilted into their fi nal positions. A second-fl  oor platform is constructed on top of the ground-floor walls, and the process of wall construction is repeated. The attic floor and roof are added.

The advantages of the platform  frame over the balloon frame are several: It uses short, easily handled lengths of lumber for the wall framing. Its vertical hollow spaces are  automatically Þ restopped by the  platform framing at each floor.  Its  platforms are convenient working  surfaces for the carpenters who build  the frame. The major disadvantage  of the platform frame is that each  platform constitutes a thick layer of
wood whose grain runs horizontally.  This leads inevitably to a relatively  large amount of vertical shrinkage  in the frame as excess moisture dries  from the wood, which can cause dis- tress in the exterior and interior finish surfaces.

A conventional platform frame is made entirely of nominal 2-inch  members, which are actually 1½  inches (38 mm) in thickness. These are ordered and delivered cut to  the nearest 2-foot (600-mm) length,  then measured and sawn to exact  length on the building site. All  connections are made with nails,  using either face nailing, end nailing, or toe nailing as  required by the characteristics of  each joint. Nails are driven either by  hammer or nail gun. In either case,  the connection is quickly made be- cause the nails are installed without  drilling holes or otherwise preparing the joint.

Each plane of structure in a  platform frame is made by aligning a number of pieces of framing  lumber parallel to one another at specified intervals, nailing these to crosspieces at either end to main- tain their spacing and flatness, and then covering the plane of framing with  sheathing, a facing layer of  boards or panels that join and stabilize the pieces into a single struc- tural unit, ready for the application of finish materials inside and out (Figure 5.4). In a floor structure, the parallel pieces are the floor  joists,  and the crosspieces at the ends of the joists are called headers, rim joists,  or  band joists. The sheathing on a floor is known as the  subfl  oor. In a wall structure, the parallel pieces  are the  studs, the crosspiece at the  bottom of the wall is the sole plate or  bottom plate, and the crosspiece at the  top (which is doubled for strength if  the wall bears a load from above) is  called the top plate. In a sloping roof,  the rafters are headed off by the top  plates at the lower edge of the roof  and by the ridge board at the peak.

Figure 5.4 The basic components of platform frame construction. (a) Walls are framed with repetitive vertical studs that are connected at the top and bottom by horizontal plates. (b) Floors are framed with repetitive joists that are connected at their ends by headers. (c) Roofs are framed with rafters. Every surface is sheathed with wood panels of either plywood (as shown) or, more commonly, less expensive OSB.

Openings are required in all  these planes of structure: for windows  and doors in the walls; for stairs and  chimneys in the floors; and for chimneys, skylights, and dormers in the  roofs. In each case, these are made  by heading off the opening: Open- ings in  floors are framed with headers and trimmers, which
usually must be doubled to support the higher loads placed on them by  the presence of the opening. In walls,  sills head off the bottoms of window  and door openings, while trimmer  studs on the sides provide support to loadbearing headers across the tops.

Sheathing, a layer of wood panels nailed over the outside face of  the framing, is a key component of  platform framing. The end nails that  connect the plates to the studs have  little holding power against uplift  of the roof by wind, but the sheath- ing connects the frame into a single  strong unit from foundation to roof.  The rectilinear geometry of the parallel framing members has no useful  resistance to wracking by lateral forces  such as wind, but rigid sheathing  panels brace the building effectively  against these forces. Sheathing  also furnishes a surface to which  shingles, boards, and ß  ooring  are  nailed for finish surfaces. In build- ings constructed without sheathing,  or with sheathing materials that are  too weak to brace the frame, such  as insulating plastic foam, diagonal  bracing must be applied to the wall  framing to impart lateral stability.

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Heavy timber cannot span as far or with such delicacy as steel, and it cannot mimic the structural continuity or smooth shell forms of concrete, yet many people respond more positively to the idea of a timber building than they do to one of steel or concrete. To some degree, this response may stem from the color, grain figure, and warmer feel of wood. In part, it may also come from the pleasant associations that people have with the sturdy, satisfying houses our ancestors erected from handhewn timbers only a few generations
ago. Most people today live in dwellings where none of the framing is exposed. Exposed ceiling beams in
one's house or apartment are considered an amenity, and shopping or restaurant dining in a converted mill  building of heavy timber and masonry construction is generally thought  to be a pleasant experience. There  is something in all of us that derives satisfaction from seeing wood beams  at work.

In economic reality, heavy timber must compete successfully on the basis of price with other construction materials, and as our forests have diminished in quality and shipping costs have risen, timber is no longer an automatic choice for building a mill or any other type of structure. For many  buildings, however, heavy timber is an economical alternative to steel and concrete, particularly in situations  where the appearance and feel of its large wooden members will be highly  valued by those who use it, in regions  close to commercial forests, or where code provisions or fire insurance premiums create a financial incentive.

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The table in Figure 1.2 shows the range of height and area limits for buildings of different occupancies built of Heavy Timber (Type IV) construction and of Light Frame  (Type V) construction (applicable when heavy timber framing  and light frame construction are mixed). Notice that when consid- ering any particular occupancy,  the allowable heights and areas for Heavy Timber construction are  comparable to those for 1-Hour-Protected Noncombustible (Type IIA) construction (steel, concrete,  or masonry) and are greater than  those for protected and unprotected Type V construction or unprotected steel (Type IIB). These limits  are an explicit recognition of the inherent fire-resistive qualities of heavy timber framing.

Figure 1.2 Height and area limitations of build- ings of various types of construction, as defi ned in the 2006 IBC. (Portions of this publication reproduce tables from the 2006 International Building Code

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Estimate the nominal depth of wood roof decking at 1 /45 of its span. Estimate the depth of wood floor decking at  1 /35 of its span. Standard nominal depths of wood decking  are 2, 3, 4, 6, and 8 inches (actual size 38, 64, 89, 140, and  184 mm).

Estimate the depth of solid wood beams at 1 /15 of their span and the depth of glue-laminated beams at  1/ 20 of their span. Add a nominal 6 inches (150 mm) to these depths for  girders. The width of a solid wood beam or girder is usually  1 /4 to  1 /2 of its depth. The width of a glue-laminated beam  typically ranges from  1 /7 to 1 /4 of its depth.

Estimate the depth of timber triangular roof trusses at  1 /5 to  1 /2 of their span and the depth of bowstring trusses at  1 /2 to  2/ 3 of their span.

To estimate the size of a wood column, add up the total  roof and ß oor area supported by the column. A nominal 6- inch (actual size 140 mm) column can support up to about  400 square feet (37 m2) of area, an 8-inch (actual size 184  mm) column 1000 square feet (93 m2), a 10-inch (actual  size 235 mm) column 1500 square feet (140 m2), a 12-inch (actual size 286 mm) column 2500 square feet (230 m2),  and a 14-inch (actual size 337 mm) column 3500 square  feet (325 m2). Wood columns are usually square or nearly  square in proportion.

For actual sizes of solid timbers in conventional units,  see Figures 3.22 and 3.23. Standard sizes of glue-laminated  timbers are given in Chapter 3. For a building that must  qualify as Type IV Heavy Timber construction under the  IBC, minimum timber sizes are given in Figure 4.7.

Figure 3.23 A complete chart of nominal and actual dimension for both framing lumber and finish lumber

These approximations are valid only for purposes  of preliminary building layout, and must not be used  to select final member sizes. They apply to the normal  range of building occupancies such as residential, office, commercial, and institutional buildings. For manufacturing and storage buildings, use somewhat larger  members.