Engineering and Construction

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Foundations for light framing, originally made of stone or brick, are  now made in most cases of sitecast  concrete or concrete block masonry  (Figures 5.5-5.11). These materials  are highly conductive of heat and  usually must be insulated to meet  code requirements for energy con- servation (Figures 5.8, 5.9, 5.12, and  5.13). Where concrete and masonry  construction methods are not practical, such as in extremely cold regions, foundations may be constructed entirely of preservative-treated wood  (Figures 5.14 and 5.15). Such  permanent wood foundations can be con- structed in any weather by the same crew of carpenters that will frame the building; they are readily insulated in the same manner as the frame  of the house they support; and they easily accommodate the installation of electrical wiring, plumbing, and interior finish materials.

Figure 5.5 Step One in the construction of a simple platform frame building: establishing the position, shape, and size of the building on the site. After the corners of the foundation have been staked, batter boards are erected safely beyond the limits of the excavation area. String lines crossing over the corner stakes are stretched from the batter boards, and their positions on the boards are preserved with notches or nails in the boards. Later, after excavation, the strings are stretched again, and the foundation corners can be relocated at the bottom of the excavation. This drawing begins a series of isometric drawings that will follow the erection of a wood light frame building step by step throughout the course of this chapter.

Figure 5.6 Typical details for a sitecast concrete foundation and basement for a platform frame building. Details A, B, and C are keyed to the circled portions of the drawing in Figure 5.7.

Figure 5.7 Step Two in the construction of a typical platform frame building: excavation and foundations. The letters A, B, and C indicate portions of the foundation that are detailed in Figure 5.6.

Figure 5.8 Most building codes require thermal insulation of the foundation. These are three different ways of adding insulation to a  cast-in-place concrete or concrete masonry basement and one way of insulating a crawlspace foundation. The crawlspace may  alternatively be insulated on the outside with panels of plastic foam. The interior batt insulation shown in B is commonly used but  raises questions about how to avoid possible problems arising from moisture accumulating between the insulation and the wall.

Figure 5.9 Some typical concrete slab-on-grade details with thermal insulation. In areas where termites are common, there must be a metal fl ashing (termite shield) similar to that shown in Figure 5.18 C or other interruption that passes through the foam insulation above ground level to prevent termites from tunneling undetected through the insulation to reach the wood structure. This applies to the foam-insulated basement details in Figure 5.8 as well.

Insulating  concrete formwork (ICF) foundation  systems using permanent insulating  forms are easy to construct, eliminate the need for formwork removal, and provide integral insulation  (Figure 14.12).  Proprietary  precast  concrete foundation systems relying on factory-fabricated reinforced  concrete panels are rapidly erected  on site and can be used to construct  basement structures of consistent  strength and quality. They may be  manufactured with insulation integral to the precast panel or designed  to readily accept insulation added in  the field.

All basements also need to be carefully dampproofed and drained  to avoid flooding with groundwater
and to prevent the buildup of water pressure in the surrounding soil  that could cause the walls to cave in  (Figures 5.6, 5.7, and 5.11).

Figure 5.10 Erecting formwork for a sitecast concrete foundation wall. The footing has already been cast and its formwork removed. It is visible in front of the worker at the left.

Figure 5.11 Masons construct a foundation of concrete masonry. The fi rst coat of parging, which is portland cement plaster applied to help dampproof the foundation, has already been applied to the outside of the wall, and the drainage layer of crushed stone has been backfi lled in place. The projecting pilaster in the center of the wall will support a beam under the center of the main fl oor. After a second coat of parging, the outside of the foundation will be coated with an asphaltic dampproofi ng compound. (Reprinted with permission of the Portland Cement Association from Design and Control of Concrete Mixtures, 12th edition; Photos: Portland Cement Association, Skokie, IL)

Figure 5.12 A concrete masonry foundation with polystyrene foam insulation applied to its outer surface.

Figure 5.13 Following completion of the exterior siding, a worker staples a glass fi  ber reinforcing mesh to the foam insulation on the exposed portions of the basement wall. Next, he will trowel onto the mesh two thin coats of a cementitious, stucco- like material that will form a durable fi nish coating over the foam.

Figure 5.14 Erecting a permanent wood foundation. One worker applies a bead of sealant to the edge of a panel of preservative- treated wood components as another prepares to push the next panel into position against the sealant. The panels rest on a horizontal preservative-treated plank, which, in turn, rests on a drainage layer of crushed stone. Wood founda- tions can be constructed in any weather and can be insulated in the same way as the superstructure of the building.

Figure 5.15 The difference in color makes it clear where the preservative-treated wood foundation leaves off and the untreated superstructure of the building begins.

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A wood light frame building can be designed to mini- mize waste in several ways. It can be dimensioned to utilize full sheets and lengths of wood products. Most small  buildings can be framed with studs 24 inches (610 mm) o.c.  rather than 16 inches (406 mm). A stud can be eliminated  at each corner by using small, inexpensive metal clips to support the interior wall finish materials. If joists and rafters are aligned directly over studs, the top plate can be a  single member rather than a double one. Floor joists can  be spliced at points of inflection rather than over girders;  this reduces bending moments and allows use of smaller  joists. Roof trusses often use less wood than conventional  rafters and ceiling joists.
Laminated strand lumber and rim joists, wood I-joists,  laminated veneer lumber beams and headers, glue-laminated girders, parallel strand lumber girders, and OSB sheathing are all materials that utilize trees more efficiently  than solid lumber. Finger-jointed studs made up of short  lengths of scrap lumber glued together may replace solid  full-length wood studs.

Framing carpenters can waste less lumber by saving  cutoffs and reusing them rather than throwing them  automatically on the scrap heap. In some localities,  scrap lumber can be recycled by shredding it for use  in OSB production. The burning of construction scrap should be discouraged because of the air pollution it  generates.

Although the thermal efficiency of wood light frame  construction is inherently high, it can be improved  substantially by various means, as shown in Figures 7.17-7.21. Wood framing is much less conductive of heat than  light-gauge steel framing. Steel framing of exterior walls is  not a satisfactory substitute for wood framing unless the heat  flow path through the steel framing members can be broken  with a substantial thickness of insulating foam.

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 fl  ush with the interior and exterior surfaces of the studs, with an insulated space between. This detail is thermally effi cient but may not provide suffi cient nailing for interior fi  nish 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 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 fi nish. (b) For maximum thermal effi ciency, one wall frame ends with a 2 X 4 stud fl ush 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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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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Wood light frame construction is the most versatile of all building systems. There is scarcely a shape it can-
not be used to construct, from a plain rectilinear box to cylindrical towers to complex foldings of sloping
roofs with dormers of every description. During the century and a half since it first came into use, wood
light framing has served to construct buildings in styles ranging from reinterpretations of nearly all the
historical fashions to uncompromising expressions of every contemporary architectural philosophy. It
has assimilated without difficulty during this same period a bewildering and unforeseen succession of
technical improvements in building: central heating, air conditioning, gas lighting, electricity, thermal in-
sulation, indoor plumbing, prefabricated components, and electronic communications cabling.

Light frame buildings are easily and swiftly constructed with a minimal investment in tools.  Many observers of the building industry have criticized the supposed inefficiency of light frame construction, which is carried out largely by hand methods on the building site, yet it has successfully  fought off competition from industrialized building systems of every sort, partly by incorporating their  best features, to remain the least expensive form of durable construction. It is the common currency  of small residential and commercial buildings in North America today.

Wood light frame construction has its deficiencies: If ignited, it burns rapidly; if exposed to  dampness, it decays. It expands and contracts by significant amounts in response to changes in  humidity, sometimes causing chronic difficulties with cracking plaster, sticking doors, and buckling floors. The framing itself is so unattractive to the eye that it is seldom left exposed in a building. These  problems can be controlled, however, by clever design and careful workmanship, and there is no arguing with success: Frames made by the monotonous repetition of wooden joists, studs, and rafters  are likely to remain the number one system of building in North America for a long time to come.

Figure 5.1 Carpenters apply plywood roof sheathing to  a platform-framed apartment building. The ground floor is a concrete slab on grade.  The edge of the wooden platform of the upper floor is clearly visible between the  stud walls of the ground and upper floors. Most of the diagonal bracing is temporary,  but permanent let-in diagonal braces occur between the two openings at the lower left  and immediately above in the rear building. The openings have been framed incorrectly,  without supporting studs for the headers.