Engineering and Construction

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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.

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Long curved timbers for making heavy timber vaults and heavy timber domes are  easily fabricated in laminated wood  and are widely used in athletic arenas,  auditoriums, suburban retail stores,  warehouses, and factories (Figures  4.29-4.31). Arched structures, like  rigid frames, exert lateral thrusts that  must be countered by tie rods or suit- ably designed foundations.

Figure 4.29 Semicircular laminated wood arches support the timber roof of the Back Bay Station in Boston. Notice the use of horizontal steel tie rods to resist the thrust of the arches at  the base.

Figure 4.30 A typical hinged foundation connection for an arch or dome, made of welded steel plates. The hinge pin allows for rotation between the arch and the foundations, which avoids many kinds of forces that might otherwise be placed on the structure by wood shrinkage and foundation movement.

Figure 4.31 This laminated wood dome spans 530 feet (161.5 m) to cover a 25,000-seat stadium and convention center in Tacoma, Washington.

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The cruck (Figure 4.2), cut from a  bent tree, was a form of  rigid frame or  portal frame. Today's rigid frames are glue-laminated to shape and find wide use in longer-span buildings.  Standard configurations are readily available (Figures 4.22-4.24), or the  designer may order a custom shape.  Rigid frames exert a horizontal  thrust, so they must be tied together  at the base with steel tension rods, also  called  tie rods. In laminated wood construction, rigid frames are often called arches, acknowledging that the  two structural forms act in very nearly  the same manner.

Figure 4.2 European timber house forms generally followed a progression of development from crude pit dwellings, made of earth and tree trunks, to cruck frames, to braced frames. The crucks (curved timbers), hewn by hand from appropriately shaped trees, were precursors to the laminated wood arches and rigid frames that are widely used today.

Figure 4.22 Three-hinged arches of glue-laminated wood carry laminated wood roof purlins. The short crosspieces of wood between the purlins are temporary ladders for workers.

Figure 4.23 Typical details for three-hinged arches of laminated wood. The tie rod is later covered by the fl oor slab.

Figure 4.24 The shear plates in the crown connection of the arch, shown here in a larger-scale detail, are recessed into grooves in the wood and serve to spread any force from the steel rod across a much wider surface area of wood to avoid crushing and splitting.

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With large, old-growth trees no longer readily available, very large timber  beams are usually built up of laminations or strands rather than sawn. Such beams are stronger and more  dimensionally stable than sawn wood  beams and can be made in the exact  size and shape desired (Figures 4.20  and 4.21). Hybrid or FRP reinforced  glulam beams, can be useful when especially lon- ger span are needed or heavy loads  must be supported.

Figure 4.20 Installing tongue-and-groove roof decking over laminated beams and  girders.

Figure 4.21 This roof uses glue-laminated beams to support proprietary long-span trusses made of wood with steel tube diagonals.

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The heavy timber structure poses some special problems for the designer because the exposed framing and decking do not provide the concealed cavities that are present in light frame structures and other conventional building systems. Roof  thermal insulation cannot be hidden in spaces between ceiling joists  or roof rafters and, instead, must be placed on top of the roof deck. If the  roof is nearly flat, it can be insulated and roofed in the manner shown for  low-slope roofs in Chapter 16, or, if  the roof is steeply pitched, a nailing surface for the shingles or other roofing must be added on the outside of  the insulation. Electrical wiring for lighting fixtures that are mounted on the underside of the decking must  either run through exposed metal  conduits below the deck, which may  be visually unsatisfactory, or be channeled through the insulation above the deck. Ductwork for heating and  cooling must remain exposed. If  the walls and partitions are made of  masonry or stressed-skin panels, special arrangements are necessary for  installing wiring, plumbing, and me- chanical system components in the  walls as well (Figure 4.19).

Figure 4.19 When floors and roofs are heavy timber framed, lighting, communications wiring, ductwork, fi re sprinklers, and other services are exposed to view.

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A heavy timber frame building with an exterior masonry or concrete bearing  wall is normally braced against wind and seismic forces by the shear resistance of its exterior walls, working  together with the diaphragm action of its roof and floor decks. In areas of high seismic risk, the walls must be  heavily reinforced both vertically and horizontally, and the decks may have to be specially nailed or overlaid with plywood to increase their shear resistance, as well as specially anchored  to the perimeter walls. In buildings with framed exterior walls, diagonal bracing or shear panels must be provided. Seismic upgrades to historic  heavy timber and masonry buildings often require the insertion of new steel-braced frames or reinforced  concrete shear walls in order to meet contemporary lateral force resistance requirements.

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Building codes require that Type IV Heavy Timber buildings have floors and roofs of solid wood construction without concealed cavities. Figure 4.18 shows different types of decking used for these purposes. Minimum permissible thicknesses of decking are given in Figure 4.7. To meet code requirements, floor decking must also be covered with a finish  floor consisting of nominal 1-inch (19-mm)  tongue-and-groove boards laid at right angles or diagonally to the struc- tural decking. In some circumstances,  ½-inch (13-mm) plywood or other composite wood panels are also per- mitted as the finish layer.

Figure 4.18

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Where heavy timber beams join masonry or concrete walls, three problems must be solved: First, the beam  must be protected from decay caused by moisture seeping through the wall.  This is done by leaving a ventilating  airspace of at least ½ inch (13 mm) between the masonry and all sides of the beam except the bottom, unless the beam is chemically treated  to resist decay. The second and third  problems have to be solved together:  The beam must be securely anchored  to the wall so that it cannot pull away under normal service, yet it must be able to rotate freely so that it does not pry the wall apart if it burns through and collapses during a severe fire (Figure 4.12). Traditional and more contemporary methods of accomplishing these dual needs are shown  in Figures 4.11 and 4.13, respectively.

Figure 4.11 Two alternative details for the bearing of a beam on masonry in traditional Mill construction. In each case, the beam end is fi recut to allow it to rotate out of the wall if it burns through (Figure 4.12), but it is also anchored against pulling away from the wall by means of either lag screws or a lug on the iron bearing plate.

Figure 4.12 A timber beam that burns through in a  prolonged fi re is likely to topple its sup- porting masonry wall (left) unless its end  is fi recut (right). In this illustration, the beam is anchored to the wall in each case  by a steel strap anchor.

Figure 4.13 An example of contemporary heavy timber construction, based on steel plate connectors and an insulated cavity wall.

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Where the edges of ß oors and roofs of a heavy timber frame are supported on concrete or masonry, special attention must be given to the potential for differential shrinkage between  the outer walls and the interior wood  column supports. Wood, compared  to masonry or concrete, expands  and contracts more with changes in  moisture content, particularly in the  direction perpendicular to its grain.

These changes occur over a period of  years as large timbers gradually dry  and seasonally with changes in ambient conditions. A heavy timber frame  building is detailed to minimize the  effects of this differential shrinkage  by eliminating cross-grain wood from  the interior lines of support. In traditional Mill construction, cast iron

Figure 4.7 Minimum sizes for solid wood members used in Type IV Heavy Timber construction, as specifi ed in the IBC.

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In addition to the issues of sustainability of wood production and use that were raised in the previous chapter,  there are issues that pertain especially to heavy timber  frame construction:

It is wasteful to saw large, solid timbers from logs:
In most instances, only one or two timbers can be  obtained from a log, and it is often difficult to saw  smaller boards from the leftover slabs.

Glue-laminated timbers and composite timbers utilize  wood fiber much more efficiently than solid timbers.

Recycled timbers from demolished mills, factories,  and barns are often available. Most of these are  from old-growth forests in which trees grew slowly,  producing fine-grained, dense wood. As a result,  many have structural properties that are superior to  those of new-growth timbers. Recycled timbers may be  used as is, resurfaced to give them a new appearance,  or resawn into smaller members. However, they  often contain old metal fasteners. Unless these are  meticulously found and removed, they can damage  saw blades and planer knives, causing expensive mill shutdowns while repairs are made.

Continuous bending action of beams may be created  by splicing beams at points of inß ection rather than  over supports, as shown in Figures 4.15, 4.20 and 4.21.  This reduces maximum bending moments, allowing  timber sizes to be reduced substantially.

Timbers do not lose strength with age, although they  do sag progressively if they are overloaded. When a  heavy timber building is demolished at some time in the future, its timbers can be recycled, even if they  were obtained as recycled material for the building  that is being demolished.

A heavy timber frame enclosed with foam core  sandwich or stressed-skin panels is relatively airtight  and well insulated, with few thermal bridges. Heating  and cooling of the building will consume relatively  little energy.

The glues and finish coatings used with glue-laminated  timbers may give off gases such as formaldehyde that  can cause indoor air quality (IAQ) problems. It is  wise to determine in advance what glues and coatings  are to be used, and to avoid ones that may cause  IAQ problems.

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Large timbers, because of their greater capacity to absorb heat, are  much slower to catch Þ re and burn  than smaller pieces of wood. When  exposed to fire, a heavy timber beam,  though deeply charred by gradual  burning, will continue to support its load long after an unprotected steel  beam exposed to the same conditions has collapsed. If the fire is not  prolonged, a fire-damaged  heavy  timber beam or column can often be  sandblasted afterward to remove the  surface char and continue in service.

For these reasons, building codes  recognize heavy timber  framing that meets certain specific  requirements as having fire-resistive properties.

In the International Building  Code (IBC), for a building to be classified as Type IV Heavy Timber (HT)  construction, its wooden structural  members must meet certain mini- mum size requirements and its exterior walls must be constructed of  noncombustible materials. Minimum  permitted sizes for solid wood timbers  are summarized in Figure 4.7. Glue- laminated members used in this type of construction must meet similar  requirements. Exterior walls may be  constructed of concrete, masonry, or  metal cladding. Historically, this combination of fire-resistive wood framing  and noncombustible exterior was referred to as Mill construction, reflecting  its origins in 19th-century brick ma- sonry mill structures, or as Slow-burning construction (Figures 4.8-4.11).

Figure 4.7 Minimum sizes for solid wood members used in Type IV Heavy Timber construction, as specifi ed in the IBC.

Traditionally, the edges of the timbers  were also chamfered (beveled at 45 degrees) to eliminate the thin edges of  wood that catch fire most easily, but  this is no longer a code requirement.

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Wood beams have been used to span roofs  and floors of buildings since the beginning of  civilization. The fi rst timber-framed buildings  were crude pit houses, lean-tos, teepees, and  basketlike assemblages of bent saplings. In  earliest historic times, roof and fl oor timbers  were combined with masonry loadbearing walls to  build houses and public buildings. In the Middle  Ages, braced wall frames of timber were built for  the first time (Figures 4.1 and 4.2). The British  carpenters who emigrated to North America in  the 17th and 18th centuries brought with them  a fully developed knowledge of how to build  efficiently braced frames, and for two centuries  North Americans lived and worked almost  exclusively in buildings framed with hand-hewn wooden timbers joined by interlocking wood-to- wood connections (Figures 4.3 and 4.4). Nails were rare and expensive, so they were used only  in door and window construction and, sometimes,  for fastening siding boards to the frame.

Until two centuries ago, logs could be converted to boards and timbers only by human muscle power. To make timbers, axemen  skillfully scored and hewed logs to reduce them  to a rectangular profi le. Boards were produced  slowly and laboriously with a long, two-man pit  saw, one man standing in a pit beneath the log,  pulling the saw down, and the other standing  above, pulling it back up. At the beginning of  the 19th century, water-powered sawmills began to take over the work of transforming tree trunks into lumber, squaring timbers and slicing  boards in a fraction of the time that it took to  do the same work by hand.

Most of the great industrial mills of 19th-century  North America, which manufactured textiles,  shoes, machinery, and all the goods of civilization,  consisted of heavy sawn timber fl oors and roofs  supported by masonry exterior walls (Figures 4.5 and 4.6). The house builders and barn raisers of the  early 19th century switched from hand-hewn to sawn timbers as soon as they became available. Many  of these mills, barns, and houses still survive, and  with them survives a rich tradition of heavy timber  building that continues to the present day.

Figure 4.1 Braced wall framing was not developed  until the late Middle Ages and early Renaissance, when it was often exposed  on the face of the building in the style of construction known as halftimbering.  The space between the timbers was fi  lled with brickwork or with wattle and daub, a  crude plaster of sticks and mud, as seen here in Wythenshawe Hall, a 16th-century  house near Manchester, England.

Figure 4.2 European timber house forms  generally followed a progression of development from crude pit dwellings,  made of earth and tree trunks, to cruck frames, to braced frames. The  crucks (curved timbers), hewn by hand from appropriately shaped trees, were  precursors to the laminated wood arches and rigid frames that are widely used  today.

Figure 4.3 The European tradition of heavy timber framing was brought to North America  by the earliest settlers and was used for houses and barns until well into the  19th century.

Figure 4.4 Traditional timber framing has been  revived in recent years by a number of builders who have learned the old  methods of joinery and updated them with the use of modern power tools and  equipment. (a) Assembling a bent (a plane of columns, beams, rafters, and  braces). (b) The completed bents are laid out on the fl oor, ready for raising. (c)  Raising the bents, using a truck-mounted crane, and installing fl oor framing and  roof purlins (smaller, secondary framing members that span across the primary  beams or rafters). (d) The completed frame. (e) Enclosing the timber-framed  house with sandwich panels consisting of waferboard faces bonded to an  insulating foam core.

Figure 4.5 Most 19th-century industrial buildings in North America were constructed of  heavy timber roofs and fl oors supported at the perimeter on masonry loadbearing  walls, a construction method that came to be known to as Mill construction.  This impressive group of textile mills stretches for a distance of 2 miles (3 km) along the Merrimac River in Manchester, New Hampshire.

Figure 4.6 The generously sized windows in the mills provided plenty of daylight to work  by. Columns were of wood or cast iron. Most New England mills, like this one,  were framed very simply: Their fl  oor decking is carried by beams running  at right angles to the exterior walls, supported at the interior on two lines of  columns. Notice that the fi nish fl  ooring runs perpendicular to the structural  decking. Overhead sprinklers provide additional fi re safety to a construction  method that already has inherent  fire-resistive qualities.

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Wood construction has evolved into two major systems of on-site construction: The hand-hewn frames of centuries past have become the heavy timber frames of today, used both for single-family houses and for larger buildings. And from the heavy timber frame has sprung wood light frame construction, which is the dominant  system for houses, small commercial  structures, and apartment buildings  even as tall as four or five  stories.

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Adhesives are widely employed in the factory production of plywood and panel products, laminated wood, wood structural components, and cabinetwork. Some such adhesives, most notably urea-formaldehyde, can release formaldehyde or other unhealthful gases long after the manu-facturing process is complete and the products have been placed into service in buildings. When used for interior carpentry, such products can be the source of indoor air quality problems.

Where products manufactured with adhesives or binders will be used indoors, those made with a more chemically stable exterior adhesive called phenol-formaldehyde, or with alternative formaldehyde-free adhesives, should be considered. Finishes or veneers that encapsulate composites also reduce emissions from the glues and binders used in their manufacture.

Adhesives are used less on the construction site, where it is more difficult to clamp and hold glued  joints and to maintain controlled environmental conditions until the adhesive has cured. In rough carpen-try work, adhesive is most commonly used for securing subflooring panels to wood framing (Figure 5.27). In this application, mastic sealant is applied using a sealant gun. The joint between the subß oor panel and the joist framing is then clamped by nailing or screwing the panels to the framing, the fasteners serving as the primary structural connection and the adhesive acting primarily to reduce squeaks in the floor. Adhesives of various types are also used in finish carpentry, usually in combination with nails or screws, to improve the strength and stability of joints in finish trim, site-built cabinets, and other architectural woodwork.

Figure 5.27 For a stiffer, quieter fl oor, subflooring  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.

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Dozens of ingenious sheet metal and metal plate devices are manufactured for strengthening common connections in wood framing. The most frequently used is the joist hanger, but all  of the devices shown in Figures 3.45 and 3.46 find extensive use. There are two parallel series of this type of  device, one made of sheet metal for  use in light framing and one made of  thicker metal plate for heavy timber  and laminated wood framing. The  devices for light framing are attached  with nails and the heavier devices  with bolts or lag screws. Where such framing devices will be exposed to  the weather or to lumber treated  with corrosive chemical treatments,  they should be made from corrosion- resistant metal such as heavily galvanized steel or stainless steel. To  prevent galvanic corrosion between  dissimilar metals, fasteners for metal  framing devices should always be  made of the same material as the  framing device: Galvanized framing  devices should be fastened with galvanized fasteners, and stainless steel  devices should be fastened with stain-less steel fasteners.

Figure 3.45 Joist hangers are used to make strong connections in fl oor framing wherever wood joists bear on one another at right angles. They are attached with special short nails driven through the holes punched in the hangers. The heavier steel beam hangers are used primarily in laminated wood construction. Post bases serve the twofold function of preventing water from entering the end of the post and anchoring the post to the foundation. The bolts and lag screws used to connect the wood members to the heavier connectors are omitted from this drawing.

Figure 3.46 The sheet metal connectors shown in this diagram are less commonly used than those in Figure 3.45, but are invaluable in solving special framing problems and in reinforcing frames against wind uplift and earthquake forces.

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Fasteners have always been the weak link in wood construction. The interlocking timber connections of the past, laboriously mortised and pegged, were weak because much of the wood in a joint had to be removed to make the connection. In todays wood connections, which are generally based on metal devices, it is usually impossible to insert enough nails, screws, or bolts in a connection to develop the full strength of the members being joined.

Adhesives and toothed plates are often capable of achieving this strength but are largely limited to factory installation. Fortunately, most connections in wood structures depend primarily ondirect bearing of one member on an-other for their strength, and a varietyof simple fasteners are adequate forthe majority of purposes.

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Hardboard is a thin, dense panel made of highly compressed wood fibers.

It is available in various thicknesses  and surface finishes, and in some  formulations is durable against weather  exposure. Hardboard is produced  in configurations for residential siding and roofing as well as in general-purpose and pegboard panels of standard dimension.

Insulating fi  berboard  sheathing is a low-density panel, usually ½ or ¾ inch (13 or 19 mm) thick, made of  wood or vegetable fibers and binders, and coated with asphalt for water  resistance. It has some thermal insulating value and is used in wood construction chieß  y as nonstructural wall  sheathing, although some panels of  this type are strong enough to also  contribute lateral force resistance to  a building frame. Other  cellulosic fiberboard panels are made from finely  processed recycled paper waste and  are used for wall sheathing, acoustical isolation, carpet underlayment, and  even structural roof decking. Panels  of these types are low in cost, make  productive use of waste or recycled  materials, and help to conserve forest  resources.

Hardwood plywood panels, made  from birch, maple, poplar, or alder veneers, are popular for use in  cabinetry and other interior finish carpentry.

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For structural uses such as sub-flooring  and sheathing, wood panels may be specified either by thickness or by  span  rating. The span rating is determined  by laboratory load testing and is given  on the gradestamp on the back of the  panel, as shown in Figures 3.33 and  3.34. The purpose of the span rating  system is to permit the use of many  different species of woods and types  of panels while achieving the same  structural objectives. Under normal  loading conditions, any panel with  a span rating of 32/16 may be used  as roof sheathing over rafters spaced  32 inches (813 mm) apart or as sub-flooring over joists spaced 16 inches  (406 mm) apart. The long dimension of the sheet must be placed perpendicular to the length of the supporting members. A 32/16 panel may be  plywood, composite, or OSB, may  be composed of any accepted wood  species, and may be any of several thicknesses, so long as it passes the  structural tests for a 32/16 rating.

Figure 3.33 Typical gradestamps for structural wood  panels. Gradestamps are found on the back of each panel. (Courtesy of APA–The Engineered Wood Association)

Figure 3.34 A guide to specifying structural panels. Plywood panels for use in wall paneling, furniture, and other applications where  appearance is important are graded by the visual quality of their face veneers rather than their structural properties.

The designer must also select from  three  exposure durability classifi  cations  for structural wood panels: Exterior,  Exposure 1, and Exposure 2. Panels marked Exterior are suitable for  use as siding or in other applications  permanently exposed to the weather. Exposure 1 panels have fully water- proof glue but do not have veneers of  as high a quality as those of Exterior panels; they are suitable for structural  sheathing and sub-flooring, which will  be protected from the weather once  the building is finished, but must often endure long periods of wetting during  construction. Exposure 2 is suitable  for panels that will be fully protected  from weather and will be subjected  to only a minimum of wetting during  construction. About 95 percent of  structural panel products are classified as Exposure 1.


For plywood panels intended as finish surfaces, the quality of the face  veneers is of obvious concern and  should be specified by the designer  (Figure 3.32). For example, A-B  plywood (with an A grade veneer on  one face and a B grade veneer on the  other) might be specified for cabinet construction, while less expensive  C-D plywood is adequate for exterior  sheathing that will be concealed in  the finished construction. For higher-quality architectural woodwork,  fine  flitch-sliced hardwood face ve- neers may be selected rather than  rotary-sliced softwood veneers and  the matching pattern of the veneers specified.

Figure 3.32 Veneer grades for softwood plywood. Ply- wood with A grade face veneer is costly and limited in availability. It is used only in applications demanding the highest quality appear- ance. Plywood with B face veneer is used for  concrete formwork construction and for less demanding appearance applications. Plywood  with C-plugged face veneer is used for under- layment and combination subfl  ooring/under- layment, where an especially smooth substrate is required to avoid telegraphing irregularities  through the fi nish fl ooring material. Plywood for sheathing and subfl ooring is most com- monly specifi ed as “CDX,” consisting of a C grade face, a D grade face, and constructed  with exterior glue (the panel is installed with the C face oriented toward the weather). C  grade veneer is also the lowest grade permitted in any ply of an Exterior rated panel, that is, a  siding panel intended for permanent exposure to the weather.