Engineering and Construction

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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.

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Structural wood panel products fall into three general categories (Figure 3.29): Plywood panels are made up of thin layers of wood  veneer glued together. The grain on the front and  back face veneers runs in the long direction of the sheet, whereas the grain  in one or more interior crossbands  runs perpendicular, in the shorter direction. There is always an odd  number of layers in plywood, which  equalizes the effects of moisture movement, but an interior layer may  be made up of a single veneer or of  two veneers with their grains running  in the same direction. Composite panels have two parallel face veneers bonded to a core of reconstituted wood fibers. Nonveneered panels are of several  different types:

Oriented strand board (OSB) is made of long shreds (strands) of wood  compressed and glued into three to five layers. The strands are oriented  in the same manner in each layer  as the grains of the veneer layers in  plywood. Because of the length and  controlled orientation of the strands,  OSB is generally stronger and stiffer  than the other types of nonveneered panels. Because it can be produced  from small trees and even branches,  OSB is generally more economical than plywood. It is the material  most commonly used for sheathing  and sub flooring of light frame wood buildings. OSB is also sometimes  called waferboard, a name that applies  more precisely to a similar type of  panel made of large flakes of wood  that has been largely replaced by  OSB.

Particleboard is manufactured in different density ranges, and is made up  of smaller wood particles than OSB  or waferboard that are compressed and bonded into panels. It  finds use  in buildings mainly as a base material  for wood veneer and plastic laminate.



It is also used commonly as an under- layment panel to create an especially  smooth substrate for the application  of resilient flooring.

Fiberboard is a very fine-grained  board made of wood fibers  and  synthetic resin binders intended for  interior uses only. The processing of  the raw wood products in fiberboard  manufacturing is more intensive  than that in the manufacture of particleboard, resulting in a panel that  is dimensionally more stable, stiffer, better able to hold fasteners, and  superior in its working and finishing  characteristics. The most commonly  used form of fiberboard  is  medium- density fi  berboard (MDF). It is used in  the production of cabinets, furniture,  moldings, paneling, and many other  manufactured products. Though the  names are similar, care should be taken not to confuse MDF panels with  medium-density overlay (MDO) ply- wood panels, discussed below.

Figure 3.29 Five different wood panel products, from top to bottom: plywood, composite panel, waferboard, OSB, and particle- board.