PLYWOOD SHEATHING

Abstract:


 

Plywood Sheathing
In this article, Mr. Geary's discussion revolves around the virtues and versatilities of plywood, if it is "installed and protected properly." He describes the many uses of plywood, as well as the most common problems encountered with its installation: the failure to space panels correctly, which often leads to expansion problems, as well as the related problem of covering plywood, trapping moisture with it, which puts it under unnecessary stress. He offers some solutions to these problems and also gives some tips on choosing plywood, as well as basic installation tips and specific notes about good nail and building paper types to use.

 

Article:


 

PLYWOOD SHEATHING

Sheets of plywood, and sometimes other wood based panels such as particle board, are normally installed on floors as subflooring and underlayment; on walls to provide shear value and help resist stress from wind-racking and other forces; on decks of balconies and elevated pedestrian walks as a substrate to be coated with some protective covering such as urethane, lightweight concrete or other material; and on roofs, as substrate for roofing. Plywood and its relatives generally serve in an excellent manner in those types of installations, IF INSTALLED AND PROTECTED PROPERLY.

Certain precautions, prior to, during, and subsequent to installation of decking or sheathing are appropriate for prevention or reduction of future problems. One very important feature that must be adhered to is that contractors and carpenters should abide by the recommendations and suggestions made available by the American Plywood Association. One extremely critical factor, that I frequently observe has not been done correctly, is the failure of carpenters to space adjoining panels of plywood in accordance with recommendations of the American Plywood Association. The aggregate of experience, knowledge and industry research developed by that association over a period of many years should not be neglected. I frequently find, on new installations of plywood on floors, walls, balconies and roofs, that no gap has been left between adjoining ends and sides of sheets of plywood. One unfortunate result of that failure by builders is that space is not provided for possible future expansion of plywood, if moisture should contact and be absorbed into the wood. Cellulosic materials such as lumber, plywood, particle board, cardboard cartons, etc., expand upon absorption of moisture. In the case of plywood decking and subflooring, sheathing on walls, and substrate of roofs, buckling of plywood and other types of wood-based panels must be expected to occur, if a sufficient amount of moisture should contact panels of plywood, subsequent to nailing or stapling panels to the base. Expansive buckling in panels of plywood may cause similar bulges in floor covering, or raised ridges telegraphing through resilient sheet-type floor covering and resilient floor tiles over alignment of edges of panels of subflooring; cracks in cement plaster applied over plywood-sheathed walls; and serious problems related to roofing installed over wood-based panels. Some of those problems may be the result of distress within panels of plywood, to be discussed in the next paragraph, but generally such adverse conditions are a direct result of improper installation by builders.

If, after installation, it has been discovered that sheets of plywood sheathing or underlayment have been butted against others, rather than having the recommended gap between panels, the condition can be cured. A saw kerf may be cut between adjoining sheets of plywood or particle board to provide that relief. The saw kerfs must be cut before building paper is applied to walls.

Carpenters should inspect the stock of plywood panels prior to installation, to look for possible deficiencies such as delamination of plies, due to exposure to weather while in transit or in stock. Any panels that have suffered delamination should be rejected.

Panels of plywood, particle board, flakeboard and other wood-based sheets should be kept dry while in storage on the job site.

A related problem is the act of covering wet plywood or similar panels with floor covering, deck coating, plaster or roofing, to entrap moisture and impose excessive stress against floor and wall covering and, worse, roofing, as plywood or particle board panels dry in place on floors, walls or roofs after having been covered with the final decorative and protective roofing, plaster, deck coating or floor covering. Drying shrinkage of panels of sheathing may adversely affect a covering installed over the panels.

Failure to provide for expansion of sheets of plywood or particle board on floors causes another problem besides buckling along edges and in the field of panels of underlayment, as well as curled edges of panels. Failure to allow for expansion of underlayment and to provide space for movement between adjacent sheets of plywood frequently causes squeaking floors, underneath resilient floor covering and carpet. The above admonition, of course, is not the only reason floors may squeak. Incorrect nailing procedure, and incorrect sequence of nailing of panels by carpenters frequently causes squeaks in floors. Builders and their carpenters should consider recommendations of knowledgeable people in the construction industry in such matters, and provide for increase in dimension of panels of sheathing on floors, decks, walls and roofs.

A thinking carpenter will NOT nail all four cornerss of panels of plywood or other wood-based sheathing or underlayment initially. Instead, he will start nailing at one corner and proceed fan-wise across the sheet, or start at one side and proceed across the panel from one side to the other. One should never nail from the four edges toward the center of the panel.

I do not favor the use of smooth nails to secure underlayment to floor joists. Instead, I prefer that galvanized nails, cement-coated nails, or ring-shank nails be used for that purpose, to avoid nail pop and possible squeaking floors. Spacing of nails should be in accordance with the nailing schedule recommended by the American Plywood Association and the Uniform Building Code.

When installing plywood as sheathing over wood studs on walls, the American Plywood Association recommends that panels of plywood be attached with the long dimension of sheets at right angles to framing members; that is, in horizontal alignment. The purpose is to have panels of sheathing cross and be fastened to the greatest possible number of studs. Vertical installation of panels against studs does not achieve that desired result. Horizontal edges of plywood should be supported by blocking secured between studs.

Successive courses of plywood should have the ends of panels offset, perhaps half the length of a panel or at least one stud space from ends of sheets of plywood in adjacent courses, to gain improved shear value. One should avoid long runs of continuing or matching junctures between panels.

I have encountered situations in which thinner plywood sheathing had been applied to second or upper levels of buildings than had been attached to the first level of the structure. That created a condition in which there was an abrupt change in thickness of the substrate behind plaster. Paper backing flared outward from the thinner panels of plywood to the thicker sheets, at that alignment. The described condition may cause tearing of the weather-barrier backing at that alignment, with resultant loss of integrity of the barrier. In addition, that abrupt change in thickness of substrate causes an abrupt reduction in thickness of applied plaster. The result may be an essentially straight-line fracture in the completed plaster membrane. It is obvious that conditions of that nature should be avoided by builders, to reduce the hazard of possible leakage of rain water into the structure, plus cracking. If it is deemed advisable that there should be a variation in thickness of sheathing, the builder should install a tapered shim to slope gradually from the thinner to thicker sheathing.

One should never expect marine plywood not to be affected by water, unless marine plywood has been fully and properly coated with sufficient good quality water-resistant paint properly applied to prevent contact of moisture with wood at any location. The several laminations in marine plywood are only wood, and thus are subject to possible ill-effect from contact with moisture. The glue line of marine plywood is much more resistant to water than the glue line of normal plywood. The proper type of plywood for specific uses should be selected from tables of groups and ratings prepared by the American Plywood Association.

The American Plywood Association has recommended that builders allow one-eighth inch (1/8") spacing at panel ends and one-quarter (1/4") at panel edges, unless otherwise recommended by the panel manufacturer, for roof sheathing, subflooring and wall sheathing. For all uses, I recommend that all nails be ring-shank, specially coated or galvanized, to provide improved grip and greatly reduce or prevent the possibility of nail pop. The nailing schedule recommended by the American Plywood Association should be adhered to, --- that is, nails to be driven six inches (6") on center long panel edges and twelve inches (12") on center at intermediate supports, using 6d common nails for panels one-half inch (1/2") and less in thickness, and 8d nails for panels greater than one-half inch in thickness.

The Uniform Building code requires that two layers of treated building paper or asphalt-saturated felt be applied over wood sheathing on walls that are to receive cement plaster (stucco), to protect the wood substrate from moisture and delayed expansion of wood. Building paper must be lapped properly, shingle-fashion, to deflect possible intruded water downward and away from wood sheathing.

 

 

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