Installation Guide

ENGINEERED WOOD CONSTRUCTION GUIDE

■

FORM NO. E30V

■

WWW.APAWOOD.ORG

Roof Construction

Plywood Under Special Coatings

Chemical coatings for roofs have increased the range of design possibil ities, particularly in larger commercial structures

with contoured or steeply pitched roof surfaces exposed to view.

The plywood thickness and span recommendations in Table 35 for ply wood under special coatings assume installa-

tion with the panel continuous over two or more spans with the long dimension or strength axis perpendicular to

supports and liquid coatings applied directly to the plywood. Check local building codes for any required deviations

from these recommendations. Allowable roof live load is based on the same deflection criteria as described in Table 30

for APA panel roof sheathing.

Exterior plywood is recommended for use under special coatings for roofs. Where the coating requires a very smooth

base, use APA A-C Exterior or APA B-C Exterior plywood. Where maximum smoothness is not essential, use APA C-C

PLUGGED Exterior. Tongue-and-groove plywood (Performance Category 15/32 or larger) or lumber blocking at panel

edges is recommended. A 1/8-inch space is recommended at all edge and end joints unless otherwise indicated by panel

manufacturer. If high-performance coatings are to be used for finish, check coating manufacturer’s recommendations

for panel joint treatment. Nail size, type and spacing recommendations are also given in Table 35.

Grades recommended in Table 35 should also be specified for the top layer when the structural wood deck is to be over-

laid with a sepa rate plywood layer to serve as substrate for special roof coatings. A 1/8-inch space is recommended at

all edge and end joints unless otherwise indicated by panel manufacturer. Although minimum 1/4-inch plywood may

be used over structural decks, Performance Category 15/32 or larger panels should be considered for best performance

over uneven surfaces or when rain or high humidity is anticipated prior to application of roof coating. Use corrosion-

resistant fasteners sized and spaced as recommended in Table 35.

APA Panel Soffits

Recommended spans for open and closed APA panel soffits are given in Tables 36 and 37. The recommendations in Table

36 for open soffits also apply to combined roof/ceiling construction. Panels are assumed continuous over two or more

spans with the long dimension or strength axis across supports for both applications. For appearance purposes in

open soffit construction, provide blocking, tongue-and-groove edges, or other suitable edge support. Panels will sup-

port at least 30 psf live load, plus 10 psf dead load.

For open soffit and nonstructural ceiling construction, panels designated Exposure 1 are recommended as a mini mum

(check local building code) where appearance is not a major consideration.

TABLE 35

PLYWOOD THICKNESS AND MAXIMUM SPANS FOR ROOF DECKS UNDER SPECIAL COATINGS

(a)

Grade

Minimum

Plywood

Performance