(a) Final allowable load is the lesser of the loads as determined by deflection and stress.

(b) Actual span of stiffeners taken as 3-1/2 inches less than center-to center spacing of purling.s

(c) Deflection limitations: Span/240under live load only; Span/180under total load, assuming a dead load of 10 psf.

(d) Loads limited by stress are based on tow conditions of duration of load: 2 months, such as for snow (1.15); and 7 days (1.25); includes effects of 10 psf dead load.

Long Span Systems

Both preframed panel systems and direct application of sheathing to secondary or primary framing are common approaches in long span roof construction. Bay spacing and type of framing govern the choice.

Experience shows that panels over supports 48 inches on center often yield maximum economy. Panels with a Span rating 48/24 are goof for at least 35 psf snow load and met the requirements for most guaranteed or warranted roofs.

Figure 24 illustrates typical connections for engineered flat roof members.

Plywood Under Special Coatings

Chemical coatings of roofs have increased the range of design possibilities, particularly in larger commercial structures with contoured or steeply pitched roof surfaces exposed to view.

The plywood thickness and span recommendations in Table 29 for plywood under special coatings assume installation with the long dimension of the panels perpendicular to supports and liquid coatings applied directly to the plywood. Check local building codes of any required deviations from these recommendations. Allowable fo0r live load is based on the same deflection criteria as described in table 24 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 (1/2 inch or thicker) or lumber blocking at panel edges recommended. A 1/8-inch space is recommended at all edge and end joints unless otherwise indicated by panel manufacture. If high-performance coating s are to be used for finish, check coating manufacturer's recommendations for panel joint treatment. Nail size, type and spacing recommendations are also give in Table 29.

Grades recommended above should also be specified when the structural wood deck is to be overlaid with a separate 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