APA Residential & Commercial - Page 59

Heavy Timber Construction

Model building codes also recognize Heavy Timber wood construction systems, which can simplify from or floor construction while providing superior fire resistance.e Heavy Timber construction does not constitute one-hour fire resistance, however.

Under fire conditions, large size timber members develop a surface char layer which acts as insulation to slow the burning process . In addition, Heavy Timber construction does not permit concealed wall or ceiling spaces where fire can spread. Years of fire service experience shows that the structural performance of Heavy Timber constructions systems under fire conditions is markedly superior to most unprotected "noncombustible" (steel) structures.

See Table 35 for minimum structural member sizes required by middle building codes for Heavy Timber construction. structural glued laminated timber (glulams) also qualifies of Heavy Timber construction systems when members conform to required sizes.

Insurance rating bureaus and all of the model building codes accept 1-1/8-inch tongue-and-groove plywood with exterior glue  (Exposure 1 ) as an alternative to two-inch nominal tongue-and-groove lumber decking in heavy timber roof lumber decking in heavy timber roof construction.

Typical construction (Figure 30) consists of tongue-and-groove plywood APA RATED STURD-I-FLOOR 48 oc (2-4-1) Exposure 1 (or 1-1/8-inch tongue-and-groove plywood APA RATED SHEATHING Exposure 1- Check local availability before specify8ng). Heavy timber beams must be 4x6 minimum and are normally spaced 48 inches on center. For an exposed ceiling with improved appearance, 1-1/8-inch textured plywood APA RATED SHEATHING/CEILING DECK panels can be specified.

Heavy timber floors may also be constructed with 15/32-inch plywood of three-inch planes.

For additional information on fire-resistant construction, see APA Design/Construction Guide: Fire-Rated Systems, Form W305.

Wind-resistive Roofs

Wind ratings are based on roof system's performance in wind uplift test. System meeting U.L. requirements are assigned a semi-wind-resistive classification (Class 30 or 60) or full-wind-resistive classification (Class 90).

Many of the fire-rated assemblies also can qualify for these rating.s Heavy Timber usually is accorded semi-wind-resistive recognition.

Three are over 70 U.L. listed roofing/roof deck systems with plywood sheathing for fully-wind-resistive Class 90 ratings. One of the systems, NM519, is illustrated in Figure 31. Another is NM 520, a panelized roof deck of 15/32-inch APA RAGED SHEATHING 32/16 Exposure 1 plywood marked PS1 (CDX, 4 plies minimum, all-Group 1