Page 409 - 2018_IRC

ROOF-CEILING CONSTRUCTION

5. Flame spread index and smoke-developed index. dance with Section R802.1.5.6 or R802.1.5.7. Interior
6. Method of drying after treatment. fire-retardant-treated wood designated as Type A shall
be tested in accordance with the provisions of this sec-
7. Conformance to applicable standards in accor- tion.
dance with Sections R802.1.5.5 through
R802.1.5.10. R802.1.5.10 Moisture content. Fire-retardant-treated
wood shall be dried to a moisture content of 19 percent
8. For FRTW exposed to weather, or a damp or wet or less for lumber and 15 percent or less for wood struc-
location, the words “No increase in the listed tural panels before use. For wood kiln dried after treat-
classification when subjected to the Standard ment (KDAT) the kiln temperatures shall not exceed
Rain Test” (ASTM D2898). those used in kiln drying the lumber and plywood sub-
R802.1.5.5 Strength adjustments. Design values for mitted for the tests described in Section R802.1.5.6 for
untreated lumber and wood structural panels as speci- plywood and R802.1.5.7 for lumber.
fied in Section R802.1 shall be adjusted for fire-retar- R802.1.6 Cross-laminated timber. Cross-laminated tim-
dant-treated wood. Adjustments to design values shall ber shall be manufactured and identified as required by
be based on an approved method of investigation that ANSI/APA PRG 320.
takes into consideration the effects of the anticipated
temperature and humidity to which the fire-retardant- R802.1.7 Engineered wood rim board. Engineered wood
treated wood will be subjected, the type of treatment rim boards shall conform to ANSI/APA PRR 410 or shall
and redrying procedures. be evaluated in accordance with ASTM D7672. Structural
capacities shall be in accordance with ANSI/APA PRR
R802.1.5.6 Wood structural panels. The effect of 410 or established in accordance with ASTM D7672. Rim
treatment and the method of redrying after treatment, boards conforming to ANSI/APA PRR 410 shall be
and exposure to high temperatures and high humidities marked in accordance with that standard.
on the flexure properties of fire-retardant-treated soft-
wood plywood shall be determined in accordance with R802.1.8 Prefabricated wood I-joists. Structural capaci-
ASTM D5516. The test data developed by ASTM ties and design provisions for prefabricated wood I-joists
D5516 shall be used to develop adjustment factors, max- shall be established and monitored in accordance with
imum loads and spans, or both for untreated plywood ASTM D5055.
design values in accordance with ASTM D6305. Each R802.2 Design and construction. The roof and ceiling
manufacturer shall publish the allowable maximum assembly shall provide continuous ties across the structure to
loads and spans for service as floor and roof sheathing prevent roof thrust from being applied to the supporting
for their treatment. walls. The assembly shall be designed and constructed in
accordance with the provisions of this chapter and Figures
R802.1.5.7 Lumber. For each species of wood treated,
the effect of the treatment and the method of redrying R606.11(1), R606.11(2) and R606.11(3) or in accordance
after treatment and exposure to high temperatures and with AWC NDS.
high humidities on the allowable design properties of R802.3 Ridge. A ridge board used to connect opposing raf-
fire-retardant-treated lumber shall be determined in ters shall be not less than 1 inch (25 mm) nominal thickness
accordance with ASTM D5664. The test data developed and not less in depth than the cut end of the rafter. Where
by ASTM D5664 shall be used to develop modification ceiling joist or rafter ties do not provide continuous ties
factors for use at or near room temperature and at ele- across the structure, a ridge beam shall be provided and sup-
vated temperatures and humidity in accordance with ported on each end by a wall or girder.
ASTM D6841. Each manufacturer shall publish the R802.4 Rafters. Rafters shall be in accordance with this sec-
modification factors for service at temperatures of not tion.
less than 80°F (27°C) and for roof framing. The roof
framing modification factors shall take into consider- R802.4.1 Rafter size. Rafters shall be sized based on the
ation the climatological location. rafter spans in Tables R802.4.1(1) through R802.4.1(8).
Rafter spans shall be measured along the horizontal pro-
R802.1.5.8 Exposure to weather. Where fire-retar- jection of the rafter. For other grades and species and for
dant-treated wood is exposed to weather or damp or other loading conditions, refer to the AWC STJR.
wet locations, it shall be identified as “Exterior” to
indicate there is not an increase in the listed flame R802.4.2 Framing details. Rafters shall be framed not
1
spread index as defined in Section R802.1.5 when sub- more than 1 / inches (38 mm) offset from each other to a
2
jected to ASTM D2898. ridge board or directly opposite from each other with a
collar tie, gusset plate or ridge strap in accordance with
R802.1.5.9 Interior applications. Interior fire-retar- Table R602.3(1). Rafters shall be nailed to the top wall
dant-treated wood shall have a moisture content of not plates in accordance with Table R602.3(1) unless the roof
over 28 percent when tested in accordance with ASTM assembly is required to comply with the uplift require-
D3201 procedures at 92-percent relative humidity. Inte- ments of Section R802.11.
rior fire-retardant-treated wood shall be tested in accor-

384 2018 INTERNATIONAL RESIDENTIAL CODE ®
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