Page 322 - 2018_IRC

WALL CONSTRUCTION

TABLE R608.7(2)
REDUCTION FACTOR, R , FOR BUILDINGS WITH MEAN ROOF HEIGHT LESS THAN 35 FEET
a
1
REDUCTION FACTOR R 1 , FOR MEAN ROOF HEIGHT
MEAN ROOF HEIGHT b, c Exposure category
(feet)
B C D
< 15 0.96 0.84 0.87
20 0.96 0.89 0.91
25 0.96 0.93 0.94
30 0.96 0.97 0.98
35 1.00 1.00 1.00
For SI: 1 foot = 304.8 mm, 1 degree = 0.0175 rad.
a. See Section R608.7.1.1 and Note c to Table R608.7(1A) for application of reduction factors in this table. This reduction is not permitted for “minimum”
values.
b. For intermediate values of mean roof height, use the factor for the next greater height, or determine by interpolation.
c. Mean roof height is the average of the roof eave height and height of the highest point on the roof surface, except that for roof slopes of less than or equal to
1
2 / 8 :12 (10 degrees), the mean roof height is permitted to be taken as the roof eave height.
TABLE R608.7(3)
REDUCTION FACTOR, R , FOR FLOOR-TO-CEILING WALL HEIGHTS LESS THAN 10 FEET a, b
2
STORY UNDER FLOOR-TO-CEILING HEIGHT c ENDWALL LENGTH
CONSIDERATION (feet) (feet) ROOF SLOPE REDUCTION FACTOR, R 2
Endwalls—for wind perpendicular to ridge
< 5:12 0.83
15 7:12 0.90
12:12 0.94
One story or top story of two story 8
< 5:12 0.83
60 7:12 0.95
12:12 0.98
< 5:12 0.83
15 7:12 0.86
12:12 0.89
First story of two story 16 combined first and second story
< 5:12 0.83
60 7:12 0.91
12:12 0.95
Sidewalls—for wind parallel to ridge
< 1:12 0.84
5:12 0.87
15
7:12 0.88
12:12 0.89
One story or top story of two story 8
< 1:12 0.86
5:12 0.92
60
7:12 0.93
12:12 0.95
< 1:12 0.83
5:12 0.84
15
7:12 0.85
12:12 0.86
First story of two story 16 combined first and second story
< 1:12 0.84
5:12 0.87
60
7:12 0.88
12:12 0.90
For SI: 1 foot = 304.8 mm.
a. See Section R608.7.1.1 and Note d to Table R608.7(1A) for application of reduction factors in this table.
b. For intermediate values of endwall length and roof slope, use the next higher value or determine by interpolation.
c. Tabulated values in Table R608.7(1A) and (1C) for “one story or top story of two story” are based on a floor-to-ceiling height of 10 feet. Tabulated values in
Table R608.7(1B) and (1C) for “first story of two story” are based on floor-to-ceiling heights of 10 feet each for the first and second story. For floor-to-ceiling
heights between those shown in this table and those assumed in Table R608.7(1A), (1B) or (1C), use the solid wall lengths in Table R608.7(1A), (1B) or (1C),
or determine the reduction factor by interpolating between 1.0 and the factor shown in this table.

2018 INTERNATIONAL RESIDENTIAL CODE ® 297
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