Emergency escape opening figures
Openings are measured by net clear opening, the hole a person climbs through. Sash size is not the measure; measuring the sash is a listed common error. The 2018 IRC figures:
- Minimum net clear area (R310.2.1): 5.7 sq ft. Openings at grade floor may be 5.0 sq ft.
- Minimum net clear height: 24 in.
- Minimum net clear width: 20 in.
- Maximum sill height (R310.2.2): the bottom of the clear opening no more than 44 in above the floor. 48 in is too high; 24 in is the opening height, not a sill height.
The area trap
Meeting both minimum dimensions does not meet the area. 24 in × 20 in = 480 sq in, and 480 ÷ 144 = 3.33 sq ft, short of both 5.7 and 5.0. Always convert square inches to square feet by dividing by 144.
To find the height needed at a 20 in width above grade floor: 5.7 × 144 = 820.8 sq in; 820.8 ÷ 20 = 41.04, so about 41 in. A 36 in height only satisfies the 5.0 sq ft grade-floor figure (720 ÷ 20).
A grade-floor window 30 in wide by 26 in high gives 780 sq in, or 5.42 sq ft. That passes the 5.0 sq ft grade-floor minimum, and both dimensions exceed 24 in and 20 in. The 5.7 figure applies only to openings not at grade floor, and 30 in is not a height minimum.
Snow load design limit
R301.2.3 sets where the prescriptive code stops. Where the ground snow load exceeds 70 psf (3.35 kPa), the building must be designed by accepted engineering practice instead of the prescriptive provisions of Chapters 5, 6 and 8.
- A site listed at 75 psf is over the limit, so the whole building leaves the prescriptive path.
- Engineering only the Chapter 8 roof is wrong; the section names Chapters 5, 6 and 8 together.
- Upsizing rafters from the tables is still a prescriptive fix and is not permitted above 70 psf.