The opening is above grade, so it needs a net clear opening area of not less than 5.7 square feet, with a clear width of not less than 20 inches and a clear height of not less than 24 inches. The unit he is looking at will give a clear opening width of 34 inches. What is the least clear opening height that will satisfy the area requirement?
- A20 inches, which is the smallest clear dimension the code states for an opening of this kind
Twenty inches is the minimum clear width, not the minimum height, and it satisfies neither the height minimum nor the area. Mixing up which minimum goes with which dimension is the most common slip in this calculation.
- B24 inches, which is the least clear height the code will accept in any escape opening
Twenty-four inches meets the dimensional minimum and misses the area by about 5 square inches. That is exactly the trap: the two requirements are independent and both have to be met.
- About 24 1/4 inches, since 5.7 square feet is 820.8 square inches and that over 34 is 24.14
- DAbout 21 1/4 inches, working from the 5.0 square feet that applies to an opening of this kind
Five square feet is the reduced area allowed at grade floor openings and below-grade openings served by a well. This bedroom is on the second floor, where the 5.7 square foot figure applies.
Why C is correct
Required area is 5.7 times 144, or 820.8 square inches. Divided by the 34-inch clear width, the clear height must be at least 24.14 inches, about 24 1/4. That also clears the 24-inch dimensional minimum, which by itself would leave the opening 4.8 square inches short on area.
What this question is testing
Whether the candidate converts area to square inches, solves for a missing dimension, and checks the result against the separate dimensional minimums instead of substituting one for the other.
On the job
Ordering the wrong sash is expensive because the rough opening is framed around it, so the arithmetic is worth doing at the counter rather than at final. The measurement is taken on the clear opening the sash actually leaves, which for a double-hung is roughly half the unit and for a casement is close to the full frame. Escape windows are the item most likely to be caught late and most likely to require framing to fix.
Memory technique
Twenty wide, twenty-four tall, and still 5.7 square feet to find.
Exam tip
Area and the two dimensional minimums are three separate tests. Run all three.
Where to look it up
IRC Chapter 3, emergency escape and rescue openings, minimum opening area and dimensions.