Thermal & moisture protection: NASCLA Commercial General Building Practice Questions
Roofing systems and slope, roofing squares and pitch math, flashing, underlayment, dampproofing and waterproofing, insulation and R-values, vapor retarders and air barriers, and joint sealants.
Question 1
In roofing, what is a "square"?
Correct answer: 100 square feet of roof surface
Estimating math (roofing)
Roofing is bought and priced by the square (100 sq ft); three-tab shingles commonly come three bundles to the square.
Question 2
Calculation
A low-slope roof measures 120 ft by 85 ft. How many squares of membrane is that, before waste and laps?
Correct answer: 102 squares
Estimating math (roofing)
Divide square feet by 100. Adding the two sides (205) is a perimeter-style mistake.
Worked solution
1
Formula
Area = 120 ft × 85 ft = 10,200 sq ft
2
Values
Squares = area ÷ 100
3
Result
Squares = 102 squares
Question 3
Calculation
A roof rises 6 inches for every 12 inches of horizontal run. What is its slope expressed as a percent?
Correct answer: 50%
Estimating math (roof slope)
Rise over run, times 100. A 6:12 slope is also called a 1/4 pitch (rise over the full span of 24 inches).
Worked solution
1
Formula
Slope = rise ÷ run
2
Values
Slope = 6 ÷ 12 = 0.50
3
Result
Slope = 50%
Question 4
Calculation
A gable roof spans 32 ft wall to wall and has a 1/4 pitch (pitch = rise ÷ span). What is the rise from wall plate to ridge?
Correct answer: 8 ft
Estimating math (roof pitch)
Pitch uses the full span; slope uses the run (half the span). Here the run is 16 ft, so the slope is 8:16, or 6 in 12.
Worked solution
1
Formula
Rise = pitch × span
2
Values
Rise = 1/4 × 32 ft
3
Result
Rise = 8 ft
Question 5
Calculation
A rafter has a horizontal run of 12 ft and a rise of 5 ft. Ignoring the overhang, what is its length along the slope?
Correct answer: 13.00 ft
Estimating math (rafters)
The rafter is the hypotenuse of a right triangle. Adding run and rise gives 17 ft, which is too long.
Worked solution
1
Formula
Length = √(run² + rise²)
2
Values
Length = √(12² + 5²) = √(144 + 25) = √169
3
Result
Length = 13.00 ft
Question 6
Calculation
A gable roof has a plan (horizontal) area of 2,400 sq ft. For a 6 in 12 slope the slope factor is 1.118 (given). How many squares of roofing cover the sloped surface, before waste?
Correct answer: 26.83 squares
Estimating math (roofing)
The sloped surface is larger than the plan area; multiply by the slope factor. Dividing by it (21.47) shrinks the roof.
Worked solution
1
Formula
Sloped area = plan area × slope factor = 2,400 × 1.118 = 2,683.2 sq ft
2
Values
Squares = 2,683.2 ÷ 100
3
Result
Squares = 26.83 squares
Question 7
Under the IBC, what is the minimum roof slope for asphalt shingles?
Correct answer: 2 units vertical in 12 units horizontal
IBC 2021 1507.2.2
Asphalt shingles need at least 2:12; from 2:12 up to 4:12 the code calls for double underlayment because water moves slowly on low slopes.
Question 8
Which roof membrane is a single-ply thermoplastic whose seams are joined with hot air welding?
Correct answer: TPO
Trade practice (low-slope roofing)
TPO and PVC are thermoplastics with heat-welded seams; EPDM is a rubber membrane with taped or adhered seams, and BUR is layers of felt and bitumen.
Question 9
What is a modified bitumen roof?
Correct answer: Polymer-modified asphalt sheets (APP or SBS)
Trade practice (low-slope roofing)
Polymer modifiers make the asphalt more flexible (SBS) or more heat-resistant (APP); open-flame torch work needs fire-watch precautions.
Question 10
What is the purpose of counterflashing at a roof-to-wall joint?
Correct answer: To lap over the base flashing
Trade practice (flashing)
Base flashing turns up the wall from the roof; counterflashing set into the wall laps down over it so water cannot get behind.
Question 11
What is a cricket (saddle) on a roof?
Correct answer: A small sloped saddle that diverts water around an obstruction
Trade practice (flashing)
Without a cricket, water and debris pond on the high side of a wide obstruction.
Question 12
Why do roofs with parapets usually need overflow (secondary) drains or scuppers?
Correct answer: Clogged drains could let water pond and overload the roof
IBC 2021 1502.2
Ponding water is heavy and can collapse a roof; secondary drainage set above the primary drain inlet prevents it.
Question 13
What is the difference between dampproofing and waterproofing of foundation walls?
Correct answer: Dampproofing resists moisture from soil without hydrostatic pressure; waterproofing resists water under hydrostatic pressure
IBC 2021 1805.2
Where a high water table puts water pressure on the walls, the code requires waterproofing; otherwise dampproofing is enough.
Question 14
What does a higher R-value mean for insulation?
Correct answer: More resistance to heat flow
Trade practice (insulation)
R-value measures thermal resistance; layers in series add their R-values. U-factor is its inverse for a whole assembly.
Question 15
Calculation
A wall has layers with R-values of 0.68, 13.0, 0.45 and 0.17 (given). What is the total R-value of the wall?
Correct answer: 14.30
Estimating math (insulation)
R-values of layers in series simply add. The overall U-factor would then be 1 ÷ 14.30 = 0.07.
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Practice questions are written in our own words and cite code sections by number. Always study from your own copy of the code and the official candidate bulletin.