The roof is framed at 2 units vertical in 12 units horizontal, and both units are mounted flat on the deck with the flashing kit lapped into the roofing and no curb beneath them. How should the inspector judge this installation?
- AAccept it; the flashing kit supplied with the unit is the manufacturer's tested detail and applies at any roof slope
A flashing kit is tested for the mounting it is made for. Deck-mount and curb-mount kits are different products, and the slope determines which one applies.
- BAccept it; curbs are required only where a skylight is site built rather than a listed factory-built unit
The curb requirement is keyed to roof slope and applies to factory-built unit skylights. Site-built skylights carry their own separate requirements.
- Reject it; on a roof flatter than 3 units in 12 a unit skylight is mounted on a curb at least 4 inches above the roof plane
- DReject it; unit skylights are not permitted on any roof flatter than 4 units in 12 and must be relocated to a steeper plane
Unit skylights are permitted on shallow roofs, on a curb. Prohibiting them outright is stricter than the code and would rule out a common and well-performing installation.
Why C is correct
Unit skylights on roofs with a slope flatter than 3 units vertical in 12 units horizontal must be mounted on a curb that extends not less than 4 inches above the plane of the roof, so the frame and its seals sit above water and snow that stand on a shallow roof.
What this question is testing
Whether the candidate knows the curb requirement for unit skylights is triggered by roof slope, recalls both the slope threshold and the curb height, and recognizes that mounting type governs which flashing applies.
On the job
Low-slope dormers, porch roofs and additions are where this comes up, and the skylight is usually already in when the inspector arrives. The check takes a level, a rule and a look at whether the frame sits proud of the roof surface. The consequence of getting it wrong is a leak at the head of the unit that appears with the first wet snow and is blamed on the skylight rather than on the mounting.
Memory technique
Shallow roof, tall curb.
Exam tip
Flatter than 3 in 12 means a curb, and the curb is at least 4 inches tall.
Where to look it up
IRC Chapter 3, glazing, the skylight section covering unit skylights and curb mounting.