Roofing Contractor Scope and Trade Knowledge
What the GS-21 scope covers
The New Mexico GS-21 classification covers roofing work: roof coverings, underlayment, flashing, roof-deck attachment, waterproofing, roof drainage, and the sheet-metal work incidental to roofing - drip edge, valley metal, vent caps, counterflashing. It does not reach structural framing, plumbing changes beyond the roof penetration, or electrical work for rooftop equipment.
Slope decides the system
- 4:12 and steeper: standard steep-slope assemblies.
- 2:12 to 4:12: shingles only with the manufacturer's low-slope detail, typically double underlayment and a locked fastening pattern.
- Under 2:12: shingles are not permitted; use a low-slope membrane (TPO, EPDM, modified bitumen, built-up).
Water must be handed off in order
Each layer laps over the one below. On a sidewall the sequence is underlayment, step flashing woven with each course, wall weather barrier, then counterflashing. A kick-out flashing at the bottom of a roof-to-wall termination throws water into the gutter instead of into the wall.
Ventilation math
The code requires net free vent area of 1/150 of the attic floor area, relaxed to 1/300 where intake and exhaust are balanced in the same ratio or a vapor retarder is present.
Worked example
A 1,200 sq ft attic needs 1,200 ÷ 150 = 8 sq ft of net free area, split 50/50: 4 sq ft (576 in²) of intake and 4 sq ft of exhaust. A ridge vent rated 18 in² per linear foot needs 576 ÷ 18 = 32 linear feet - often more than the available ridge, so additional exhaust vents are required.
Traps
Quoting gross vent opening instead of net free area; forgetting double underlayment between 2:12 and 4:12; nailing above the manufacturer's nail line, which voids the wind warranty; relying on sealant instead of a kick-out flashing.