Roofing Systems, Materials, and Installation Practices
Slope drives every decision
Roof slope is expressed as rise over run (a 4:12 roof rises 4 inches per 12 inches horizontal). Slope determines which covering is legal and how it must be underlaid. Asphalt shingles are permitted down to a 2:12 slope, but from 2:12 up to 4:12 the code requires a double layer of underlayment; above 4:12 a single layer is standard. Below 2:12 shingles are not allowed - use a low-slope membrane such as modified bitumen, TPO, EPDM or built-up roofing.
Components that get inspected
- Deck: sound, code-compliant sheathing, re-nailed where fasteners are inadequate
- Underlayment: felt or synthetic, lapped per manufacturer and code
- Flashing: step flashing at sidewalls with counterflashing, apron flashing at headwalls, pan flashing at chimneys and skylights
- Valleys: open metal or woven, consistent across the plane
- Drip edge at eaves and rakes, plus edge metal set in mastic on rakes
- Ventilation: net free vent area of roughly 1/150 of the attic floor area (1/300 where balanced intake and exhaust with a vapor retarder are provided)
Fastening and Oklahoma conditions
Shingles go in the manufacturer's nail line; four fasteners are typical, six in high-wind zones. Hail and wind dominate Oklahoma losses, so impact-rated (UL 2218 Class 4) shingles and manufacturer high-wind systems with sealed starter courses are common specifications. Re-roofing is limited: a new covering may generally be applied over one existing layer, not two.
Worked example
A porch measures 3:12. Shingles are permitted, but the installer must use double underlayment and confirm the manufacturer allows 3:12 application. Measuring 2:12 instead would push the job to a membrane system.
Traps
Nailing high or overdriving, skipping drip edge, using shingles on a 1:12 shed roof, and stacking a third shingle layer on a house that already has two.