Roofing Materials, Systems and Installation Practices
Roofing Materials
Two big families
Roofing materials divide into steep-slope products, which shed water by gravity and rely on overlap, and low-slope membranes, which must be watertight by themselves. Questions often test whether you can match a product to an allowable slope and to its governing standard.
Asphalt shingles. Three-tab (single-layer, lighter) versus laminated/architectural (multi-layer, heavier, usually higher wind ratings). Reference standards are ASTM D3462 for shingles, ASTM D3161 or D7158 for wind resistance, and UL 790 for Class A fire rating. Underlayment is asphalt-saturated felt (ASTM D226) or a synthetic polymer sheet; underlayment is not the waterproofing layer except where a membrane is specified.
Metal. Exposed-fastener panels are cheap and common on ag buildings; standing-seam panels use concealed clips and allow thermal movement. In coastal Louisiana, substrate and coating matter more than panel profile: Galvalume or hot-dip galvanized steel with a Kynar-type finish resists salt, while mill-finish aluminum and electro-galvanized steel do not. Fasteners must be metallurgically compatible with the panel to avoid galvanic corrosion.
Tile, slate, shake. Heavy materials require a structural check and specific fastening; clay and concrete tile typically need a slope of 2.5:12 or steeper with double underlayment.
Low slope. Built-up roofing, modified bitumen (SBS versus APP), single-ply (TPO, EPDM, PVC), spray foam, and coatings.
Traps. Confusing SBS (hot-mop, cold adhesive, self-adhered) with APP (torch-applied); assuming any shingle can go on any slope; specifying dissimilar fasteners; ignoring bundle coverage printed on the wrapper.
Rule of thumb. One square = 100 ft² of finished roof. Add roughly 10% waste for a simple gable and 15% or more for hips, valleys and dormers.
Systems and Installation Practices
Sequence drives watertightness
A roof is a system: deck, underlayment, flashing, drainage and ventilation. Correct sequence is deck repair, underlayment, drip edge and flashing, then the field material, then ridge and hip caps.
Slope rules. Asphalt shingles are permitted at 2:12 and steeper; from 2:12 up to 4:12 a double underlayment layer is required. Below 2:12 you are in low-slope territory and must use a membrane system. Metal panel minimum slope follows the manufacturer. Remember that the eave drip edge goes over the underlayment and the rake drip edge goes under it.
Flashing. Step flashing at sidewalls, lapped one shingle course at a time, with counterflashing set into a reglet. Valleys either open metal or woven, but never a single layer of shingle. Pipe boots, skylight curbs, and chimney crickets are chronic leak points that exam questions target.
Fastening. Nails go in the manufacturer's nail line, high enough to be covered by the next course but low enough to hold, driven flush, not overdriven or angled. Typical minimum is four nails per shingle, six in high-wind areas; steep-slope and high-wind requirements follow the manufacturer's printed instructions.
Low slope. Multiple plies with staggered laps, positive drainage to drains or scuppers, crickets behind large penetrations, and terminations at parapets.
Ventilation. Net free vent area of roughly 1/150 of the vented ceiling area, or 1/300 when intake and exhaust are balanced and a vapor retarder is present.
Traps. Re-covering over deteriorated flashing, forgetting starter course, mixing nail types, and assuming a "typical" detail applies to a non-typical condition.
Worked example. A 3:12 roof plan calls for architectural shingles: shingles are allowed, but the spec must show two underlayment layers, and the detail sheet must show the eave drip edge over the underlayment.