Roofing Systems, Materials, and Installation Methods
Slope drives every decision
Express slope as rise over run in inches (x:12). At 2:12 or less the roof is low-slope and must receive a membrane system - built-up, modified bitumen, single-ply (TPO, EPDM, PVC), or spray polyurethane foam. Asphalt shingles are permitted from 2:12 upward only when a double layer of underlayment or a self-adhered membrane is installed; below 2:12 shingles are not allowed. From 4:12 up you are in steep-slope territory: shingles, tile, shakes, and most metal panels.
The stack, bottom to top
Deck, then underlayment, then flashing, then covering. Underlayment laps run about 2 in. on the sides and 4 in. at the ends, with end laps staggered. Tile and shake roofs generally need a full 30-lb felt or synthetic layer across the whole deck, plus battens or counter-battens on steeper pitches.
Flashing and water shedding
Leaks start at transitions and penetrations: step flashing at sidewalls (one piece per shingle course, lapped about 2 in.), counterflashing, kick-out flashing where a wall meets a gutter, drip edge at eaves and rakes, and valley metal. Everything that stops water must lap shingle-fashion - upper courses over lower.
Ventilation
Provide net free vent area of roughly 1/150 of the attic floor area; 1/300 is allowed when intake and exhaust are balanced and a vapor retarder is present. Intake at the eaves must be balanced with exhaust at or near the ridge.
Traps
Questions love the 2:12 and 4:12 thresholds, confusing squares with square feet of footprint, and forgetting that old flashing is replaced rather than reused.
Rule of thumb: under 4:12, double the underlayment; at or under 2:12, no shingles - membrane only.