Roofing Materials, Components, and System Selection
Slope drives everything
System selection starts with slope, because slope decides how fast water leaves the roof. Roofs at 3:12 or steeper are steep-slope; 2:12 or flatter are low-slope membrane roofs. Each product has its own listed minimum: asphalt shingles 2:12 (with a double underlayment layer from 2:12 up to 4:12), wood shingles about 3:12 and shakes 4:12, clay or concrete tile roughly 2.5:12 to 3:12 depending on profile and underlayment, slate 4:12, and standing-seam metal as low as 1:12 (lower still for structural panels with continuous sealant and no laps). Below the listed minimum, a product depends on sealant rather than gravity, which is a warranty and leak problem.
Compare more than price
Weigh dead load (concrete tile can reach 8–12 lb/sf versus 2–4 lb/sf for asphalt shingles), fire classification (Class A assembly), wind uplift rating, impact resistance (UL 2218 Class 4), and expected service life. Components must be compatible: underlayment, flashing, fasteners, and sealants from the same system, or a manufacturer may deny a claim. Climate matters too - freeze-thaw, hail, salt air, and wildfire exposure all narrow the choices.
Traps
Questions love single-component swaps: three-tab shingles nailed on a 2:12 porch, or a membrane run up a 6:12 slope where it ponds behind laps. Watch for tile placed on a deck never checked for load, and for mixing brands of self-adhered membrane and cap sheet.
Worked example
A 2.5:12 shed roof over a porch: shingles are out. Use a two-ply modified bitumen or standing-seam metal with continuous sealant; either way, confirm the eave detail handles water backing up under ice.