Roofing Systems, Materials and Components
Roofing Systems
Two basic families
Roof systems split into steep-slope (roughly 4:12 and steeper) and low-slope (3:12 and flatter). Steep-slope coverings - asphalt shingles, metal panels and shingles, clay and concrete tile, slate, wood shakes - shed water by gravity and depend on layered underlayment as a secondary barrier. Low-slope systems - built-up roofing, modified bitumen, EPDM, TPO and PVC membranes - rely on continuous waterproof membranes, sealed laps and positive drainage.
Slope governs material choice
The most-tested rule is that slope decides what may be installed. Asphalt shingles are limited to a 2:12 minimum, and between 2:12 and 4:12 the code requires a double layer of underlayment; many manufacturers require 4:12 for standard warranty coverage. Below 2:12 you must move to a membrane or a specialized low-slope product. Low-slope membranes need drainage slope, commonly about 1/4 inch per foot toward drains or scuppers.
Support and assemblies
Deck type matters: wood structural panels on one- and two-family dwellings, structural concrete or metal decks on commercial work. Ventilation and insulation strategy - vented attic versus insulated roof deck - changes which assembly is acceptable.
Trap
Questions often describe a shallow roof and ask which covering is allowed, or offer shingles at 3:12 without mentioning the double underlayment. Read the slope first, then the assembly requirements.
Rule of thumb
Estimate area with the pitch multiplier. A 6:12 roof multiplies plan area by about 1.12, so a 30 ft by 40 ft footprint (1,200 sq ft) yields roughly 1,200 x 1.12 = 1,344 sq ft, or about 13.4 squares before waste.
Materials and Components
Shingles and ratings
Asphalt shingles are rated by fire class (most are Class A) and wind resistance (commonly 60 to 130 mph), and by warranty type. Three-tab shingles are lighter and less expensive; laminated architectural shingles are heavier and usually rated for higher wind. Fiberglass mat has largely replaced organic felt mat.
Underlayment
Underlayment is the secondary water barrier. Options include No. 15 and No. 30 asphalt-saturated felt and synthetic sheet goods; synthetics are lighter, resist tearing and are often required by manufacturers on steep slopes. Ice-dam protection membrane belongs at eaves and in valleys in cold climates such as Michigan.
Flashing and edges
Flashing includes drip edge, valley metal, step flashing at sidewalls, apron and headwall flashing, counterflashing, chimney flashing with a cricket, and pipe boots. Materials must be compatible: bare aluminum corrodes in contact with alkaline pressure-treated lumber, and dissimilar metals in direct contact cause galvanic corrosion.
Fasteners and accessories
Shingle nails are typically minimum 12-gauge with a 3/8-inch head, hot-dip galvanized or stainless where corrosion is a concern, long enough to penetrate sheathing at least 3/4 inch or through the deck. Staples and electro-galvanized nails are common failure points. Accessories include starter strip, hip and ridge cap, ridge vents and exterior-rated sealants.
Traps
Questions mix up 15 lb and 30 lb felt, or place ice barrier across the whole roof. Another classic is a fastener that is too short, or a nail that will corrode in treated wood.
Rule of thumb
Match every accessory to the covering. Never use a product the shingle manufacturer does not list for the assembly, because warranty claims follow the published details.