Installation Practices, Steep-Slope and Low-Slope Application
Steep-slope sequence
Installation runs bottom-up: drip edge, ice-dam protection, underlayment, valley treatment, starter course, field shingles, flashing, then cap and ventilation. Two placement rules generate exam items. In an ice-dam region - IRC Chapter 9 applies where the average January temperature is 25°F or lower, which covers essentially all of Illinois - a self-adhered ice barrier runs from the eave edge up the slope to a point at least 24 inches inside the exterior wall line, and it must also cover valleys and other problem areas. Second, at the eave the underlayment laps over the drip edge, while at the rake the drip edge laps over the underlayment. Between 2:12 and 4:12, shingles require double underlayment; below 2:12 they are not permitted at all.
Fastening and detailing
Nail within the manufacturer's nail line, about 1 inch above the exposure line and 1 inch from each end, driving flush - not overdriven or angled. Four nails per shingle is the baseline; six is common in high-wind zones. Valleys may be open metal, woven, or closed-cut; open metal is used on low slopes and where flow is heavy. Step flashing at sidewalls is interleaved with each course and hidden behind the siding or housewrap; counterflashing at masonry is set into a reglet. Ridge vent must be balanced by soffit intake, or the system simply does not vent.
Low-slope
Positive drainage of at least 1/4 inch per foot is the design floor. BUR is commonly four plies, TPO seams are welded with a minimum overlap and probed for voids, and base flashing terminates at least 8 inches above the roof surface with a nailed and sealed counterflashing above it. Crickets go on the high side of curbs and chimneys.
Worked example
A three-tab shingle with 5-inch exposure yields 80 shingles per square; at four fasteners each, that is 320 nails per square - a number worth memorizing for material takeoffs and inspection checks.