Pitch turns footprint into roof area. After that it is hips, valleys and ridge — the lines where shingles get cut on an angle and the offcut goes in the skip.
Starter and ridge cap are separate. Neither comes out of the field bundle count, and forgetting them is the classic second trip to the supplier.
Slope makes a roof bigger than the building under it. The factor is the pitch multiplier:
multiplier = √(rise² + 144) ÷ 12
| Pitch | Multiplier | 1200 sq ft becomes |
|---|---|---|
| 3 / 12 | 1.0308 | 1,237 sq ft |
| 4 / 12 | 1.0541 | 1,265 sq ft |
| 6 / 12 | 1.1180 | 1,342 sq ft |
| 8 / 12 | 1.2019 | 1,442 sq ft |
| 10 / 12 | 1.3017 | 1,562 sq ft |
| 12 / 12 | 1.4142 | 1,697 sq ft |
Hips and valleys run at a shallower angle than the main slope, so they use a different multiplier — √(rise² + 288) ÷ 12 — which is why a hip rafter is longer than you would guess from the common rafter.
The single most common ordering mistake is using one waste figure for every roof. A simple gable has two rectangles and four straight edges, and almost every shingle goes down whole. A hip roof has four planes meeting at four hips, and every course on a hip ends in an angled cut whose offcut is scrap.
| Roof | Waste | Why |
|---|---|---|
| Shed | 8% | one plane, four straight edges |
| Simple gable | 10% | two planes, no angled cuts |
| Hip | 15% | four hips of angled cuts |
| Cut-up | 18–20% | valleys, dormers, short runs |
Field shingles are sold by the square. Four other items are not, and all four are needed:
Starter course runs the eaves, and on most manufacturer instructions the rakes as well. It is a different product from field shingle and warranties often depend on it.
Ridge cap runs the ridge plus every hip. Cap bundles cover roughly 20 to 35 linear feet depending on exposure, so check the wrapper rather than assuming.
Drip edge runs every eave and every rake, in 10 ft lengths, and goes under the underlayment at the rake but over it at the eave.
Underlayment is sold by the roll in squares of coverage. Synthetic typically covers 10 squares, #15 felt 4, #30 felt 2.
Three for nearly all architectural and three-tab shingles. Heavyweight and designer products run four or five.
Codes generally permit one additional layer over one existing layer, not more, and never over a wet or uneven deck. A tear-off tells you what the sheathing is doing, and a second layer hides it.
Four in twelve for standard application. From two to four in twelve most manufacturers require a double underlayment or a full self-adhering membrane, and below two in twelve shingles are not an appropriate roof covering at all.