The whole game in roofing estimation is turning a flat house footprint into the true sloped area, then converting that into squares, bundles, and rolls. Miss the pitch and every other number is wrong. This guide runs from squares and pitch factors to flashing, ventilation, decking, and cost — distilling the 100 questions roofers and homeowners ask most into one readable pass.
Roofing runs on one unit: the square, which is 100 square feet of roof surface (a 10×10 patch). Shingles are sold to cover squares — it takes 3 bundles per square, and a single bundle covers about 33.3 sq ft. To convert any area to squares, divide by 100: a 2,450 sq ft roof is 24.5 squares.
The catch that trips up every first-timer: you can't buy shingles off the house footprint. The footprint is flat, but the roof slopes up, so its real surface is larger. Before applying pitch, add the eave overhang (the edge past the walls) and rake overhang (the sloped gable edge) to your footprint dimensions.
Pitch is written as rise-over-run: a 4/12 roof climbs 4 inches for every 12 inches across; a 6/12 is a medium residential slope. To turn footprint area into true sloped area, you multiply by a pitch factor — a number from the Pythagorean theorem that scales the flat projection up to the slope.
| Pitch | Factor | Pitch | Factor |
|---|---|---|---|
| 3/12 | 1.0308 | 8/12 | 1.2019 |
| 4/12 | 1.0541 | 9/12 | 1.2500 |
| 5/12 | 1.0833 | 10/12 | 1.3017 |
| 6/12 | 1.1180 | 12/12 | 1.4142 |
| 7/12 | 1.1577 | 12/12 = 45° = √2 | |
Pitch also dictates the system: low-slope roofs (1/12 to 2/12) need membrane coverings rather than shingles to stay watertight, while steep-slope roofs (over 9/12) require ropes and harnesses to install safely.
1,500 × 1.0541 = 1,581 sq ft of roof = 15.81 squares. Shingles = 15.81 × 3 = 47.4 → about 48 bundles before waste. Add 10% and you're near 53.
Both 3-tab and architectural shingles use the same 3 bundles per square — the difference is weight, not coverage, which matters for delivery and structural load. Standard 3-tab runs 200–220 lb per square; heavy architectural can hit 250–350+ lb. A factory pallet holds 39–42 bundles (13–14 squares).
Two accessories are sold by linear foot, not area:
The visible part of each shingle (the exposure, typically 5″ for 3-tab) sets the overlap and shows up again when you count step flashing.
Underlayment is the water-resistant membrane rolled over the deck before shingles. Coverage varies a lot by type, and overlaps eat into it:
| Material | Coverage per roll | Notes |
|---|---|---|
| #15 felt | 4 squares (400 sq ft) | Lose ~10% to 2–4″ overlaps |
| #30 felt | 2 squares (200 sq ft) | Thicker, heavier |
| Synthetic | 10 squares (1,000 sq ft) | Lighter, more durable |
| Ice & water shield | 36″ × 65 ft ≈ 195 sq ft | Self-adhering waterproofing |
To count rolls, divide roof area by the roll yield and round up — a 28-square roof in synthetic is 28 ÷ 10 = 2.8 → 3 rolls. Because parallel rows overlap 2–4″, knock about 10% off effective coverage. Ice & water shield is a self-adhering membrane along eaves and valleys; code typically wants it from the eave edge to at least 24″ inside the exterior wall line (usually a 2-row run), plus a full 36″ strip down every valley before felt goes on.
Waste covers cuts, breakage, and mistakes — and on a roof it's driven almost entirely by how many angles the design has:
| Roof type | Waste factor |
|---|---|
| Simple gable (no valleys/dormers) | 5–10% |
| Hip roof (diagonal cuts) | 15% |
| Multiple valleys, dormers, turrets, skylights | 20% |
| Conical turret | 30% |
Coming up short stops the job with the deck exposed to weather while you wait on a delivery. For a tear-off, also size your dumpster up front — a single layer off a 20-square roof weighs 4,000–5,000 lb, so a 15–20 cu yd dumpster is typical.
Code (IRC) sets the 1:300 rule: 1 square foot of net free ventilation for every 300 square feet of attic floor. Split that area roughly 50/50 between intake and exhaust — unbalanced airflow traps heat and moisture, causing winter ice dams and summer sheathing rot. Ridge vents (continuous slots at the peak, sold in ~4-ft tracks) handle exhaust; soffit vents under the eaves pull in intake air. Measure ridge length to count ridge-vent sections.
Almost everything in this category is linear-foot or piece-count math. Drip edge (L-shaped metal along eaves and rakes) comes in 10-ft strips — total the linear feet and add 2–3″ overlap per seam. Flashing seals every place the roof meets something else:
For fasteners, code wants 4 nails per shingle (6 in high-wind zones), which works out to roughly 2.25–2.5 lb of roofing nails per square. A 25-square roof needs 25 × 2.5 = 62.5 lb.
Decking is the structural OSB or plywood layer over the rafters. Sheets are 4×8 = 32 sq ft, so divide roof area by 32 and add 10% for edge and valley trimming.
Plan 1–2 H-clips per panel span between rafters to stiffen the joints. Fasten with 8d common nails every 6″ on panel edges and 12″ at intermediate supports. Let damp sheets dry before underlayment so you don't trap moisture, and never deck over rotted boards — tear back to sound framing first. A radiant-barrier deck sheet (foil-faced, installed foil-down) is an energy-efficiency upgrade option.
The area math is identical — pitch factor and all — but the order unit changes:
Total cost stacks squares, bundles, rolls, nails, and flashing against local prices, plus delivery and tax.
| Item | Typical figure |
|---|---|
| Architectural shingles (material) | $100–$160 per square |
| Replacement labor | $150–$350 per square |
| Crew install rate | 20–35 squares per day (4–5 crew) |
| Pallet | 39–42 bundles (13–14 squares) |
| Tear-off dumpster | 15–20 cu yd for a 20-square layer |
Labor scales with pitch, stories, and tear-off difficulty, so a steep cut-up roof costs far more per square than a simple gable. Solar mount brackets and other specialty flashings are counted as individual pieces on top of the base estimate.
Manual math and online tools differ because software uses satellite/aerial data (EagleView, RoofSnap) to capture exact slopes and valley lines, while hand math leans on an average pitch. The fix for complex roofs is the same split-and-sum habit: treat each dormer as its own mini-roof — area it with its own length, width, and pitch factor, then add to the main total — and line every dormer-to-roof intersection with ice & water shield plus woven step flashing. Open valleys use exposed metal with shingles trimmed back; closed valleys weave shingles across, which uses slightly more material.
Verify the horizontal house lines with a tape, the slope with a pitch gauge, then cross-check your totals against the supplier's delivery invoice before construction starts. (And for maintenance: clean moss with a zinc-based cleaner or diluted bleach and a gentle rinse — never a pressure washer, which strips the protective granules.)