How a roof replacement works: the steps and what to ask your roofer
A shingle roof replacement goes tear-off, deck repair, drip edge and underlayment, flashing, shingles from the eaves up, ridge vents and cap, then cleanup and a magnet sweep for nails. Each step should be a line in the written quote.
This is the homeowner’s side of the job: what to see in the quote and check from the ground. Measurements come from plans or a professional roof report, and the climbing belongs to the roofer.
Worked example
Example: a 30 × 20 ft footprint at 6 in 12, with 10% extra and 3 bundles to a square.
This example gives 23 shingle bundles.
- Section 1 (gable): 30 × 20 ft footprint = 600 sq ft × 1.118 = 670.82 sq ft
- 670.82 × (1 + 10/100) ÷ 33.3333 = 22.14 shingle bundles before rounding.
- Underlayment: 670.82 sq ft ÷ 935.63 sq ft a roll = 0.72, rounded up (GAF FeltBuster, 48 in × 250 ft, after its 3 in side laps and 6 in end laps; double coverage below 4 in 12). The extra for cuts is for the shingles only.
- Starter strip along the eaves and rakes: 104.72 ft ÷ 120 ft a bundle (GAF Pro-Start) = 0.87, rounded up.
- Drip edge: 104.72 ft ÷ 9.83 ft a piece (10-ft pieces lapped 2 in, as the residential code requires) = 10.65, rounded up.
- Ridge cap over the ridges and hips: 30 ft ÷ 25 ft a bundle (GAF Seal-A-Ridge) = 1.2, rounded up.
- Nails: 7.38 squares × 256 a square (64 shingles a square, 4 nails each, the maker’s standard pattern; 6 nails each in high-wind areas), rounded up to the next hundred.
- Vents: 600 sq ft of attic ÷ 150 = 4 sq ft (576 sq in) of net free ventilating area (International Residential Code R806.2).
Other calculators for this job
Every step of a roof project: measure, the list, what it costs and the quotes.
Describe the roof before pricing it
List each roof plane with its pitch, then the eaves, rakes, hips, ridges and valleys; the roofing calculator works these out from the footprint. Name the shingle you have and the one you want. Keep a porch, addition or low section separate: asphalt shingles need a slope of at least 2 in 12, with extra underlayment below 4 in 12, and anything flatter takes a membrane roof.
Agree on tear-off and decking
Ask whether the old layers come off, who protects the gutters and plantings, and who hauls the debris. Rotten decking only shows once the shingles are off, so get the price per sheet of new decking in the contract instead of a guessed percentage, so a surprise under the shingles has a price agreed in advance.
See the edges and flashing in writing
The quote should name the drip edge at the eaves and rakes, the underlayment, an ice and water barrier at the eaves and valleys in cold climates, step and counter flashing at walls and chimneys, and a boot for every pipe. GAF’s Steep-Slope Pro Field Guide treats each of these as its own part of the roof. “New shingles” alone does not say whether the flashing is reused or replaced.
Match the valley to the shingle
GAF Technical Advisory Bulletin R-136 says laminated Timberline shingles go in open or closed-cut valleys, never woven ones. That rule belongs to that product line; another shingle has its own. Ask which valley the quote includes and check its length on the material list.
Get the vents and nailing right
The building code’s minimum (IRC R806.2) is 1 sq ft of net free ventilating area for every 150 sq ft of attic floor, or 1 for every 300 only when both conditions of its exception are met: 40 to 50% of the vent area high on the roof, within 3 ft of the ridge, with the rest in the lowest third of the attic, and in climate zones 6 to 8 a Class I or II vapor retarder in the ceiling. The roofing calculator uses 1 in 150 unless you choose the exception. Ask which vents the roofer will fit and their open area. Shingles are nailed to the maker’s pattern, with more nails per shingle in high-wind areas; ask for the GAF field guide’s pattern for yours.
Close out the job
Before the final payment, walk the yard with the roofer; a magnet sweep should pick up stray nails. Ask for the shingle name and color, both warranties (the maker’s for the shingles and the roofer’s for the work), the permit sign-off if one was needed, and photos of the deck and flashing before they were covered. If it leaks later, note where and when the water shows: it often gets in higher up the roof than the stain.