Flat roof replacement cost calculator

What counts as a low-slope roof

Nobody in the trade builds a truly flat roof. What the code calls low slope is anything under 2:12 — the IBC writes it as “low-slope roofs (roof slope < 2:12)” — and 2:12 is also the least pitch asphalt shingles are allowed on. Below that line water stops running over a lapped course and starts sitting on it, so the covering has to be one continuous sheet: a single-ply membrane, modified bitumen or built-up plies.

Membranes still need fall. The IBC sets a design slope of at least 1/4:12 (2%) for built-up, modified bitumen and single-ply roofs, with coal-tar built-up allowed down to 1/8:12. A quarter inch per foot is the number to check with a level before you price anything. If you are not sure which side of 2:12 a porch or addition sits on, run it through the roof pitch calculator first.

How to use this calculator

1. Enter the roof area in square feet, or switch to length × width for a plain rectangle. A low-slope roof needs no pitch factor, so plan area is roof area. For an L-shaped roof, add the rectangles up on the roof square calculator.
2. Pick the membrane. Each one carries its own installed band and service life.
3. Set the layers to strip. Zero means a recover over the existing roof, which the code allows only in narrow cases (see below).
4. Guess the share of deck that has to go. Ten percent is a sensible placeholder until the old roof is open. Soft spots underfoot are the tell.
5. Leave the permit on unless you know your town does not ask for one.

Worked example: a 1,200 sq ft garage and porch roof

A detached two-car garage and a wraparound porch, together 1,200 sq ft or 12 squares, at a quarter inch per foot. One layer of old roofing comes off and roughly 120 sq ft of sheathing is rotten along the low edge where the water sat. In TPO the membrane line is $4,800 – $16,800, tear-off adds $1,200 – $6,000, the deck patch $240 – $960 and the permit $150 – $600. All in, $6,390 – $24,360, or $5.33 – $20.30 a square foot.

The width of those ranges is the point. The national average for a flat roof is $8,000, but a small garage roof in modified bitumen and a big porch roof in heavy TPO with a long warranty are not the same purchase. The low column is a plain roof with easy access. The high column is the thick membrane, a hard-to-reach roof and a manufacturer warranty. Rot found once the old roof is open is priced as in the roof decking replacement guide, per sheet, after it is found.

What moves the price

The membrane and its thickness

Every membrane in the dropdown starts near the same floor and splits at the top. Modified bitumen tops out early because it is a two-ply torch or self-adhered job on a small roof. TPO, EPDM and PVC run much higher because the same product name covers a thin, mechanically fastened sheet and a thick, fully adhered one with a long manufacturer warranty. Get the thickness in mils written on the quote. The TPO vs EPDM comparison covers the seam method, which matters more than the material name.

What comes off first

The tear-off line uses Fixr’s flat-roof figure for removal and disposal, $1 to $5 a square foot for each layer — a wider band than a shingle tear-off on the roof replacement cost calculator, because flat roofs strip heavier. A built-up roof under gravel, or insulation board that has soaked up years of leaks, comes off in wet slabs; on a roof like that, budget from the high column.

The deck under it

Water that sits on a roof finds every fastener hole and every failed seam, which is why an old flat roof so often comes with deck work. Replacement runs $2 to $8 a square foot. Nobody can see it until the old roof is up, which is why a fair quote prices it per sheet as an allowance rather than as a fixed number.

Insulation, edges and holes

Insulation upgrades, tapered boards to fix ponding, new flashing at parapets and curbs, and every pipe, vent and skylight through the roof are priced per job, not per square foot. Each can add hundreds or thousands, so they are left out of the calculator rather than guessed. Ask for them as separate lines. The flashing guide shows what each detail is. Labour alone runs $3 to $7 a square foot inside the membrane price. That is why a roof you can only reach through a house, or one with a crane lift, lands in the high column. Permits, where required, are usually $150 to $600. The permit guide covers who pulls it.

Repair or replace

A flat roof repair averages about $700: a split seam re-welded, a blister cut out and patched, a pipe boot re-flashed. If the leak is one spot and the membrane around it is still flexible, repair it. The roof leak repair cost guide has the per-fix prices. Replace instead when leaks show up in more than one place, the seams are letting go all over, or the roof is at the end of its service life. Built-up and modified bitumen usually get there first.

A recover, a new membrane over the old roof, saves the tear-off line and is legal under the IBC only when the existing roof is sound. It is not permitted where the existing roof or its insulation is water-soaked, or where the roof already has two or more coverings on it. Core a sample before you price it. A recover over wet board traps the water and rots the deck from the inside. The overlay vs tear-off comparison works through that decision, and the roof replacement cost guide covers the pitched part of the house if both are going at once.