Flat Roof Calculator: EPDM, TPO and PVC

Membrane covers the roof plus every wall it climbs

The takeoff starts with the roof itself, then adds every place the sheet climbs: walls, parapets, curbs and the lip over the edge. Waste goes on last. Most of the mistakes in homemade rubber roof estimates come from forgetting the walls.

• Roof field = length x width
• Wall flashing = wall length x flashing height (never less than 8 in)
• Edge wrap = open edge length x 6 in, so the sheet can turn over the nailer
• Curbs = 16 sq ft each (a 4 x 4 ft curb, flashed 12 in up)
• Membrane = (field + walls + edges + curbs) x 1.10, and rolls = membrane / roll size, rounded up
• Sheets across = 1 + (width minus one roll width) / (roll width minus lap), rounded up. Seams = (sheets across minus 1) x length, plus end laps if the roof runs past 100 ft
• Polyiso boards = roof area x 1.05 / 32, per layer (two layers over 2.5 in)

The default numbers make a worked example. A 40 x 30 ft garage roof is 1,200 sq ft, with 140 ft of edge. Forty feet of that edge runs into the house wall and is flashed 12 in up, which adds 40 sq ft. The other 100 ft gets 6 in of edge wrap, which adds 50 sq ft. One skylight curb adds 16 more. That's 1,306 sq ft, times 1.10 = about 1,437.

With 20-ft-wide EPDM, one 20 x 100 sheet (2,000 sq ft) covers it, cut into two runs with a single 40-foot taped seam down the middle. Two inches of polyiso is 1,200 x 1.05 / 32 = 39.4, so 40 boards. On a glued-down roof that's 8 screws and plates each, 320 total.

For cost, 1,200 sq ft of EPDM at $4 to $10.50 is $4,800 to $12,600, plus tear-off at $1 to $1.50 a foot ($1,200 to $1,800). The total is $6,000 to $14,400.

What does a flat roof cost per square foot?

In this tool, EPDM costs $4 to $10.50 a square foot, TPO $6.50 to $11 and PVC $7.50 to $13, with tear-off priced as its own line. Thickness moves you around inside each range: 45 mil toward the bottom, 80 or 90 toward the top.

These match the numbers in our own guides, with one adjustment for tear-off. The EPDM guide (EPDM is what most people mean by a rubber roof) quotes $5 to $12 a sq ft with tear-off already included. This tool adds tear-off separately, so it takes that out and uses $4 to $10.50 for the rubber, insulation, flashing and labor. TPO sits at $6.50 to $11, tear-off extra.

PVC is the most expensive. Its guide says $8.50 to $14.50 with tear-off included, so the tool uses $7.50 to $13 and lists tear-off separately. Grease from a restaurant vent doesn't bother it.

Tear-off, insulation and flashing add to the price

The add-ons come from the flat roof guide. Tear-off costs $1 to $1.50 a foot per layer. Thick polyiso to meet today's energy code adds $2 to $4 a foot, and tapered board can double that insulation line, so the tool adds another $2 to $4.

A roof crowded with skylights and HVAC units runs 20 to 30 percent more, almost all of it flashing labor. Rotten decking is extra, at $80 to $120 a sheet. Local quotes vary a lot, so treat this as a check on bids, not a bid.

Membrane goes at least 8 inches up every wall

The minimum turn-up at walls and curbs is 8 inches, measured above the finished roof, in the federal spec sections for TPO roofing (VA 07 54 23 on WBDG) and the EPDM section next to it. On a short parapet, the sheet goes up and over the top. A termination bar clamps it at the top edge, and the tool counts a foot of bar for every foot of wall. Open edges get metal (drip edge or gravel stop), with the membrane lapped onto it.

Wider sheets mean fewer seams

EPDM comes in very large sheets. Carlisle's Sure-Seal runs up to 50 ft wide by 200 ft long, and the tool offers 10, 20 and 50 ft widths. Wider sheets mean fewer seams, and seams are where rubber roofs fail (the tape lets go long before the rubber does). The tool figures a 3-inch lap with primer and seam tape.

TPO and PVC come in narrower rolls: 10 or 12 ft for TPO and 10 ft for most PVC. That means more seams, but they are hot-air welded into one piece, and a good weld is stronger than the sheet next to it. The tool uses a 6-inch lap, which leaves room for screws in the seam on mechanically attached jobs.

Mechanically attached roofs need screws in every seam

Glued-down roofs only need screws for the insulation. Mechanically attached TPO or PVC also gets a row of screws and plates inside every seam. Johns Manville's bulletin on wood decks sets the minimum at 10-ft sheets with screws 12 in apart in the field, and 5-ft half sheets at 6 in around the perimeter. The tool uses that pattern, taking the perimeter strip as one half sheet wide. The real pattern comes from a wind uplift calculation for your building and could be denser.

Insulation screws follow GenFlex's fastening chart: 5 per 4x8 board under a mechanically attached sheet. Under a glued sheet it's 16 for boards under 1.5 in, 12 up to 1.9 in, and 8 at 2 in and thicker. With two layers, only the top one is counted, since its screws go through both.

To check the footprint before relying on these numbers, the roof cost calculator can pull a building outline from your address. If you're choosing between a single-ply membrane and torch-down, read TPO vs modified bitumen first.