Flat roofing is priced by the square (100 sq ft) like pitched roofing, but the components are entirely different: membrane, insulation board, adhesive, flashings, and drainage. No pitch factor applies.
Flat Roof Area
Area = L × W (no pitch factor). Add 10% for flashings, termination bars, and perimeter details.
Example
50×80 commercial flat roof = 4,000 sq ft ÷ 100 = 40 squares + 10% = 44 squares billed
Waste factor: 10% for membrane; 12% for insulation
Measure the full roof field (L × W). Separately measure the perimeter — parapet walls, edge details, and curbs are priced by linear foot, not sq ft. Count penetrations (HVAC curbs, drains, pipes) — each is priced individually at $150–$400.
Code-required R-values vary by climate zone. Most commercial flat roofs require R-20 to R-30 of polyiso insulation. 2-inch polyiso ≈ R-13; 3-inch ≈ R-20. Insulation is priced per square at $60–$120/sq depending on thickness. It's often the largest single cost on a flat roof job.
TPO (60 mil): $80–$140/sq installed. EPDM (60 mil): $70–$120/sq installed. Modified bitumen (two-ply): $90–$150/sq installed. Manufacturer warranty level affects which products are required. Always specify mil thickness in your estimate — 45 mil vs. 60 mil is a significant cost and warranty difference.
Perimeter edge metal (drip edge or coping): $12–$25/LF. Wall flashings: $15–$25/LF. Penetration flashings: $150–$400 each. These are frequently underestimated on flat roof jobs — measure the perimeter carefully.
Manufacturer's warranties on flat roofing (10, 15, 20 year) require certified installer status. If you're certified, include the warranty as a selling point and line item. If not, price the warranty registration separately or partner with a certified applicator.
Scan the roof with a moisture survey before estimating re-roofing
An infrared or nuclear moisture survey ($0.05–$0.10/sq ft) identifies wet insulation that must be removed before installing new membrane. Discovering wet insulation mid-job without a plan destroys your margin.
Price penetrations individually — they take time
An HVAC curb with new curb flashing, crickets, and a pitch pocket takes 1–2 hours. Multiply that by 8–10 curbs on a commercial job and you've got 10–20 hours of penetration work that disappears into a flat sq ft price.
Specify membrane thickness in writing
60 mil membrane costs 20–30% more than 45 mil and performs significantly better. Specify the thickness in your estimate. If a client is comparing your 60-mil TPO bid to a competitor's 45-mil bid, they're not comparing the same product.
TPO flat roof installation runs $8–$16/sq ft installed in 2026, depending on insulation thickness, number of layers removed, and market. This includes insulation, membrane, flashings, and fasteners. EPDM is slightly less; modified bitumen (torch-down) is similar to TPO.
A 40-square (4,000 sq ft) flat roof with two-man crew takes 2–4 days depending on tear-off complexity, insulation layers, and number of penetrations. Mechanical attachment is faster than fully adhered systems. Always allow an extra day for unusual penetrations or complex drainage.
TPO uses heat-welded seams (faster, requires expensive welder) and typically costs 5–15% more than EPDM. EPDM uses adhesive or tape seams. For estimating: TPO has higher material cost but faster seam installation; EPDM has lower material cost but slower adhesive seam work. Both require similar perimeter and flashing pricing.
Yes — for re-roofing over an existing flat roof, a core sample ($150–$300 lab fee) identifies how many layers exist, whether wet insulation is present, and the deck substrate. Wet insulation must be removed — discovering it mid-job without a pre-authorized rate creates a major scope dispute.
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