What a flat roof costs
In 2026, single-ply membrane replacement runs $5.50 to $9.00 per square foot installed for typical residential and light-commercial work. The two dominant systems are TPO (white thermoplastic, heat-welded seams, $6–$9/sq ft) and EPDM (black rubber, taped/glued seams, $5.50–$8/sq ft). PVC runs a dollar or two above TPO and earns it in restaurant or chemical-exposure settings. A 6,000 sq ft commercial roof typically lands between $35,000 and $55,000; a 400 sq ft home addition might be a $3,000–$4,500 job because minimum crew and mobilization costs dominate small projects.
What you're really paying for
Seams and edges. Flat roofs fail at seams, penetrations, and parapet terminations — almost never in the field of the membrane. Heat-welded TPO/PVC seams are as strong as the sheet itself; EPDM's taped seams are good but more installer-dependent. Ask any bidder how many linear feet of seam your roof has and how they finish terminations.
Insulation. Commercial codes generally require rigid polyiso above the deck (R-25 to R-30 in most climate zones), which can add $1.50–$3.00/sq ft. Residential additions over conditioned space face the same logic. Cheap bids often carry thin insulation — compare R-values, not just totals.
Drainage. "Flat" roofs must still slope about ¼ inch per foot to drains or scuppers. Ponding water ages every membrane and voids many warranties; fixing drainage with tapered insulation is money better spent than a thicker membrane.
Tear-off versus overlay
Most codes allow one recover over an existing flat roof if it's dry (verified by moisture scan). A recover saves $1.50–$2.50/sq ft in tear-off and disposal. Wet insulation, multiple existing layers, or a deteriorated deck force a full tear-off — which is also the only way to fix slope problems properly.
Lifespan and maintenance
Expect 20–30 years from TPO/PVC and 25–35 from EPDM when drainage is right. Flat roofs reward (and quietly require) semi-annual maintenance: clear the drains, check seams and sealant at penetrations, and walk the roof after major storms. Membrane thickness matters at bid time — 45-mil is builder grade, 60-mil is the sweet spot for most buildings, and 80-mil earns its premium under heavy foot traffic or rooftop equipment. The calculator above prices a mid-grade 60-mil membrane system with standard insulation, adjusted to your state's labor index; bids that come in far under it are usually 45-mil quotes with minimal insulation, which is a different product, not a better price.