Metal, tile & flat
Flat roof membranes: EPDM, TPO and PVC quantities
A flat roof is a membrane, a slope to the drains and as few seams as possible. Here is how to count each one.
Flat is never quite flat
“Flat” roofs are low-slope roofs: below about 2/12, where shingles are not allowed, down to a quarter inch of fall per foot. They are covered with a continuous membrane rather than overlapping pieces, because water does not run off fast enough to be shed. Even so, the deck must slope to drains, scuppers or gutters: ¼ inch per foot is the usual minimum for new roofs, and most membrane warranties exclude damage from water that ponds for more than about 48 hours. Tapered insulation boards are the usual way to build the slope on a flat deck.
Three single-ply membranes
| EPDM | TPO | PVC | |
|---|---|---|---|
| Material | Synthetic rubber, black (or white) | Thermoplastic polyolefin, usually white | Polyvinyl chloride, usually white |
| Seams | Taped (or glued) | Hot-air welded | Hot-air welded |
| Common roll widths | 10 and 20 ft (larger for commercial) | 10 and 12 ft | 10 and 12 ft |
| Thickness | 45 and 60 mil | 45, 60 and 80 mil | 50 to 80 mil |
| Expected life (surveys) | 20 to 30 years | 15 to 25 years | 20 to 30 years |
| Strengths | Proven, forgiving, wide sheets, DIY-friendly | Reflective, welded seams stronger than tape | Welded seams, resists grease and chemicals |
Rolled roofing (mineral-surfaced asphalt) and modified bitumen are the asphalt-based alternatives: cheaper and shorter-lived, common on sheds, porches and garages. The rolled roofing calculator covers rolled roofing.
Counting a membrane
A membrane roof is one sheet if a roll is wide enough, or several strips joined by seams. Each strip is cut from a roll, overlaps the next by the seam lap, and the whole sheet turns up the parapets or over the edge to the fascia.
A 40 by 24 foot roof in EPDM
12-inch turn-ups all round, 10 × 100 ft rolls, 3-inch seams.
- Sheet: 42 × 26 = 1,092 sq ft
- Net width: 9.75 ft; strips: 3 of 42 ft; two per roll, so 2 rolls
- Seams: 2 × 42 = 84 ft, one roll of seam tape; bonding adhesive about 19 gallons
The same roof in one 20-foot-wide sheet would have one seam instead of two. On a small garage, an 18 by 16 foot roof fits one 20 × 50 sheet with no seam at all. Both from the flat roof membrane calculator.
Seams are where flat roofs leak
The sheet itself rarely fails before its time; seams, edges and penetrations do. Taped EPDM seams depend on clean, primed rubber and pressure from a roller; welded TPO and PVC seams depend on the welder’s temperature and speed and are probed after welding. Every foot of seam avoided is a foot that cannot leak, which is why the widest sheet the crew can handle is the right sheet.
Attachment
- Fully adhered: bonding adhesive on the deck (or insulation board) and the membrane. Smooth, wind-resistant, the usual choice on houses. Adhesive coverage is printed on the can.
- Mechanically attached: plates and screws along the seams, hidden by the next sheet. Faster and cheaper; common on larger roofs.
- Ballasted: loose-laid and held down with stone. Only on structures designed for the weight.
Edges, walls and drains
At the edges the membrane turns up a parapet and is held by a termination bar and counter flashing, or runs over the edge onto metal edge trim. Drains and scuppers get pre-formed boots or target patches; so do pipes and curbs. These accessories come from the membrane maker’s catalogue and are what makes the system warrantable. Water leaving the roof over an edge goes into a gutter sized like any other: the gutter length calculator counts it.
Over an old flat roof
Many flat roofs are re-covered rather than torn off: a new membrane over the old one, with a cover board or insulation between them. Codes allow it only if the old roof is dry and sound and there is at most one existing layer, and a moisture survey (a core cut, or an infrared scan) should show the old roof is not holding water. Trapped water under a new membrane rots the deck from inside and voids the new warranty. When in doubt, tear off: the extra disposal is cheaper than a second new roof.
Low slope, not flat
Between about 2/12 and 4/12, shingles are allowed with double underlayment, and many roofers lay self-adhering membrane under them, as the ice and water shield guide explains. Below 2/12, a membrane is the roof. Felt vs synthetic underlayment covers the double-coverage rule in between.
Cost and life
Published 2025 cost surveys put installed single-ply membrane roofing at roughly $500 to $1,000 per square on houses, depending on thickness, insulation and access. The replacement cost calculator includes it, and how long roofs last compares membranes with pitched roofs. The area itself is simple: length times width of the deck, which the roof area calculator gives as a footprint at zero pitch.