Minimum Fall for a Flat Roof
“Flat” roofs still need a fall. The US code requires ¼/12 (about 2%) for membrane roofs, and UK guidance in BS 6229 requires a minimum finished fall of 1:80 once the roof is built, so ponding water drains away.
Minimum pitch by system
| Roofing system | Minimum pitch | Conditions | Source |
|---|---|---|---|
| Built-up roofing (US) | ¼/12 (1.2°) | Coal-tar built-up roofs: ⅛/12 | IRC R905.9.1 |
| Modified bitumen (US) | ¼/12 (1.2°) | — | IRC R905.11.1 |
| EPDM / thermoset single-ply (US) | ¼/12 (1.2°) | — | IRC R905.12.1 |
| TPO / PVC thermoplastic single-ply (US) | ¼/12 (1.2°) | — | IRC R905.13.1 |
| Mineral-surfaced roll roofing (US) | 1/12 (4.8°) | — | IRC R905.5.2 |
| Any flat roof covering (UK, finished fall) | 1:80 fall (0.7°) | — | BS 6229 |
Design fall vs finished fall (UK)
BS 6229 sets a minimum finished fall of 1:80. Older editions recommended designing every flat roof at 1:40 so that deflection and building tolerances still left 1:80 at completion. The 2025 revision removed that blanket rule: the design fall now comes from structural analysis or survey data, and 1:60 is recognised as a starting point in many cases.
Frequently asked questions
What is the minimum roof pitch for flat roofs (EPDM, TPO, felt)?
“Flat” roofs still need a fall. The US code requires ¼/12 (about 2%) for membrane roofs, and UK guidance in BS 6229 requires a minimum finished fall of 1:80 once the roof is built, so ponding water drains away. Full list: Built-up roofing (US): ¼/12 (1.2°); Modified bitumen (US): ¼/12 (1.2°); EPDM / thermoset single-ply (US): ¼/12 (1.2°); TPO / PVC thermoplastic single-ply (US): ¼/12 (1.2°); Mineral-surfaced roll roofing (US): 1/12 (4.8°); Any flat roof covering (UK, finished fall): 1:80 fall (0.7°).
Can I use flat roofs (EPDM, TPO, felt) below the minimum pitch?
No. Below the minimum the covering is not code-compliant and manufacturers will not warrant it, because water drains too slowly and can be driven under laps. The lowest option on this page is any flat roof covering (UK, finished fall) at 1:80 fall (0.7°); below that, choose a different roofing system from the minimum pitch chart.