A flat roof is a roof with a very slight pitch (1-5°) that is covered with a waterproof layer such as EPDM, bitumen, or PVC. Despite the name, a flat roof is never completely flat — there is always a slight fall for water drainage.
Characteristics
| Property | Flat roof |
|---|---|
| Pitch | 1-5° (1.5-9% fall) |
| Roof covering | EPDM, bitumen, PVC |
| Construction | Joists + roof decking |
| Trusses | Not needed |
| Gutter | Often on one or two sides |
Flat roof vs. pitched roof
| Flat roof | Pitched roof | |
|---|---|---|
| Construction | Simpler (no trusses) | More complex |
| Construction cost | Lower | Higher |
| Waterproofing | More critical (low pitch) | Easier (water runs off quickly) |
| Maintenance | More (leak checks) | Less |
| Usable as terrace | Yes (with correct build-up) | No |
| Loft space | None | Yes |
Build-up of a flat roof
Warm roof (most common)
The insulation sits above the structure, below the waterproof layer:
1. Supporting structure — Timber joists or concrete slab
2. Roof decking — Plywood or OSB board
3. Vapour barrier — Prevents condensation forming in the insulation
4. Insulation — PIR boards (with built-in fall)
5. Waterproof layer — EPDM, bitumen, or PVC
Cold roof (outdated, not recommended)
Insulation sits between the joists, waterproof layer on the decking. Risk of condensation. No longer used in new builds.
Fall
The fall is crucial: standing water is the greatest enemy of a flat roof. Guidelines:
- Minimum 1.5-2% fall (1.5-2 cm per linear metre)
- Towards the drainage — Gutter or rainwater outlet
- Creating fall with: angled joists, tapered insulation boards (PIR with sloped top), or uneven joist layers
Flat roof in DIY projects
Garage with flat roof
A popular choice thanks to the simple construction:
- Joists from wall to wall (or on a steel beam for large spans)
- Roof decking (18 mm plywood)
- Tapered insulation + EPDM
- Gutter on the low side
Garden building/shed with flat roof
- Modern appearance
- Easier to build than a gable roof
- Watch the maximum joist length (span) — over 4-5 metres you need intermediate support or heavier joists
Common mistakes
- No fall — Water pools (“bathtub effect”), accelerating wear on the roof covering and potentially causing leaks
- Building a cold roof — Condensation risk. Always build a warm roof.
- EPDM not properly bonded — Separation at the seams is the most common cause of leaks
Related terms
- EPDM
- Bitumen
- Fall (roof gradient)
- Gutter
- Fascia board
- PIR insulation
- Mono-pitch roof