Description
Built-up felt is still the most widely specified flat roof covering in Britain, and for good reason: two bonded layers of SBS-modified bitumen give you a genuinely redundant waterproof system, where the cap sheet takes the weather and the underlay beneath it is a complete second line of defence. This is that two-layer system — the charcoal mineral cap sheet and the glass-fibre-cored underlay that goes under it — available by the roll or by the trade pallet.
- Two-layer torch-applied system: glass-fibre-cored underlay plus SBS polyester-cored mineral cap sheet
- Charcoal slate mineral finish — the granules take the UV so the bitumen beneath doesn't
- Complies with BS 8217:2005 when installed as a multi-layer system
- Buy single rolls for one roof, or full pallets at trade quantity
Specification
| Cap sheet roll | 8m x 1m, charcoal slate mineral finish |
|---|---|
| Cap sheet core | Polyester, saturated and coated in SBS-modified bitumen |
| Underlay roll | 16m x 1m (16m²), light sand upper finish |
| Underlay core | Glass fibre, saturated and coated in bitumen |
| Standard | BS 8217:2005 as part of a multi-layer system |
| Expected life | In excess of 15 years, correctly installed and maintained |
| Pallet quantities | Cap sheet 25 rolls; underlay 30 rolls |
Frequently asked questions
How many rolls do I need? Work from your roof area, then add for laps — felt is lapped at both side and end joints, so usable coverage is meaningfully less than the nominal roll area. As a rough guide, one 16m² underlay roll and two 8m² cap sheet rolls cover a small single garage. Measure, add roughly 10–15%, and round up to whole rolls.
Do I need both layers? Yes. The cap sheet is not a standalone covering — the BS 8217 compliance, and the redundancy that makes felt reliable, both depend on it being bonded over an underlay. A single layer is a false economy on a roof you want to forget about for fifteen years.
What does installing it involve? Torch-on felt is bonded with a propane roofing torch, melting the bitumen backing as the roll is unrolled. It is a skilled, hot-works job with real fire risk — most insurers and many local authorities have specific requirements around hot works on roofs. If you are not experienced with a roofing torch, this is a job for a contractor, or choose a cold-applied system instead.
How does it compare with EPDM? Felt is the traditional built-up system: proven, repairable in patches, and installed hot. EPDM is a single-piece cold-applied rubber membrane with a longer typical service life and no hot works. Both are sound choices — felt often wins on familiarity and repairability, EPDM on longevity and DIY-friendliness.
What goes underneath? A sound, dry, well-supported deck. On a warm roof build-up the insulation and vapour control layer go beneath, in that order from the deck up.
Prefer a cold-applied system with no torch? See our 1.2mm EPDM rubber membrane. Building a warm roof? Lay our polythene vapour control layer and PIR insulation board beneath it first.
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