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Felt vs Breathable Membrane — Roofing
Compare traditional bituminous roofing felt with modern breathable roof membranes for UK pitched roofs, covering BS 5534 requirements and condensation risk.
Side-by-Side Comparison
| Feature | Traditional Felt (Type 1F) | Breathable Membrane |
|---|---|---|
| Material cost | £0.40-0.80/m² | £0.80-1.50/m² |
| Ventilation needed | Yes — eaves + high level | No (in most cold roofs) |
| Vapour resistance | High (HR class) | Low (LR class — vapour open) |
| Condensation risk | Higher if ventilation inadequate | Lower — moisture escapes |
| Lifespan | 20-30 years | 30-50 years |
| UV resistance | Poor — days | Good — up to 3 months |
| Total cost incl. ventilation | Higher (ventilation products) | Often lower overall |
| BS 5534 classification | HR underlay | LR underlay |
When Traditional Felt Is Appropriate
- Matching existing underlay on partial re-roofs for consistency
- Heritage or conservation area requirements
- Budget repairs where ventilation is already adequate
- Specific manufacturer systems requiring HR underlay
When to Choose Breathable Membrane
- All new-build pitched roofs (standard practice)
- Full re-roofs on existing buildings
- Loft conversions where ventilation space is limited
- Properties with condensation problems in the loft
- Cost saving by eliminating ventilation products
Related Calculators
Use the Felt and Batten Calculator for material quantities, or the Roof Ventilation Calculator for ventilation sizing.
Next up: related pages
How We Calculate This
Frequently Asked Questions
Generally no. Breathable membrane (vapour-permeable underlay) to BS 5250 allows moisture vapour to pass through, eliminating the need for eaves-to-ridge ventilation in most cold roof constructions. Traditional felt is vapour-impermeable and requires 10mm continuous eaves ventilation plus 5mm high-level ventilation. However, warm roof constructions and rooms-in-roof may still need specific ventilation detailing.
Slightly. Breathable membrane costs £0.80-1.50/m² vs £0.40-0.80/m² for Type 1F bituminous felt. However, savings on ventilation products (eaves vents, tile vents, ridge vents) easily offset this, typically saving £2-5/m² of roof area. The total roofing cost is often cheaper with breathable membrane once ventilation savings are factored in.
BS 5534:2014+A2:2018 requires that all underlays be tested to BS EN 13859-1 for water tightness, tensile strength and tear resistance. The standard classifies underlays as HR (high resistance to water vapour — traditional felt) or LR (low resistance — breathable membrane). LR underlays can be draped unsupported between rafters, while HR underlays must be supported or laid taut.
Breathable membrane is suitable for most pitched roofs. Exceptions include roofs with impermeable coverings (fully torched felt, metal sheet without counter-battens), roofs over swimming pools or high-humidity environments, and some heritage buildings where the conservation officer may require traditional materials. Always check the membrane manufacturer guidance for specific applications.
Traditional bituminous felt (Type 1F) typically lasts 20-30 years before becoming brittle and cracking. Quality breathable membranes have expected lifespans of 30-50 years. Premium membranes with UV stabilisation can be left exposed for up to 3 months during construction without degradation, while traditional felt can degrade in weeks of UV exposure.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.