Question 1

Do I still need to ventilate the roof with breathable membrane?

Accepted Answer

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.

Question 2

Is breathable membrane more expensive than traditional felt?

Accepted Answer

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.

Question 3

What does BS 5534 say about underlays?

Accepted Answer

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.

Question 4

Can I use breathable membrane on any roof?

Accepted Answer

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.

Question 5

How long does each underlay last?

Accepted Answer

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.

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

Felt vs Breathable Membrane — Roofing

Side-by-Side Comparison

When Traditional Felt Is Appropriate

When to Choose Breathable Membrane

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