What is roofing felt underlay?

Roofing felt underlay is a layer of water-resistant sheet material fixed directly to the rafters beneath the tiling battens and tiles or slates. It acts as a secondary barrier against wind-driven rain that penetrates between tiles, and it channels any water that gets through down to the eaves and into the gutter.

What is the difference between breathable membrane and traditional felt?

Traditional bitumen-based roofing felt (BS 747) is non-breathable - it traps moisture in the roof void and requires a ventilated air gap beneath it. Breathable (vapour-permeable) membrane allows water vapour to pass through while remaining waterproof, so it can be laid tightly against insulation in a warm roof without requiring a ventilated void. BS 5534 now specifies requirements for both types.

Does roofing felt need to be replaced when re-roofing?

Yes. When a roof is stripped for re-tiling, the underlay should always be replaced. Old bitumen felt becomes brittle and splits over time. Replacing with a modern breathable membrane when re-roofing is best practice - it improves performance, can simplify ventilation requirements, and is required by BS 5534 for new installations.

How is felt underlay fixed to rafters?

Underlay is draped over the rafters horizontally, starting at the eaves and working up to the ridge, with each course overlapping the one below by at least 150mm. It is temporarily held with clout nails or staples before the tiling battens are fixed through it into the rafters. The underlay must sag slightly between rafters to drain water effectively into the gutter.

Felt Underlay - Construction Glossary

Definition

Felt underlay is a layer of water-resistant sheet material fixed directly to the rafters beneath the tiling battens and tiles or slates. It provides a secondary line of defence against wind-driven rain that penetrates between tiles, channels condensation and any water ingress down to the eaves and into the gutter, and helps reduce wind lift on the tiles above.

In practice

There are two main types in use on UK roofs. Traditional bitumen felt (formerly BS 747 Type 1F) is non-breathable and requires a ventilated airspace between the felt and any insulation to prevent condensation. Breathable (vapour-permeable) membranes allow water vapour to escape while remaining waterproof, making them compatible with warm roof and hybrid insulation arrangements without a separate ventilated void.

Underlay is laid horizontally starting at the eaves and working up to the ridge, each course overlapping the one below by at least 150mm. The felt must sag 10-15mm between rafters to channel water effectively - an underlay pulled drum-tight will pond water and fail prematurely. At the eaves, the underlay must lap into the gutter rather than terminate behind the fascia. BS 5534 (2014) updated the fixing and lapping requirements for all pitched roof underlays.

Building Regulations

Relevant PartsPart C (moisture), Part F (ventilation)

StandardBS 5534:2014 - Slating and tiling for pitched roofs

Minimum lap150 mm horizontal, 100 mm at vertical side laps

Ventilation requirementNon-breathable felt requires 25 mm air gap above

BS 5534:2014 is the key standard for pitched roof slating and tiling in the UK, covering underlay specification, lapping, and fixing. Approved Document C requires the roof to resist penetration of moisture; Approved Document F covers ventilation of the roof void. When re-roofing, replacing old felt with a compliant breathable membrane is best practice and simplifies ventilation requirements.

Full Building Regulations guidance

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