Roofing Membranes & Breathable Felt Explained

The underlay, often called felt, sarking, or membrane, is the second line of defence under your tiles or slates. It catches any water that passes the outer covering, manages condensation risk, and provides temporary weather protection during construction. Getting the specification wrong costs far more to rectify than the few pounds per square metre difference between products.

The Two Fundamental Types

Roofing underlays divide into two performance categories, HR and LR, according to their water vapour resistance:

Non-breathable (traditional bitumen felt), typically a glass-fibre or polyester reinforced bitumen sheet. It is impermeable to water vapour. Moisture that reaches the underside of the tile (from condensation within the roof space) cannot pass through; it runs down the felt to the eaves. This makes roof-space ventilation essential in a cold roof: the equivalent of a continuous 10 mm opening at the eaves where the pitch is over 15° (25 mm at 15° or less), plus 5 mm at high level on steeper or wider roofs, to prevent interstitial condensation on timbers.¹ The traditional product was Type 1F felt to BS 747, a standard since withdrawn; Class 1F reinforced bitumen underlay is now made to BS 8747.¹

Breathable (low resistance / LR) underlays, woven or spunbond polypropylene or polyethylene membranes with a micro-porous structure. Water vapour passes outward; bulk liquid water is resisted. BS 5534 and BS 5250 call these LR (low resistance) underlays: a water vapour resistance of no more than 0.25 MN·s/g, an Sd value of about 0.05 m.¹ Anything more resistant, including traditional bitumen felt, is an HR (high resistance) underlay.¹ They let a cold roof manage with much less loft ventilation, though the exact requirement still depends on the roof type, where the insulation sits and how well the ceiling is sealed.¹

Ventilation Requirements

This is where most specification errors occur. The rule of thumb:

• Cold roof (insulation at ceiling level) + non-breathable felt: eaves ventilation equivalent to a continuous 10 mm opening where the pitch is over 15°, or 25 mm at 15° or less, plus 5 mm at high level where the pitch is 35° or more or the span is 10 m or wider.¹
• Cold roof + LR breathable membrane: less loft ventilation is needed, typically a continuous 3 mm eaves opening with a well-sealed ceiling, or 7 mm where the ceiling is not well sealed, and some coverings also need the batten space ventilated.¹
• Warm roof (insulation between/over rafters) + LR membrane: the loft itself needs no ventilation, but an AVCL goes on the warm side of the insulation, and the batten space may still need ventilating under an airtight covering such as fibre-cement slates.²

BS 5250:2021, the code of practice for managing moisture in buildings, gives the detailed recommendations, and NHBC Standards Chapter 7.2 covers pitched roofs on NHBC-registered homes.³ Always check the membrane manufacturer’s technical data sheet, as some LR products have specific substrate requirements.

Underlay Roles Beyond Weatherproofing

A correctly specified underlay also:

• Provides temporary weather protection between battening and tiling (important on long roofing programmes).
• Reduces wind-driven rain and snow penetration at laps and edges.
• Adds a small improvement to air tightness of the roof envelope.
• Some reflective variants (foil-faced) contribute a marginal insulating effect, though this is not a substitute for specified insulation.

Product Grades and Standards

The Sd value, the water vapour diffusion resistance expressed as an equivalent thickness of still air, is the number to check on the datasheet. An LR underlay’s is no more than about 0.05 m, so vapour passes through it freely.

Cost per Roll and per m²

Prices are UK trade supply (2025–2026), ex-VAT, for standard rolls:

As a check, Wickes sold IKO’s 120 g/m² Rubershield Eco Extra breather membrane for £74 a 50 × 1 m roll in September 2026, including VAT, which is about £1.23/m² before VAT.⁴

Material cost is a small fraction of total roof cost; specifying a premium LR product over a budget felt adds approximately £1–£2/m² to a project where total installed cost is £60–£150/m². It is rarely worth cutting here.

Lapping and Installation

BS 5534 specifies minimum laps:

• Horizontal laps (between courses): these depend on pitch and on whether the underlay is supported; unsupported underlay needs 225 mm at 12.5° to 15°, 150 mm at 15° to 35° and 100 mm at 35° and above.¹
• Vertical side laps (at roll ends): at least 100 mm.¹
• Drape the membrane, do not tension it tight; a slight sag between rafters, nominally 10 mm and no more than 15 mm, allows any water to track to the eaves rather than pooling and penetrating lap joints.¹
• At eaves, the underlay should discharge into the gutter, not be cut flush with the fascia.
• At abutments and verges, turn up or use proprietary edge sealing tape.

Building Regulations

• Part C (Resistance to moisture): the roof must protect the building from rain and from condensation, and the underlay is part of how a tiled roof does that.⁵
• Part L (Conservation of fuel and power): the underlay is not the vapour control layer. In a warm pitched roof the air and vapour control layer (AVCL) goes on the warm side of the insulation, and the underlay above it should be an LR type.¹
• BS 5534:2014+A2:2018: the code of practice for fixing and laying underlays with slates and tiles.⁶
• BS EN 13859-1: the European product standard for underlays for discontinuous roofing, look for CE-marked products citing this.⁷

Choosing the Right Membrane

For the vast majority of UK residential reroofing projects, a mid-range LR breathable underlay is the correct specification. It reduces the ventilation a cold roof needs, is compatible with insulation upgrades, and simplifies Building Control sign-off. Traditional bitumen felt remains acceptable but adds the cost and complexity of ventilation provision, and it can have a shorter service life than a quality LR membrane, so it may need replacement before the tiles above it.