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 defined in BS 5534 and BS EN 13859-1:
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 cross-ventilation mandatory — typically 25 mm open ventilation at eaves and 5 mm at ridge — to prevent interstitial condensation on timbers. The old designation was Type 1F.
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 5534:2014 classifies these as LR1 (lower resistance). In many configurations they eliminate the need for additional roof-space ventilation, though the precise ventilation requirement depends on roof type, insulation position and span.
Ventilation Requirements
This is where most specification errors occur. The rule of thumb:
- Cold roof (insulation at ceiling level) + non-breathable felt: mandatory cross-ventilation of 25 mm equivalent open area at eaves, plus counter-ventilation at ridge or high level.
- Cold roof + LR1 breathable membrane: ventilation is still recommended by BS 5250 (management of moisture in buildings) for spans over 8 m or where insulation is close to the underside of the rafters.
- Warm roof (insulation between/over rafters) + LR1 membrane: in most cases, no additional ventilation is needed because the insulation is above the cold zone.
NHBC Standards Chapter 7.2 and BS 5250:2021 provide the detailed ventilation calculation method. Always check the membrane manufacturer’s technical data sheet — some LR1 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
| Product Type | BS/EN Classification | Sd Value (m) | Typical Application |
|---|---|---|---|
| Standard bitumen felt (1F) | Non-LR | >200 | Cold roofs, requires ventilation |
| Non-woven PP breathable (LR1) | BS EN 13859-1 LR1 | <0.25 | Pitched roofs, reduced ventilation |
| High-performance HR (vapour check) | HR class | >2 | Timber frame warm roofs, AVCL |
| Reinforced bitumen (2F) | Non-LR | >200 | Flat roofs, not pitched tile use |
The Sd value (water vapour diffusion resistance) is the key number: LR1 membranes have Sd ≤ 0.25 m, which means vapour passes through freely. Non-breathable felts have Sd values well above 20 m.
Cost per Roll and per m²
Prices are UK trade supply (2025–2026), ex-VAT, for standard rolls:
| Product | Roll Size | Cost per Roll | Cost per m² |
|---|---|---|---|
| Standard bitumen felt (1F) | 20 m × 1 m | £8–£16 | £0.40–£0.80 |
| Budget breathable (LR1) | 50 m × 1.5 m (75 m²) | £45–£75 | £0.60–£1.00 |
| Mid-range breathable (LR1) | 50 m × 1 m (50 m²) | £65–£100 | £1.30–£2.00 |
| Premium breathable (LR1) | 50 m × 1 m (50 m²) | £100–£130 | £2.00–£2.60 |
| Foil-faced breathable (LR1) | 40 m × 1 m (40 m²) | £70–£110 | £1.75–£2.75 |
Material cost is a small fraction of total roof cost; specifying a premium LR1 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:
- Side laps (horizontal overlap between courses): 150 mm minimum.
- End laps (vertical overlap at roll joints): 150 mm minimum, positioned over a batten.
- Drape the membrane — do not tension it tight; a slight sag between battens allows any water to track to the eaves rather than pooling and penetrating lap joints.
- 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 resist moisture from rain penetrating to the building interior. The underlay is integral to compliance.
- Part L (Conservation of fuel and power): breathable underlays are compatible with any insulation system; vapour-check HR underlays are required as part of warm roof assemblies where they form the AVCL (Air and Vapour Control Layer).
- BS 5534:2014+A1:2015: mandatory reference for fixing and laying of underlays with slates and tiles.
- BS EN 13859-1: the European product standard for underlays for discontinuous roofing — look for CE-marked products citing this.
Choosing the Right Membrane
For the vast majority of UK residential reroofing projects, a mid-range LR1 breathable underlay is the correct specification. It eliminates the need to cut additional ventilation openings, 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 its shorter service life (20–30 years vs 50+ years for quality PP membranes) means it may need replacement before the tiles above it.
