Roofing Underlay: Breathable Membranes vs Traditional Felt

Summary

Underlay is the layer everyone forgets until it fails. Tiles and slates are the primary weather barrier, but wind-driven rain, snow and condensation all get past them — the underlay catches that water and drains it to the gutter, and (in modern roofs) lets water vapour escape from the roof space. Choosing the wrong underlay, or the right one fixed badly, causes condensation, sarking sag, and timber decay that is invisible until the ceiling stains.

The big shift in UK roofing is from impermeable bitumen felt (the old "1F" sand-surfaced felt and reinforced "HR" felt) to vapour-permeable membranes ("breathers"). The difference matters because it changes how the roof handles moisture. An impermeable-felt cold roof relies on ventilation gaps at the eaves and ridge to clear condensation (see roof ventilation). A breathable membrane lets vapour pass through the membrane itself — but only some membranes are certified to work without supplementary ventilation, and only when detailed correctly with the right air-and-vapour control below.

This article gives the practical fixing rules under BS 5534 — lap dimensions, fixing centres, drape, eaves and ridge detailing — and a clear comparison of felt vs breathable so you specify and fix the right product. The headline trap to avoid: fitting a breathable membrane and removing the ventilation, when the membrane wasn't certified for unventilated use or the ceiling below isn't airtight. That combination grows condensation, not stops it.

Key Facts

• BS 5534 governs fixing — BS 5534:2014+A2:2018, Code of practice for slating and tiling, sets underlay laps, fixing, drape and wind-uplift requirements.
• Three broad underlay types — impermeable bitumen felt (type 1F / sand-surfaced), reinforced bitumen felt (HR), and vapour-permeable (breathable) membranes (spunbond/microporous "breathers").
• Breathable membrane vapour resistance — low-resistance (LR) breathers have a vapour resistance typically below 0.25 MNs/g; this is what lets moisture escape. High-resistance products do not "breathe" usefully.
• Horizontal head lap — typically 100mm minimum at pitches at or above 15°, increasing at lower pitches and in exposed/high-wind sites; verify against the membrane maker's table and BS 5534.
• Vertical (side) lap — typically 100mm minimum, or to manufacturer's printed lap line.
• Drape — underlay should be laid with a slight drape (sag) of ~10–15mm between rafters to channel water into the gutter, but not so much it ponds or touches the ceiling insulation.
• Wind uplift — BS 5534 added stricter underlay fixing because membranes balloon and tear in wind; counter-battens or taped laps may be required in exposed zones.
• Ventilation interaction (BS 5250) — impermeable felt cold roofs need ventilation (commonly 10mm continuous at eaves, plus high-level for certain spans); breathable membranes may reduce this only when third-party certified (e.g. BBA) for the specific build-up and with an effective air/vapour barrier at ceiling level.
• Eaves detail — underlay must carry water into the gutter via an eaves carrier / support tray (felt support) so it doesn't sag and rot the fascia. See fascia soffit.
• Ridge detail — at the ridge, underlay is lapped and (on ventilated ridges) cut back to allow airflow; dry-ridge systems have specific underlay detailing.
• UV degradation — most underlays are not UV-stable for long exposure; cover with tiles/slates within the maker's stated exposure window (often a few months).
• Tile/slate fixing still required — underlay is secondary; primary fixing of tiles/slates to BS 5534 (clips, nails, mechanical fixing by zone) is unchanged. See pitched roof repairs.

Quick Reference Table

Detailed Guidance

How underlay actually works on the roof

Water gets past tiles and slates in three ways: wind-driven rain through laps, capillary action, and condensation forming on the cold underside of the covering. The underlay collects all of that and drains it down to the eaves and into the gutter. On a cold roof (insulation at ceiling level), the roof space above is cold, so warm moist air from the house that leaks up will condense on the cold underlay — which is why ventilation, or a breathable membrane, matters.

The two strategies:

• Impermeable felt + ventilation: the felt stops vapour, so you must vent the roof space to carry that vapour out before it condenses (BS 5250). Eaves and ridge vents do this.
• Breathable membrane: the membrane lets vapour pass through it to the outside, so in principle less (or no) ventilation is needed — but only if the membrane is certified for that use and the ceiling below is airtight enough to control how much vapour reaches the roof.

Specifying breathable vs felt

The trap: fitting a breathable membrane and sealing up all the ventilation, on a roof where the ceiling leaks warm moist air, and the membrane isn't certified for unventilated use. That moves the condensation risk to the cold side of the membrane and into the battens/rafters. If in doubt, ventilate — a breathable membrane with ventilation is safe; an uncertified unventilated build-up is not.

Fixing to BS 5534 — laps, drape and uplift

   RIDGE
    │  ── underlay lapped at ridge, vented if ventilated ridge
    │
    ▼ run UP the slope, each course lapping the one below
   ┌─────────────────────────────┐  head lap ~100mm (pitch-dependent)
   │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│  slight drape between rafters
   ├─────────────────────────────┤  side lap ~100mm / to printed line
   │░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░│
   └─────────────────────────────┘
   EAVES ── underlay onto eaves carrier → into gutter

• Lay up the slope, each course lapping over the one below so water always runs onto, never under, the next sheet.
• Head (horizontal) lap typically 100mm at standard pitches, more at low pitch and in exposed sites — follow the BS 5534 table and the membrane maker's printed lap line.
• Side (vertical) lap typically 100mm or to the printed lap line.
• Drape: allow a slight sag between rafters (~10–15mm) so water is channelled to the gutter, but keep it clear of insulation below.
• Wind uplift: in exposed zones, BS 5534 may require taped laps and/or counter-battens to stop the membrane ballooning and tearing — this is the headline change the 2014+ standard brought in.
• Batten over the underlay to BS 5534 batten spec (graded battens, correct size and fixing).

Eaves and ridge detailing

• Eaves: the underlay must be supported over the fascia by an eaves carrier (felt support tray) so it drains into the gutter and doesn't sag, pond and rot the fascia/soffit. See fascia soffit. A continuous eaves ventilation strip is fitted here on ventilated/felt roofs.
• Ridge: on a ventilated ridge, the underlay is cut back / lapped to allow airflow out at the top of the slope; on a dry-ridge system follow the kit's underlay detail. Continuous high-level ventilation here works with the eaves vent to drive airflow.
• Valleys, hips and abutments: dress underlay into valleys and up abutments under the flashing so water is carried over the underlay, not behind it.

The ventilation link — read this before you seal a roof up

Underlay choice and ventilation are one decision, not two. The rules live in BS 5250 (condensation control) and are summarised in roof ventilation and roof insulation:

• Cold roof, impermeable felt: ventilate — commonly a 10mm continuous equivalent gap at the eaves, with high-level ventilation for larger spans / certain pitches.
• Cold roof, breathable membrane: ventilation can be reduced or omitted only where the membrane is certified (e.g. BBA) for that build-up and there is an effective air-and-vapour control layer (a reasonably airtight ceiling). Otherwise, ventilate as for felt.
• Warm roof: insulation at rafter level with the membrane above — different rules again; follow the system certification.

If you cannot confirm certification and ceiling airtightness, the safe default is to ventilate.

Frequently Asked Questions

Is breathable membrane always better than felt?

Not always. Breathable membrane is the modern standard and is lighter, stronger in wind (when taped/counter-battened), and lets vapour escape. But it only removes the need for ventilation if it's third-party certified for unventilated use and the ceiling below is airtight. For a like-for-like repair on an existing felt roof, or a traditional/listed roof, bitumen or reinforced felt may be the correct spec. Match the existing system unless you're re-roofing fully.

Do I still need roof ventilation if I use a breathable membrane?

Maybe — and the safe answer is usually yes. Ventilation can only be reduced or removed when the specific membrane and build-up are certified for unventilated use (e.g. BBA) and the ceiling is airtight enough to control vapour. If you can't confirm both, ventilate the roof to BS 5250 anyway. A breathable membrane with ventilation is safe; an uncertified, unventilated, leaky-ceiling build-up causes condensation in the battens and rafters.

What lap do I need on roofing underlay?

Typically 100mm head lap and 100mm side lap at standard pitches, increasing the head lap at low pitches and in exposed/high-wind sites. Always check the BS 5534 lap table and the membrane manufacturer's printed lap line for the specific pitch and exposure zone.

Can I leave underlay exposed before tiling?

Only briefly. Most underlays are not UV-stable and degrade if left exposed beyond the maker's stated window (often a few months, sometimes less). Get the tiles or slates on within that window, and don't leave a "membrane-only" roof over winter expecting it to last.

Why does the underlay sag (drape) between the rafters?

A deliberate slight drape (~10–15mm) channels any water that gets onto the underlay down into the gutter rather than letting it pool. Too little drape and water can be forced over the laps; too much and it ponds or touches the insulation. Lay it with a gentle, even sag.

Regulations & Standards

• BS 5534:2014+A2:2018 — Code of practice for slating and tiling (including shingles): underlay laps, fixing, drape and wind-uplift design.

• BS 5250:2021 — Management of moisture in buildings (formerly "control of condensation in buildings"): roof ventilation and the use of vapour-permeable underlays.

• Approved Document C (Site preparation and resistance to contaminants and moisture) — resistance to moisture for roofs.

• Approved Document F (Ventilation) — building ventilation that interacts with roof-space condensation control.

• BS EN 13859-1 — flexible sheets for waterproofing: underlays for discontinuous roofing — definitions and characteristics.

• BBA / third-party certification — product-specific certificates stating whether a breathable membrane may be used in an unventilated roof and how it must be detailed.

• BS 5534 slating and tiling (BSI) — underlay fixing, laps and wind uplift

• BS 5250 moisture in buildings (BSI) — roof ventilation and vapour-permeable underlay use

• NFRC (National Federation of Roofing Contractors) technical guidance — underlay selection and roof ventilation

• BBA (British Board of Agrément) — product certificates for breathable membranes and unventilated roofs

• roof ventilation — eaves/ridge ventilation that pairs with felt and uncertified membranes

• roof insulation — cold vs warm roof build-ups that drive the underlay/ventilation decision

• pitched roof repairs — tile and slate fixing over the underlay to BS 5534

• fascia soffit — eaves carrier and vented soffit detail the underlay drains onto