Roof felt — more accurately called sarking membrane or underlay — is the secondary line of defence beneath tiles or slates. It is not designed to keep the roof watertight on its own, but it catches any water that drives through gaps in the primary covering and directs it to the gutters, while also reducing heat loss and draughts at ceiling level.
Traditional bitumen-impregnated felt was introduced from the 1960s onwards and became near-universal in UK roofing by the 1980s. Many roofs built in that era now have felt that is 40–60 years old — well beyond its design life — and are starting to show the consequences.
How Long Does Roof Felt Last?
| Material type | Typical lifespan | Notes |
|---|---|---|
| Traditional bitumen felt (1B or 1F grade) | 20–40 years | Becomes brittle; common in homes built 1960s–1990s |
| Heavy-duty reinforced bitumen (HR) | 30–50 years | Thicker specification, slower degradation |
| Breathable (air-permeable) membrane | 40–60+ years | Modern standard; resistant to UV degradation |
| High-performance vapour-permeable (VP) | 50+ years | Premium product; used in cold-roof and warm-roof applications |
Bitumen felt exposed to cyclic temperature changes eventually embrittles, cracks, and sheds. In warm attic conditions (which can exceed 60°C on a summer day in a south-facing loft), degradation accelerates significantly.
Signs That Roof Felt Is Failing
From the loft:
- Visible daylight through the roof (cracks or holes in the felt)
- Felt hanging down in sags between rafters (has detached from battens or become heavy with accumulated moisture)
- Fine powdery black debris on the loft floor directly below the roof — this is degraded bitumen
- Damp patches on loft joists or rafters that cannot be traced to a specific tile or flashing failure
- Strong musty smell in the loft on damp days
From outside:
- A roofer lifting one or two tiles and finding felt that crumbles or tears on handling
- Tiles that move slightly — in a well-felted roof, the felt provides some resistance to tile movement
At ceiling level:
- Staining on ceilings directly under the roof void, particularly after windblown rain, when no obvious tile damage exists
- Drips during or immediately after prolonged driving rain from directions that would put water past the tile laps
If the felt is simply old and brittle but no water is entering, replacement is still advisable within five years — once it starts allowing water in, secondary damage (to battens, rafters, insulation, and ceiling finishes) accumulates quickly.
What Replacement Involves
Roof felt replacement cannot be done without stripping the primary covering. The process is:
- Remove roof tiles or slates carefully (salvageable tiles can be reused, reducing material cost)
- Remove existing battens (often rotted at the ends if felt has failed)
- Strip old felt from rafters
- Fix new breathable membrane over the rafters, lapping joints per manufacturer guidance
- Fix new treated softwood counterbattens and tile battens
- Relay tiles or slates
- Re-bed and re-point ridge, hips, and verges
This is a full roof strip and relay — structurally simple but labour-intensive. Access scaffolding is always required.
Breathable vs Traditional Felt
Modern breathable (air-permeable) membranes have significant advantages over traditional bitumen felt:
Traditional bitumen felt:
- Vapour-impermeable — traps moisture in the roof void
- Requires a ventilation gap between felt and insulation to prevent condensation
- Requires additional ventilation at eaves and ridge
Breathable membrane:
- Allows water vapour to pass outward while remaining impermeable to liquid water
- Can be laid in contact with insulation without a ventilation gap (subject to manufacturer specification)
- Typically approved by NHBC and meets current Building Regulation requirements under Approved Document C
For most residential pitched roofs, a breathable membrane to BS EN 13859-1 (underlays for discontinuous roofing) is the correct modern replacement. The most common UK specification is a lightweight class 1 or class 2 vapour-permeable underlay.
Roof Felt Replacement Cost (UK, 2026)
Costs vary significantly based on roof complexity (number of hips, valleys, dormers), pitch, tile salvageability, and access.
| Roof size / property type | Supply & fit cost range (2026) |
|---|---|
| Small detached / semi (40–60 m²) | £800–£1,600 |
| Standard semi-detached (60–80 m²) | £1,200–£2,500 |
| Detached (80–120 m²) | £1,800–£4,000 |
| Complex roof with dormers / multiple hips | Add £300–£800 per dormer or hip valley |
| New tiles (if originals not reusable) | Add £15–£40/m² supply |
| New replacement slates | Add £25–£60/m² supply |
Day rates for experienced roofers in 2026 run at £200–£350/day in the South-East and £160–£260/day in the Midlands and North. A typical semi-detached will take a two-man team three to five days.
Scaffold hire for a standard two-storey property costs £600–£1,200 for a two-week hire — this is usually the largest single cost in addition to the roof work itself.
Building Regulations and Permitted Development
Replacing like-for-like felt without altering the roof structure or covering is generally not notifiable under Building Regulations, provided the work is a repair rather than a material alteration. However, if the insulation specification is changed as part of the same project (which is a sensible opportunity to improve it), Part L may apply and improvement to minimum insulation values will be required.
If the tile or slate type is being changed — particularly in a conservation area or on a listed building — permitted development rules and/or listed building consent may apply. Check with the local planning authority before proceeding.
