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How to Install Roof Felt
Step-by-step guide to installing roofing felt underlay, covering roll direction, overlaps, drip edge details, and fixing to rafters and battens.
Tools and Materials
Tools Required
- Roofing ladder and scaffold
- Staple gun (hammer tacker or pneumatic)
- Stanley knife or scissors
- Tape measure and chalk line
- Hammer
- PPE: harness, hard hat, non-slip boots
Materials Required
- Breathable membrane or bituminous felt (add 15% for laps and waste)
- Stainless steel staples
- Counter battens (if required by membrane manufacturer)
- Tile battens (25 x 38 mm or 25 x 50 mm)
- Batten nails (65 mm stainless steel)
- Eaves carrier / felt support tray
- Butyl tape for sealing laps (if specified)
Before You Start
- Ensure safe access — scaffold and roof ladders are essential; never work on a roof without fall protection
- Check rafters are straight and true — plane down any high spots
- Install any required valley or hip boards first
- Choose a dry, calm day — wind makes handling large rolls on a roof dangerous
- Check BS 5534 requirements for your exposure zone (1-4) as fixing specifications vary
Step-by-Step Installation
- Step 1 — Fit eaves carrier trays. Clip plastic eaves carrier trays (felt support trays) to the fascia board. These support the underlay at the eaves and direct water into the gutter.
- Step 2 — Roll out the first course. Starting at the eaves (bottom), roll the membrane horizontally across the roof from one end to the other. Allow the membrane to overhang into the eaves carrier tray by 25-50 mm. Pull taut (breathable) or allow 10-15 mm drape (non-breathable).
- Step 3 — Staple to each rafter. Fix the membrane to every rafter with staples at 150-300 mm centres. Do not over-tension the membrane between rafters.
- Step 4 — Lay successive courses. Work up the roof, overlapping each course by at least 100 mm (150 mm recommended). The upper course always overlaps the lower. Use a chalk line to keep courses parallel to the eaves.
- Step 5 — Seal end laps. Where rolls join within a course, overlap by 150 mm minimum and ensure the joint falls on a rafter. On breathable membranes, seal the end lap with butyl tape if specified by the manufacturer.
- Step 6 — Ridge detail. At the ridge, drape the membrane over the ridge board (breathable) or stop 25 mm short (non-breathable to allow ventilation). On a breathable membrane roof, the membrane from each side should overlap at the ridge.
- Step 7 — Fix counter battens (if required). For some breathable membranes, fix counter battens (25 x 38 mm or 38 x 50 mm) vertically over each rafter on top of the membrane. This creates an additional drainage and ventilation channel.
- Step 8 — Fix tile battens. Nail tile battens horizontally across the counter battens (or rafters) at the gauge required for your tile type. Use stainless steel or sherardised nails. Check the gauge with a batten gauge.
Common Mistakes
- Using non-breathable felt without ventilation — causes condensation and timber rot
- Draping breathable membrane — it should be taut, not sagged
- Insufficient overlaps — water tracks back under laps in driving rain
- Damaging the membrane with staples — avoid over-stapling; every hole is a potential leak
- Not extending to the gutter — water must discharge into the gutter, not down the fascia
Cost Estimate (2026)
| Item | Typical Cost |
|---|---|
| Breathable membrane (per m²) | £0.80-2.00 |
| Bituminous felt 1F (per roll) | £15-25 |
| Tile battens (per m) | £0.50-1.00 |
| Stainless steel batten nails (per kg) | £8-15 |
| Total (50 m² roof, breathable membrane, DIY) | £150-300 |
Related Calculators
Use the Felt and Batten Calculator for material quantities, or the Roof Tile Calculator for tile counts and batten gauge.
Next up: related pages
Frequently Asked Questions
Breathable membrane (vapour-permeable underlay) allows moisture vapour to pass through from inside to outside, eliminating the need for ventilation at eaves and ridge in most cases. Non-breathable (bituminous) felt traps moisture and requires 25 mm ventilation gaps at eaves and 5 mm at ridge per BS 5250. Breathable membrane is now standard for new builds and re-roofs.
Horizontal (side) laps should be a minimum of 100 mm, with 150 mm recommended. Vertical (end) laps where rolls join should be at least 150 mm and must fall on a rafter or counter batten for support. At hips and valleys, allow at least 300 mm overlap. All laps should shed water downhill — upper sheets always overlap lower sheets.
Non-breathable bituminous felt must be draped with 10-15 mm sag between rafters to allow any water that penetrates the tiles to drain to the eaves. Breathable membrane should be laid taut and flat across the rafters without draping, as per BS 5534. Always check the manufacturer's installation guide for the specific product.
Use stainless steel or galvanised staples (minimum 20 mm crown, 25 mm legs) at 150-300 mm centres along each rafter. For breathable membrane, some manufacturers require counter battens fixed over the membrane before tile battens. On exposed sites, increase fixing frequency and use cap fixings at lap joints.
In most cases, breathable membrane eliminates the need for eaves and ridge ventilation. However, the membrane must be HR (high resistance to water vapour) rated, and the loft must have adequate background ventilation if habitable. Always check Part C and BS 5250 requirements for your specific roof construction. Non-breathable felt always requires ventilation.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.