How to Install Roof Felt

Tools and Materials

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

1. 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.
2. 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).
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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)

Related Calculators

Use the Felt and Batten Calculator for material quantities, or the Roof Tile Calculator for tile counts and batten gauge.