Roofing felt, also called underlay, is the secondary waterproof barrier beneath the tiles or slates on a pitched roof. Installing it correctly is essential for the long-term weathertightness of the roof structure, as it prevents water that penetrates the tile covering from entering the roof space.

Types of Roofing Felt

There are two main types of roofing underlay. Traditional reinforced bituminous felt, also called BS 747 felt, is the standard roofing felt used on most domestic roofs. It is a heavy-weight product that provides a secondary barrier and prevents wind-driven rain and dust from entering the roof space. Breathable membrane underlays, introduced in the 1990s, allow water vapour to pass through from inside the roof space while preventing liquid water from entering, which improves ventilation and reduces condensation risk. Breathable membrane is now the recommended product for most new and replacement roofing work and may be required to comply with current Building Regulations for habitable loft spaces.

Laying the Felt

Start at the eaves, laying the first strip of felt horizontally with its lower edge overhanging the fascia by 50mm so that water runs into the gutter rather than onto the fascia board. Fix the felt with galvanised clout nails at regular intervals along both the top and bottom edges, and at each rafter crossing. The nails should be close enough to hold the felt flat and prevent wind lifting it before the battens are installed.

Lay subsequent strips working up toward the ridge, each strip overlapping the one below by a minimum of 75mm. In exposed locations or on low-pitched roofs, an overlap of 100 to 150mm provides more reliable secondary protection. At the ridge, the felt from each slope should overlap by at least 150mm or be covered by a strip of felt laid over the ridge to create a sealed junction.

At Valleys and Abutments

At valleys where two roof slopes meet, cut the felt so that it laps into the valley and is covered by the valley flashing or valley tiles. At abutments where the roof meets a wall, the felt should turn up the wall by at least 150mm and be covered by the wall flashing.

Fixing Battens Over the Felt

Once the felt is laid, fix the tile battens over it at the gauge spacing required for the specific tile being laid. The battens press down the felt and provide the nailing surface for the tiles. The batten size and fixing should follow the BS 5534 standard for slating and tiling, which specifies the required batten size for the rafter spacing and tile weight.

Summary

Roofing felt is laid horizontally from the eaves upward with a minimum 75mm overlap between strips, fixed with galvanised clout nails. Breathable membrane is the recommended modern alternative to traditional bituminous felt. At the ridge, valley, and abutments, the felt must be lapped or covered to maintain the secondary waterproofing. Battens are fixed over the felt at the correct tile gauge to complete the roof structure.