Sarking felt — also called roofing underlay, underfelt or roof felt — is the membrane installed beneath the tiles on a pitched roof. Most homeowners are unaware it exists until a roofer mentions it during a repair or re-roof survey. It matters considerably more than it might seem.
The felt layer beneath roof tiles serves three functions. First, it provides secondary weather protection — when tiles are temporarily displaced by wind, storm damage or bird interference, the felt prevents water from entering the roof structure immediately. It is not intended as a primary weathering layer, but as a backup. Second, it controls condensation within the roof void — warm, moist air from the house rising into the cold loft space will condense on cold surfaces. Correctly specified underlay manages where this condensation goes. Third, it prevents wind-driven rain from being pushed up between tile courses in extreme weather conditions.
Properties built or re-roofed before approximately 1995 typically have impermeable black bitumen felt. This acts as a vapour barrier — moisture cannot pass through it in either direction. On a cold roof (where the insulation is at ceiling joist level and the loft is ventilated), this is manageable because ventilation carries moisture out of the roof void. On an unventilated roof, impermeable felt traps moisture in the roof structure, leading to batten rot and rafter deterioration over time. Original impermeable felt from the 1960s and 1970s is now 55–65 years old and typically brittle, cracked and non-functional.
Modern breathable underlay — manufactured to BS EN 13859-1 — allows moisture vapour to pass through from the roof structure to the outside while preventing liquid water from entering. This significantly improves moisture management in the roof void without requiring the precise ventilation gaps that impermeable felt demands. Breathable membrane is the standard specification for all re-roof work and is what we install as standard on every roof replacement we carry out in Guildford.
The most common felt failure modes we encounter across Guildford are:
Failed felt alone is not a reason to re-roof if the tiles and structure are otherwise sound. However, felt condition is assessed during any re-roof survey and is one of the factors we consider when advising on the repair vs replacement decision. A roof with failed felt and nail sickness is a stronger candidate for re-roof than one with failed felt alone. A roof with good felt and isolated tile damage is a repair candidate.
When a Guildford property does require re-roofing, installing breathable membrane as the new underlay — rather than repeating the impermeable felt specification — is one of the most cost-effective upgrades available and is standard in all our re-roof work.
Unsure about your felt condition? We assess felt condition as part of every survey visit in Guildford. Call 01483 346 440 for a free inspection and honest recommendation.
The condition of sarking felt in Guildford's housing stock varies systematically by era. Properties built before 1970 almost universally have original bitumen felt — either lapped traditional felt or the early thin reinforced varieties. This felt has a service life of 25–50 years depending on ventilation conditions and moisture exposure, meaning it is now either at or significantly past end of life on pre-1970 Guildford properties. When a pre-1970 Guildford property is re-roofed, the condition of the original felt is always investigated from the loft and documented before the replacement scope is finalised.
Properties built from the mid-1990s onwards have breathable membrane underlay — the polyethylene-based products that replaced traditional felt in UK housebuilding. These membranes, when correctly lapped and fixed, have indefinite service lives under normal conditions and do not deteriorate in the same way as bitumen felt. Re-roofing a 1990s or 2000s Guildford property where the breathable membrane remains intact does not require underlay replacement — just tile and batten replacement over the sound existing membrane.
A loft inspection shows the felt condition from below. Failed traditional felt is visible as: discolouration and brittleness, sagging between rafters, splits and holes, and sometimes daylight visible through cracks. Breathable membrane that has failed shows delamination, tears at batten fixing points, or sections where the membrane has pulled free from laps.
Breathable membrane is included as standard in our re-roof quotes — it is a relatively modest material cost. On a standard 3-bed Guildford semi, breathable membrane throughout adds approximately £300–£500 to the total re-roof cost and is the correct specification for any complete re-roof regardless of the tile material chosen.
Breathable membrane is classified as a vapour-permeable underlay that reduces but does not eliminate the need for roof ventilation. Current building regulations require counter-batten installation to provide an air gap above the breathable membrane — this is standard on all our re-roofs. We do not install breathable membrane direct to rafters without the counter-batten air gap.
Not effectively — sarking felt or breathable membrane runs from ridge to eaves under the battens and tiles. It cannot be replaced in sections or without removing the full tile covering. If the underlay is the primary concern (rather than the tiles), the most economical approach is to schedule a full re-roof rather than attempting partial felt replacement.
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