What Is the Sheet Under Our Tiles, and Does It Need Replacing?

The underlay is the second line of defence on a pitched roof. How it works, how it fails and what a failed one means for your roof.

Tiles and slates shed most of the rain, but wind driven rain and snow still get past them. The underlay, laid over the rafters and under the battens, catches that water and carries it down into the gutter. It also reduces wind uplift on the tiles.

Very old roofs may have no underlay at all, or torching, a lime mortar applied to the underside of the slates.

Bitumen felt versus breathable membrane

Roofs from roughly the 1950s to the 1990s usually have black bitumen felt. It keeps water out but also traps moisture in the loft, so it relies on eaves and ridge ventilation. Over time it becomes brittle and sags or tears between rafters, especially at the eaves where sunlight reaches it.

Modern breathable membranes let water vapour escape while still keeping liquid water out, which lowers the condensation risk, though most roofs still need some ventilation designed in.

From inside the loft look for daylight through torn felt, pieces of felt lying on the insulation, water staining on the rafters and felt that has perished into the gutter at the eaves. Damp patches on bedroom ceilings after windy rain are another clue.

Can it be replaced on its own?

Underlay sits under the battens and tiles, so replacing it across a slope means stripping and relaying the covering. A strip at the eaves can sometimes be renewed with an eaves carrier tray. Where the underlay is failing across the whole roof, it is often part of the case for a re roof, which our signs you need a new roof guide covers.

Is torn roof felt an emergency?

Not usually on its own, because the tiles are still shedding most water. It is worth planning a repair before it lets wind driven rain into the loft.

Does a breathable membrane mean I need no loft ventilation?

Not necessarily. It reduces the risk but most roofs still benefit from some ventilation, depending on the membrane and insulation.

Why is the felt rotten only at the bottom of the roof?

The eaves section is exposed to sunlight and gutter water, so it perishes first. An eaves carrier tray is a common fix.

My old slate roof has no felt at all. Is that a problem?

Not automatically. Many old roofs have performed for a century without it. It becomes relevant when the roof is being relaid.

Seen torn felt in the loft?

Send us a photo or book a look. We will tell you whether it is an eaves fix or a sign the roof is nearing the end.

Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.