Breathable roof membrane: what it is and when to replace it

What roof underlay is for

Tiles and slates shed most of the rain, but not all of it. Strong wind can push water and fine snow up between the laps, and a cracked or slipped tile lets it straight through. The underlay sits across the rafters, under the battens, and carries that water down to the eaves so it drips into the gutter instead of onto the loft insulation.

It also stops wind getting underneath the covering and helps keep dust and debris out of the loft. A roof can look perfect from the street while the underlay beneath it has perished, which is why it is worth understanding what is up there.

Bitumen felt versus a breathable membrane

Older roofs usually have a heavy black bitumen felt underlay. It keeps water out well when new but lets almost no vapour through, so the loft relies on ventilation at the eaves and ridge to carry moisture away. Over decades the felt becomes brittle, especially where it sags between rafters at the eaves, and it can tear or crumble to nothing in the gutter line.

A breathable roof membrane is a lightweight synthetic sheet that is waterproof to liquid water but allows water vapour to pass through. That reduces the risk of condensation forming on the underside, because moisture rising from the house can escape rather than being trapped. Breathable does not mean ventilation can be ignored. The manufacturer's instructions and the roof design decide whether the loft still needs eaves or ridge ventilation, and that should be checked rather than assumed.

• Bitumen felt: heavy, low vapour movement, depends on loft ventilation
• Breathable membrane: light, vapour permeable, reduces condensation risk
• Both: must be lapped correctly and dressed into the gutter at the eaves

Signs the underlay has failed

From inside the loft, look for daylight through torn felt, shredded strips hanging between rafters, or felt that has crumbled away at the eaves. Damp or stained timbers, wet insulation and drips after windy rain point the same way. From outside, flaking black fragments in the gutter are a common clue on older roofs.

Some of these signs overlap with condensation, which is a different problem with a different fix. Our guide to roof insulation explains how insulation and ventilation affect moisture in the loft, and a roofer can tell the two apart on inspection.

What roof felt replacement involves

The underlay sits beneath the battens, so it cannot be replaced from inside the loft or by lifting a few tiles. Renewing it properly means stripping the covering, removing the battens and old felt, laying new membrane with the correct laps, fitting new battens and relaying the tiles or slates. In practice that is a re-roof, which is why underlay is normally renewed as part of a roof replacement.

Where only a small area has failed, such as the eaves strip that has rotted into the gutter, a local repair can sometimes be made by lifting the lower courses. Whether that is worthwhile depends on the condition of the rest of the felt and the covering. A full replacement is also the right moment to consider insulation and ventilation together, because the roof is open.

Choosing a membrane

Membranes differ in weight, strength, vapour resistance and whether they are approved for use without separate ventilation. The right one depends on the roof pitch, the covering, the region's wind exposure and whether the loft is cold or is being insulated at rafter level. Ask any roofer quoting you which membrane they propose, whether the loft will still be ventilated, and how the membrane is finished at the eaves, verges, ridge and around chimneys or rooflights.

Common questions