Can a survey tell if roof underlay has failed?

Underlay is a sheet that runs across the rafters under the battens, which are the slim timber laths the tiles or slates hang from. Its job is secondary: it catches wind-blown rain and snow that gets past the covering and directs it down into the gutter. It also lets the roof space breathe where a breathable type is used. Nobody sees it in daily life, and nobody replaces it unless the roof is stripped, so it can stay in place long after it has stopped working.

Two broad types exist. Older roofs often have bitumen-based felt, which hardens and cracks with age. Newer ones may have a breathable membrane, which allows moisture vapour to pass but not liquid water. A survey judges underlay by its visible condition, such as cracking, brittleness, tears and staining, rather than by its age on the calendar.

The point to remember is that a leak in the loft does not always mean a broken tile. Sometimes the covering is sound and the membrane under it has given way.

The loft gives the clearest evidence. With a torch and a ladder, a surveyor scans the underside of the roof for:

• Tears, splits or holes in the sheet, especially around pipes, hatches and chimneys.
• Sheets that sag between rafters and hold a bag of moisture or debris.
• Brittle, cracked or powdery felt that snaps when touched at an edge.
• Daylight showing through, which means the covering above may also be open.
• Dark, damp patches on battens and rafters, with the grain lifted or furred.
• Drip marks, rust on nails and tide lines on the timber, which show water has come through more than once.
• Laps that have parted, so the upper sheet no longer overlaps the lower.

None of these is conclusive alone. A single stained batten might come from a one-off slipped tile. A pattern of damp across several rafters, along with torn felt, is a much stronger sign. Our wider checklist in what a roofer looks for in your loft covers everything else we check up there.

From outside, underlay is judged by its effects. Look for slates or tiles that keep slipping on the same slope, because a sagging membrane can hold water against the battens until they rot and release the fixings. Damp patches at the eaves or on the fascia can show water running off the underlay where it ought to enter the gutter. At the eaves, underlay is meant to run into the gutter; where it stops short or has curled, water gets behind the fascia board.

Moss growing thickly at the same place year after year often means moisture is held there. At ridges, hips and valleys, the membrane is cut and lapped, and cracked mortar or lifted lead there gives water an easy path to the sheet beneath. In high wind, loose felt can flap and lift tiles, which an eye from the ground will often catch as a rippled or lifted course.

Photographs help here. Our survey pictures show the slope in close and wide shots so the pattern is clear. A quick check before an auction relies on exactly this sort of outside evidence when the loft cannot be reached.

Separating the two matters because the repairs differ. If the covering is failing but the membrane beneath is sound, individual slates or tiles can be replaced and the roof carries on. If the underlay has gone, the sound tiles above it may be perfectly good, yet the roof still leaks, and the honest fix is to lift the covering, replace the membrane and battens as needed, and relay.

Clues that the underlay is the problem include leaks that appear in dry spells after a downpour, damp in the same area despite several tile repairs, and torn sheet near the eaves with sound tiles above. Clues pointing to the covering include a visible gap, a missing tile or a broken slate directly over the stain.

Re-roofing is where underlay is renewed as a matter of course. We cover the sequence in our answer on how often to inspect a roof, where regular checks stop small underlay faults from spreading.

There are limits to what a survey can say. A surveyor can see the underside of the membrane from the loft, but not its upper face, and cannot always reach every rafter bay. Insulation laid across the joists may hide the eaves. Boarded lofts, stored belongings and tight spaces limit access. In those cases we say plainly what we could not see and what we are inferring.

If the evidence is mixed, the sensible next step is a small opening: lifting a few tiles in a suspect area, which a roofer can do in a short visit, then looking at the sheet and battens directly and closing it up again. That costs far less than stripping the roof on a guess.

Where a full renewal is needed, a Stroud re-roof generally costs between £5,000 and £12,000 according to MyBuilder in 2026, depending on size and access, and scaffold is a separate line. Our itemised quote after the survey shows underlay, battens and labour as individual items, so you can see what the membrane adds. We give a written 10-year workmanship guarantee on our work, and the roof inspections hub shows how to arrange a visit.