Should a stone roof have underlay?

Torching is lime mortar applied on the underside of the slates, bedding them against the battens and filling the gaps between courses. It kept out wind, dust and driven snow, and it held the slates steady. Our guide to lime mortar, torching and pointing on stone roofs covers the method. You can see it from the loft as a pale coat between the rafters and the stone.

Underlay is different. It is a sheet laid over the rafters and under the battens, backup protection for any water that slips beneath the stone. Old stone roofs were built without it. They relied on steep pitch, the mass and overlap of the stone, and lime beneath.

Neither approach is wrong in itself. The right one depends on the roof in front of you.

Stripping a stone roof gives a chance to put something under the slates. Approved Document L treats stripping a roof to its rafters and rebuilding it as a renovation of the thermal element. Renovate more than 50% of the roof surface, or take on a major renovation covering over 25% of the building envelope, and the whole element is expected to reach the Table 4.3 standard. That is why insulation comes up at the same moment as underlay.

A breathable membrane is one common choice, since it lets moisture pass outward while holding back liquid water. Others prefer to keep torching so that the traditional character survives. On a listed building, the decision is a matter of consent too, so we raise it with the conservation officer early.

Whatever you choose, write it down. A note of what went under the stone helps the next owner and the next roofer.

Underlay changes the way a roof breathes. Marley's guidance, citing BS 5250, separates two types. HR underlay does not breathe and needs ventilation along the eaves. LR, or breather, underlay on a well-sealed ceiling with an air-permeable covering needs much less, because moisture passes through the membrane. Stone slates leave open gaps at the laps, so the covering is air-permeable.

Wrong choices cause condensation, which rots timber and leaves a loft damp. A torched roof, by contrast, tends to be draughty and dry because air moves through it. Closing it up with the wrong membrane, or with spray foam, can upset that balance. RICS warned in its March 2023 guide that incorrect spray foam installation can lead to dampness, and by November 2024 some lenders had said they would decline properties with it, while others weigh each case.

The aim is to treat the roof as one system. We look at the loft, ceiling and eaves before suggesting anything.

Stone is heavy, at about 1,000 kg to every 10 m² on the council's figure. A membrane adds very little, but the battens that carry the stone must be sound and set out to suit the course sizes. When we compare stone with other coverings, we look at load as well as looks.

If a re-lay swaps to a covering about 15% heavier, the Planning Portal says you need approval, and Approved Document A expects the structure to be checked. Moving from stone to something lighter changes the appearance, which is not a step to take on an older roof without advice.

Shortcuts cost money here. A badly laid membrane can trap damp, and taking the roof off a second time is the largest part of any stone re-lay bill. Checkatrade's 2026 slate guide gives roughly £160 to £210 per m² for stone, so a second strip is no small matter.

If you do fit a membrane, the stone going back on must be sorted first. Our answer on how many old stone slates can be reused explains what to expect when a roof is stripped.

At survey we look at the loft, the rafters, any torching still in place and the eaves, and we photograph your roof. The written quote afterwards covers scaffold, the strip, stone, underlay if you choose it, battens, labour, waste and any repairs. Consent is explained to you, and we deal with notification to building control on notifiable work. Our workmanship is guaranteed for 10 years, in writing.

Ring 01453 933613, or reach us by WhatsApp or the enquiry form. The rest of our stone work is described on Cotswold stone roofing.

Before recommending torching or a membrane, we need to see what is already there. Is the old lime sound, or does it sit in lumps on the ceiling? Are the battens dry and firm, or soft and dark? Do the pegs hold, and does daylight show between slates? Has insulation been pushed so far down that it blocks the eaves?

The answers shape the advice. A roof whose torching is mostly intact may only need patching. A roof with failed lime, tired battens and damp timbers is a candidate for a full re-lay, and that is when the membrane question arises properly. Photographs of each finding go into our written note, so you can compare them with anything another roofer tells you and judge the advice for yourself. If the roof is sound, our advice is to leave it alone.