Roofs and flat roofs
Mortar torching has dropped onto the loft insulation under a slate roof. Does that debris mean the slates have failed, or that old bedding is loose?
Not that the slates have failed. Mortar torching dropped onto the loft insulation means old bedding is loose, which is a different sentence from a failed slate covering. Torching is the mortar that used to be buttered under the slates from the loft side. Lumps on the insulation show that bedding has come away. They do not, by themselves, show that the slates are cracked, slipped or letting rain in. The survey records the debris it could see. Ask a roofer to look at the slates if you need the covering judged. Do not read fallen mortar as a stripped roof.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
Under many older slate roofs the joints were torched with mortar from inside. As it ages, that mortar lets go and falls onto the insulation. The debris is evidence of loose bedding. A failed covering would be slates that are missing, broken or admitting water, which is a separate observation if the surveyor made it. Fallen torching can leave gaps at the laps, so it is worth understanding, but the lumps themselves are not the slates.
Ask a roofer to inspect the slate covering from outside and to say whether the lost torching needs any action, without assuming the roof must be stripped. Do not bag the mortar up and call the defect gone, and do not scrape remaining torching off the slates from the loft. If the survey also recorded damp stains, keep that note beside the debris rather than treating them as the same sentence. Loose bedding is what the insulation is holding. The slates need their own look.
What should you ask your surveyor?
- How widespread was the fallen torching, and on which slope?
- Did you see daylight, slipped slates or stains, separate from the mortar debris?
- Is any torching still attached to the slates you could see from the loft?
What are the typical next steps?
- Read the debris as loose old bedding, not as failed slates.
- Ask a roofer to inspect the slate covering in its own right.
- Do not scrape remaining torching off from the loft.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Does fallen torching mean the roof must be stripped?
No. Loose bedding on the insulation is not an instruction to strip the slates. A roofer should look at the covering before anyone decides the work.
If you vacuum the mortar, has the roof been repaired?
No. Clearing debris tidies the loft. It does not refix torching or prove the slates. The survey point was the fallen bedding.