Roofs and flat roofs
Sarking felt sags between the rafters and holds a dark line of dust. Is that sag, by itself, a roof that needs stripping?
No. Sarking felt that sags between the rafters and holds a dark line of dust is not, by that sag alone, a roof that needs stripping. The survey can record tired underfelt and a dust mark in the belly of the sag. A sag is the felt stretching between the rafters. A dust line can be dirt, old moisture tracking, or both. Neither sight is a decision to strip the covering. The tiles or slates above were not proved failed by the drape of the felt. Ask a roofer if you need the covering and the underlay judged. Do not order a strip from the sag.
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?
Sarking felt is the underlay beneath the battens. It often droops as it ages, and dust collects in the low point. That is what a dark line in a sag commonly is. It can also mark where moisture once ran. The survey can describe both without concluding that the roof covering has failed or that the felt must all come off. Stripping is a major job decided from the covering, the battens and the underlay together, not from one drape seen in the loft.
If you are unsure, ask a roofer to look at the felt and at the covering above that bay, and to say whether the sag is only age or whether water is getting past the laps. Do not pull the felt down to see the battens on a viewing. Do not read a dust line as a measured leak. Keep the survey sentence as underlay that sags and holds dust. A later opinion can recommend work. The drape itself has not already specified a strip.
What should you ask your surveyor?
- How deep was the sag, and was the dust line damp or only dirty?
- Any daylight, stain on the rafters, or drip, separate from the dust?
- Which bay of the roof was this, and was the covering seen from outside?
What are the typical next steps?
- Do not treat the sagging felt as an order to strip the roof.
- Ask a roofer to judge the underlay and the covering together.
- Do not pull the felt down on a viewing.
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 a dark line in the felt prove an active leak?
No. Dust collects in a sag, and a dark line is not by itself a live leak. The survey did not say the roof must be stripped.
If the felt is original, must it be replaced before you buy?
No. Age of the underlay is not an order to strip the roof. Ask a roofer what this sag actually needs, if anything at all.