Chimneys and parapets
The only weathering between thatch and the chimney is a mortar fillet. Why is that fillet not a lead flashing, and what leak has it not timed?
A mortar fillet is not a lead flashing because it is mortar smeared at the junction, not metal dressed into the chimney and over the thatch. Lead flashing is cut, stepped or chased into the stack so water is turned onto the covering. A fillet simply fills the angle. It cracks as the thatch moves and as the mortar weathers, and then the junction is open. The survey has not timed a leak. It recorded that the only weathering seen was the fillet. It did not water-test it and did not date any staining.
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?
Where thatch meets a chimney the junction has to move a little and still shed water. Mortar is rigid, so it fissures, and a fissure at that point lets rain run down the stack and into the coat or the roof space. That is why the detail is described as a fillet rather than as flashing. Seeing mortar tells you the metal detail was not there to be seen. It does not tell you when water last got in, or whether the bedroom ceiling has ever stained from this junction.
A thatcher, often with a chimney contractor, can say how this junction should be weathered, which may involve a proper flashing rather than another smear of mortar. Tell the solicitor the report identifies the detail, not a dated leak. The survey has not specified the new metal and has not said the stack must be rebuilt. Leave the fillet as found. Do not rake it out on a viewing, because opening the joint before a thatcher is ready only makes a wider path for rain.
What should you ask your surveyor?
- Was the only weathering you saw a mortar fillet, with no lead?
- Did you see staining on the thatch or in the roof space at that stack?
- Have you left the leak untimed because the joint was not water-tested?
What are the typical next steps?
- Call the detail a mortar fillet, not a lead flashing.
- Ask a thatcher how the chimney junction should be weathered.
- Do not treat the note as a leak with a known start date.
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
If the fillet looks freshly pointed, is it now a flashing?
Fresh mortar is still mortar. It is not lead dressed into the stack. The survey has not renamed a new fillet as flashing, and it still has not timed a leak at that junction. No later decision about the fillet was made, and this visit did not supply one.
Does moss on the thatch beside the stack prove the fillet leaks?
Moss shows a damp surface, which on thatch can have several causes. The survey has not used moss to time a leak from the fillet, and it has not water-tested the joint. What happens next for the mossy is outside this note and was not settled here.