Roofing

Should a lead valley be repaired with mortar fillets or flexible sealant like CT1

Dom MacDonald 18/11/2025 - 7.51 AM

Hi all, I’d really appreciate some expert opinions on this because I’m getting conflicting advice. I’ve got two valleys with lead troughs. The original bedding mortar, which sits under the cut tiles, has been in place for years and is still doing its job. Recently a roofer ‘repointed’ the valley by running a fresh mortar fillet down the entire length, bedding it onto the face of the tiles and partially onto the lead . Within months it started cracking and falling off into the gutter. From what I understand: Bedding mortar under the tile lasts because it’s mechanically supported and protected from water flow. A new mortar fillet smeared onto the tile faces and lead has no key, no support, and is fully exposed to freeze/thaw and water run-off. BS5534 guidance now leans away from using mortar in exposed positions unless mechanically fixed. Many roofers seem to favour CT1 / polyurethane sealant to close small gaps rather than trying to recreate large mortar fillets. So my question is: Is running a new mortar fillet down a valley still considered acceptable practice, or should the correct modern repair be to remove loose material and use a small bead of CT1/Sika only where gaps actually exist? I’m trying to work out whether the failure is due to poor preparation, or whether this technique is simply outdated and unsuitable for a lead valley. Any professional insight appreciated.

Ivory Property Services

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Grays

The roofer probably just cemented over the old cement, If you remove the tiles and lay a bed of cement down and set the tiles into the cement then repoint them in they will last for another 10-15 years

Answered18 November 2025

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