Cracked Mortar Valley: What Are the Options?

A valley is the inside corner formed where two slopes come together. Rain from both runs down that line, so it works harder than any other detail on the roof. With a mortar-bedded finish, the tiles either side are cut to follow the line and set in mortar over a lining beneath, which may be lead, felt or a board. The mortar seals the cut edges and keeps the tiles from shifting.

Mortar is stiff, and a roof is not. Frost, summer heat, wind and the slow flex of rafters all open hairline cracks, and water gets into them and prises them wider. Once the mortar has cracked, pieces drop away, leaving the cut tile edges loose and the lining exposed. Look for crumbs of mortar in the gutter, a dark stain along the valley line, or a damp mark on the ceiling beneath it.

None of that means the whole roof has failed. It means the valley detail has reached the end of what mortar can do, and the options below are about how to replace that detail so it lasts. Our cracked roof tile repair answer covers single tiles elsewhere on the slope.

Where cracking is limited, the loose mortar can be raked out and the edges repointed. That is a repair, not a cure: new mortar will flex and split just as the old did, usually sooner than you would like. It makes sense where the tiles and lining below are sound, the valley is short and you want to buy a few seasons before a larger job.

It makes less sense where mortar is falling out along several metres, tiles beside the valley have slipped, or the ceiling underneath has stained. Then a roofer should lift the cut tiles and examine the lining before any mortar goes back. Pointing over a failed lining hides the fault and does not fix it.

Ask what the cut tiles will be fixed with. Marley's guidance stopped treating a mortar bed as enough by itself where ridge and hip fittings are concerned, and wants a mechanical fixing alongside. A valley edge deserves the same thought, so ask how the tiles will stay put if the mortar lets go, and whether the repair uses stainless nails, clips or both.

A dry valley swaps the mortar for a purpose-made trough, usually GRP or a similar plastic, that channels the water. The cut tile edges rest on or beside the trough and are held with clips or fixings, not bedded. With no mortar to crack, a dry valley copes better with movement, and it goes in more quickly. It has to suit your tile and your pitch, so ask to see the system's data sheet.

A lead valley is the other route, and MyJobQuote puts the life of lead flashing beyond 80 years, a figure that is only a typical range. The sheet is laid as a gutter-like lining, with the tiles trimmed back to leave an open channel on each side. Royston's lead brochure lists Codes 4 to 8 for a pitched valley gutter, and the thickness chosen should suit the length of run and the exposure. Our lead work page explains lead codes plainly.

Lead costs more than a trough in materials, and it needs a roofer who can dress and fold it neatly. Either option is a longer-lasting answer than patching mortar again, and this kind of repair is part of our pitched roofing work.

What you pick should follow what the roofer sees once the valley is opened up. If the lining underneath is sound and the trouble is purely tired pointing, a repoint may be enough. If the lining is torn or tiles beside it have slipped, rebuilding in a dry trough or lead is the safer course. Where your tile is awkward to cut cleanly, a lead valley with its open channel may suit better than a trough.

Access matters too. A valley is rarely at the eaves, so reaching it safely often calls for a tower or scaffold, which can move the price more than the materials do. Before comparing quotes, find out which of them include VAT and the scaffolding. A firm figure only comes after a survey has shown how long the valley is and what sits beneath the mortar.

If you are chasing a leak, our answer on why a roof leaks in wind-driven rain covers other places to look, and torn roof underlay explains what may be hiding under the valley.