Roof valley repairs: lead, GRP and tiled valleys

Think of a valley as a small gutter set into the slope. Water arrives from two directions, often from a long run of roof above it, and picks up speed as it runs down. Where a chimney, dormer or bay leans on the same corner, the flow doubles again. Every part of the system must be sound: the lining itself, the laps between its lengths, the cut edges of the tiles on both sides and the line at the bottom where the water drops into the gutter.

The channel also catches whatever falls on the roof. Leaves, moss that has worked loose, bird droppings, grit from worn tiles and the seeds of plants that take hold in the damp all gather there. Debris slows the water, which backs up and finds its way sideways under the tiles. A choked valley can explain a leak that shows itself only during a downpour.

Movement and weather

A valley lining sits between two slopes that move slightly with temperature and wind. Any rigid material along that join is under strain. Sun, frost and sea weather work on the edges first, and the weakest link opens up. Near the coast, gales and driving rain test valleys harder than a sheltered street would.

The valley is not the only suspect

Not every damp corner is a valley fault. Flashings against a wall, a cracked chimney, a failed ridge end or a blocked gutter can each produce the same stain. A roofer checks all of these before putting the blame on the valley. Our guide to how roofers find a roof leak sets out the order of that search, and it is a useful read before you ring anyone.

Some signs are visible from the ground with binoculars, and some only from the loft. None of them needs you to climb.

• Stains and drips at an inside corner of a ceiling, or on the wall beside it, that grow in heavy rain.
• Damp timber in the loft along the line of the valley, with dark tide marks on the rafters beside it.
• Moss or plants growing along the valley line, which means debris and moisture have built up.
• Dark, shiny or split lead where you can see it, or a lining that has sagged away from the tiles.
• Cracked, chipped or missing tiles along the cut edge where they meet the valley.
• A gutter at the foot of the valley that is overflowing, blocked or stained down the wall below.

Binoculars help, as does a phone camera held up at arm's length from a safe position. A photograph taken with the zoom on a bright day often shows a split in a lining that the eye misses.

What the loft tells you

From inside, a valley is a diagonal timber, often a valley rafter, with the rafters on either side leaning into it. Torch the timbers below the valley line. A damp, blackened valley board or underlay stained along its length suggests the lining is leaking, not just the tiles beside it. The underlay under a valley can be the secret witness: if it is torn or wet, the water has been getting through for some time.

Photographs taken before any repair let a roofer see what you saw. Our roof repair survey visit page describes what we record when we come to look.

A lead valley is a trough of sheet metal laid over boarding or a lower layer, with the tiles or slates trimmed back to leave an open strip. Lead has been used for centuries because it moulds to the roof and lasts. MyJobQuote's cost guide quotes over 80 years for lead flashing, so a well-laid valley can outlast the tiles beside it.

When lead valleys fail, the cause tends to be one of a few things. The sheet may have split along a fold or at a lap. It may be too light a grade for the run it serves. Debris may have trapped moisture against it. A previous repair may have been done with a sealant or a patch that has since come away. Sheet lead comes in numbered codes, rising with thickness. Royston Lead's brochure reserves code 3 for soakers, starts flashings and pitched valley gutters at code 4, and lists codes 5 to 8 for the heavier work, including tapered valley and box gutters from code 5 upward.

Repair or replace

A small split in sound lead can sometimes be repaired by a lead worker, but patching a valley that is thin, cracked in several places or stained along its length is rarely worth the money. The fix usually means lifting the tiles at both sides, stripping the old lining, checking the boards and felt beneath, and laying new lead in lengths with proper laps. Our guide to lead valley replacement covers that job, and the note on the lead code for valleys explains how thickness is chosen.

Beware of sealant. A bead of mastic over a crack looks like a repair and fails in a season or two, while hiding the true condition of the lining.

GRP stands for glass-reinforced plastic, commonly called fibreglass. Laid wet over a prepared base and left to cure, it forms a single sheet with no joints along the length of the valley. That is the attraction, and the reason it gets chosen for valleys that are awkward to line in lead or where the pitch changes.

Less (March 2026 update) gives GRP 25 to 40 years on a flat roof, and a valley lining is a smaller and more sheltered version of the same thing. GRP rarely fails in the middle of the run. It fails at its edges and ends, where it meets the tiles, a wall or the gutter, because those are the points where movement tries to tear it from its base.

Typical faults

A crack at a change in direction. A lifted edge where the resin has lost its grip on dirty or damp boards. A pinhole where the layers did not wet out fully. Surface crazing that has opened into the glass fibre. Each of these can be repaired by cleaning, abrading and laying a new patch of matting and resin, provided the board underneath is sound and dry and the weather allows the resin to cure.

When a patch is not enough

If the board beneath has gone soft, or the liner has lost its bond across a large area, patching will not hold. The valley then needs stripping back to the boards and relining. The tile edges on both sides often need re-bedding at the same time, because they were cut to the old lining and carry the stress. GRP also shows up where lead was a better choice, such as steep valleys that heat up in summer sun. A roofer can advise which suits a given roof, and sometimes the answer is to switch material at the same time as the repair.

Many houses have valleys made of purpose-made valley tiles, which are curved or angled units laid end to end in the channel, with the roof tiles cut to meet them. Some older roofs have bedded valleys, where the roof tiles curve round the corner and the joint is mortared. Both depend on a hidden layer below the visible surface, and both can fail where that layer is damaged.

Valley tiles crack from frost, from a branch, or from someone walking on them. A cracked trough tile lets water pass to the felt underneath, which may hold up for a time but is not designed to carry a constant flow. Mortar fixings and pointing along the cut tiles can dry out, crumble or open and let water behind. Moss thrives in these damp junctions and its roots lift pointing and tile edges further.

What a repair involves

Repair means lifting the tiles beside the damaged section, replacing cracked valley tiles, checking the underlay and boards, then relaying the cut tiles and fixing them mechanically. Rebedding in mortar alone gives a short-lived result. BS 5534 stopped treating a mortar bed as sufficient for ridge and hip tiles in its 2014 revision, and the same reasoning favours clips and nails where tiles meet a valley.

Cutting tiles takes care. A badly cut tile leaves a gap along the valley line or sits too near the channel and traps debris. Matching old tiles is also a skill, particularly on clay roofs where each tile has a slight colour variation. Our note on swept valleys on stone roofs shows how that traditional form is built.

There is no national price for a valley repair, because the work ranges from reseating a few tiles to relining an entire channel. The fair way to think about it is through the pieces. Roofers charge around £35 to £45 an hour or £280 to £360 a day, according to Checkatrade's cost guide, whose repair tables exclude VAT at the standard 20 per cent. Lead flashing repairs are given at £50 to £70 per metre in the same guide, and that is a useful guide to how lead work is priced by length. The materials depend on the lining you have and the code of lead.

Access is often the biggest variable. A valley may sit at the back of a hipped roof, over a rear extension or above a flat roof, so it cannot be reached from a simple ladder. Checkatrade's October 2026 scaffolding table has £1,200 a month for a detached house and around £800 for a terrace, and scaffolding that stands over a public pavement needs a licence from the county council. In West Sussex that means at least 10 working days' notice, so a planned valley repair has a lead time.

What the written quote should say

Ask whether the quote covers the lining, the tiles on both sides, the boards or felt if needed, access and clearing away, and whether VAT is included. The price you receive from us comes after the survey, with photographs of your own valley, and is one total. The verdict comes first and says plainly whether the valley can be mended, held for now or needs relining, with the reasons.

For a wider view of what moves a bill, the roof repair cost guide covers it, and our roof repairs page describes the work as a whole.