Look at an L-shaped house or a roof with a back extension, and you will see a line where two slopes run down into each other. That line is the valley. It gathers the rain from both slopes, so it takes a heavier flow than any other stretch of the covering, and it is where leaves, moss and grit end up. A valley that is well built can go unnoticed for decades. A poor one is a regular source of ceiling stains.
There is more than one way to form a valley on a tiled roof. Some leave a lead or plastic channel open to view, some bury the channel under cut tiles, and some sweep the courses round in a curve. This guide describes each, how it is laid, what it tends to last and what usually goes wrong. For what repairs cost, see our answer on roof valley repair cost.
What a valley is and why it works so hard
Underneath the covering, a valley is built on a valley rafter that runs diagonally from the ridge to the eaves, with shorter jack rafters fixed to it from each slope. A board, or a pair of boards, lies along the rafter to give the lining a flat bed, and the underlay is dressed down into the channel. The covering stops either side, leaving a gap, or covers the channel completely, depending on the type.
The valley rafter carries the load of both slopes at a point where the roof changes direction, so it is a structural member as well as a drain. A rotted or undersized valley rafter shows up as a sag in the line of the valley, and it connects to the problems described in our guide to roof spread.
Why do valleys leak more than the rest of the roof?
Water arrives from two directions at once, and in heavy rain a channel can run nearly full. The lining is the only barrier, with no overlapping layers to fall back on, so a single split or lifted edge lets water straight onto the timber below. Debris adds to it, because a valley that cannot drain freely backs up and overflows the edge of the lining.
Open valleys lined with lead
The traditional open valley is a channel of lead sheet laid over the boards, with tiles or slates cut to a neat line on each side and a strip of lead left visible between them. It suits both plain tiles and interlocking tiles, and it is the form most roofers reach for when the roof has to last.
The thickness of the lead matters. Royston's brochure, read on 14 October 2026, lists Code 4 to Code 8 for pitched valley gutters, and Code 5 and above for tapered valley gutters, with heavier codes for larger roofs and longer runs. Code 5 is 2.24 mm thick and Code 6 is 2.65 mm. Lead is laid with welted or rolled joints in long runs, with the sheet allowed to move as the weather warms and cools.
Lasting power is the strength of lead. MyJobQuote gives over 80 years for lead flashing that is correctly installed, and Less quotes 50 to 100 years for lead on a flat roof. A valley is harsher than a flashing, yet lead remains the long-lived choice.
How does a lead valley fail?
Lead fails by fatigue, not by wearing out. Sheet pinned too rigidly, or run in over-long lengths, cracks along its folds as it expands and contracts. Short laps let water travel sideways under the sheet. Moss and lichen run-off can eat away along the line of the channel. Foot traffic makes dents that hold water, and a thin code can tear under a ladder. Leaves, once piled, keep the lead wet for weeks.
Preformed GRP and plastic valley troughs
A valley trough is a factory-made channel, normally in glass-reinforced plastic (GRP) or another plastic, with a raised centre rib and flat wings along both sides. It is laid on the boards, the tiles are cut and bedded to its edges, and a dry valley system uses clips and seals to hold the tiles down at the edge instead of mortar. Troughs go in fast, in long lengths with few joints, and they cost less than lead for the same run.
They suit many modern tiled roofs, especially interlocking concrete tiles, and they pair well with dry-fix ridge and hip systems. They are less welcome on historic work, where a visible plastic strip may not suit the building and the council may want lead.
What goes wrong with a trough?
Plastic and GRP age differently from lead. Sunlight makes a trough brittle over time, so the exposed edges crack or lift, and a boot or a ladder can split the central channel. Tiles cut too tight against the rib trap water and debris, while tiles cut too wide let the sun and the weather in. The nails or clips that hold the wings can work loose, and the join between two lengths is the weak point if the overlap is short or the sealant has failed.
The lifespan of a trough depends on the product, the thickness of the moulding and how much sun the roof gets. We read the manufacturer's data sheet for a given make, and the survey tells us how the trough on your roof is ageing. A written quote follows, and if the existing trough is sound, we leave it alone.
A cracked trough can sometimes be repaired, but a trough that has gone brittle along its length is better replaced than patched.
Closed valleys: mitred, laced and valley tiles
A closed valley hides the channel under the covering, so the roof shows an unbroken surface. There are three main ways of doing it.
What is a mitred valley?
The tiles from both slopes are cut at an angle and meet along the valley line in a tight mitre. Beneath them, soakers (small lead sheets) go in with each course and turn up to catch water running between the cut edges. The method is old and neat, but it relies on the soakers, which are hidden, and on mortar pointing along the join. When either fails, a ceiling stain is usually the first warning.
What is a laced valley?
Used with plain tiles, a laced valley curves the courses round from one slope to the other, interleaving them like the fingers of two hands. It needs specially shaped tiles, cut and fitted by a tiler who knows the method. Laced valleys shed water well and look fine, but they are slower to build and harder to repair.
Do valley tiles work?
Valley tiles are purpose-made curved or angled tiles that form the channel, usually in concrete and clay. They are set in mortar or fixed dry. They are cheap and quick to fit, but they have wide joints at the sides and mortar that perishes, and the channel is shallow, so heavy rain can overtop them.
Closed valleys have a smart appearance, and that is a good reason to choose one on a plain-tile roof, but the hidden parts are exactly what you cannot inspect from the ground.
Swept valleys on stone and slate
On Cotswold stone slate and some natural slate roofs, the traditional answer is the swept valley, where the courses curve round the corner with no visible lining. Cotswold District Council's technical guidance, dated April 2000, describes specially cut slates designed to form a gentle curve, with names such as bottomer, lie-by or side skew, and skew for the cut pieces. It describes close-mitred and open lead valleys as cheaper to construct, yet less satisfying to look at, and best avoided on stone.
Swept valleys need skill, a good supply of matching stone and a patient craftsman, which is why they are rare on new work outside the Cotswold edge. If your house is within reach of that country, say around Cirencester, a stone slate roof deserves a specialist's view, and we will be plain with you if the job needs one.
On a house being converted from tile to slate, the valley changes with the covering. Our page on changing from tile to slate looks at what that involves. Dormers add their own small valleys at the cheeks, with their own rules, covered in the dormer roofs guide. In a conservation area or on a listed house, the valley form is part of the character of the roof, so our guide to tiled roofs in conservation areas is worth reading before any change.
Where valleys fail and how to spot it
Almost every valley fault comes down to one of a few things. Debris blocks the channel and water spills over the lining. The lining cracks, splits or lifts. The covering beside it slips or the mortar bedding crumbles. The boards beneath rot after years of leaking, and the line sags. Or something has been done to the channel, such as a satellite bracket drilled through it.
- Brown stains on the ceiling below the line of the valley, often after heavy rain.
- Moss or green growth packed along the channel, or a visible heap of leaves.
- Tiles at the valley edge that have slipped or cracked, or mortar on the ground below.
- A visible crack, split or lifted edge in the lead or the plastic.
- A dip along the valley line, which points to rotten boards or a weakened valley rafter.
- Daylight at the valley when you look along it from the loft.
Can I clear a valley myself?
Clearing from a ladder at the eaves is within reach of many households, but climbing onto the roof is not worth the risk, and walking in a lead valley dents it. Our answer on a blocked roof valley explains what happens when one backs up. Can't see the valley from the ground? We can photograph it for you.
A damp patch below a valley is not always the valley itself. Gutters, flashings and tile cracks can produce the same mark, so a survey traces the water before anyone cuts anything open.
Choosing a valley for your roof
The right valley depends on the covering, the pitch, the age of the house and the budget. Lead is the safe choice where longevity matters or the roof is a heritage one. A preformed trough is a reasonable choice on a modern interlocking tile roof where speed and cost count. A closed form such as mitred, laced or valley tile suits a plain-tile roof where the owner wants the look, as long as someone checks it regularly. Stone and slate roofs follow their own traditions.
Should I replace the valley when I re-roof?
Almost always, yes, if the lining is original. Taking up the covering gives access to the boards and the valley rafter, and replacing a valley later means lifting the tiles beside it all over again. Fitting a new lining while the slope is already open adds little to the job. If the existing valley is sound while the rest of the roof is not, we leave it alone.
What drives the price is the length of the valley, the type of lining, the number of valleys, the amount of covering to be relaid, and access. Scaffolding is often needed, so when you compare quotes it is worth asking whether VAT and scaffolding are included.
The survey brings photographs of your own valley and a written quote, and a 10-year workmanship guarantee follows the work in writing. Whether the answer is a clear-out, a new lining or a new valley board, we put forward only the work the fault needs. Ring 01793 626096 or send the enquiry form, and see the pitched roofing hub for related guides.