Valleys carry water from two slopes into one channel, which makes them the hardest working detail on a pitched roof. Here is how they fail and how they are relined.
What a valley has to cope with
A valley collects the run off from two roof slopes and channels it into a single line, so the volume of water passing through a valley in a downpour is many times what any other part of the roof handles.
It is also where leaves, moss and grit collect. A partially blocked valley backs water up under the tiles on either side, which is why valley leaks so often appear as damp on an upstairs ceiling well away from the valley itself.
Open lead valleys are traditional and, correctly laid in short bays over valley boards with the tiles cut cleanly to the margin, will last 60 years or more. GRP preformed valley troughs are common on newer housing and are quick and effective. Tiled or swept valleys, where the covering is closed across the valley, look beautiful on clay and stone roofs and demand real skill.
Cement fillets are not a valley type, they are a repair that has already failed somewhere else. They crack, and once cracked they direct water into the joint rather than away from it.
Lead splits when the bays are too long, exactly as it does on a chimney. Valleys with an inadequate margin, meaning the tiles are cut too close to the centre line, overtop in heavy rain. Debris blocks the channel, and rotten valley boards let the lining sag so water sits in the dip.
Repeated patching is the other cause. Each patch creates an edge for water to catch on, and a valley with three patches in it is nearly always cheaper to reline than to patch a fourth time.
Tiles or slates are lifted back along both sides, the old lining and valley boards removed, and any rotten timber replaced. New boards are fitted, an underlay upstand is dressed in, and the new lead is laid in bays with proper laps and clips, or a GRP trough is bedded in.
The covering is then reset with a consistent margin of typically 100 to 125mm each side, and any tile cut to a feather edge is replaced with a properly supported cut piece rather than a sliver bedded on mortar.
Clearing and inspecting a valley is usually part of general maintenance. A patch repair to a small split runs £250 to £500. Relining a typical domestic valley of 4 to 6 metres is more often £900 to £2,000 depending on the lining material, timber condition and access.
Twice yearly clearance, particularly after autumn, is the single most effective thing you can do to keep a valley working. It costs very little and it prevents the failure mode that causes most valley leaks.
How do I know the valley is the source of a leak?
Damp appearing after prolonged or heavy rain rather than a short shower, and staining that sits on the ceiling line running back from the valley, are both typical. A loft inspection above the valley usually confirms it.
Can a valley be repaired without lifting tiles?
Very rarely. The lining runs under the tiles on both sides, so anything more than clearing debris means opening up the margins.
Is GRP as good as lead in a valley?
For a modern tiled roof a preformed GRP trough performs well and costs less. On a period property or a complex junction, lead is more adaptable and more appropriate.
Why does my valley overflow in heavy rain?
Usually a margin that is too narrow, a blockage, or a valley taking more water than its size allows. Widening the margin during relining often solves it permanently.
How often should valleys be cleared?
Twice a year on a roof with trees nearby, once a year otherwise. It is worth doing at the same time as the gutters.
Leak that only shows in heavy rain?
That pattern often points at a valley. Let us clear it, inspect it and tell you whether it needs relining or just maintaining.
Evans Roofing & Maintenance Ltd, Witney, Oxfordshire. Call 07960 482233 for a free written quote.