The short answer
The valley, the inward-angled join between two roof slopes, takes the rain from both. When moss, leaves and grit fill it, rain slows, backs up and spills over the edge or sideways under the tiles. That can soak the timber below and overload the gutter at the bottom, so a clogged valley deserves attention before the rest of the roof.
The valley is the roof's drain. When moss clogs it, the water has to go somewhere, and that somewhere is rarely where you want it.
What a valley does, and why it clogs
Rain from two slopes that fall towards each other runs down the line where they meet. That line is the valley, and it is lined with lead, another metal or a purpose-made tile so the water has a smooth path to the gutter. Because it gathers from two slopes at once, it works harder than any patch of plain tiling.
Why does moss settle in valleys?
Valleys are damp and often shaded by the roof around them. Leaves, seeds and grit wash down into them and stay. Moss takes hold in that debris, then the moss catches more debris, and the channel narrows. A valley that looked clear in spring can be half full by autumn on a house with trees nearby.
Roof valleys are also hard to see from the ground, since you are looking up at the join from below. Growth can build for years without anyone noticing, until a brown ring spreads across an upstairs ceiling or water overflows the gutter in a heavy shower. Our page on roof cleaning and moss removal covers how growth is cleared from the whole roof.
How a blocked valley causes damage
Water in a clear valley moves quickly and leaves. Water in a blocked one slows, rises and looks for the easiest way out. That is often over the edge of the valley and under the tiles beside it.
Where does the water go when a valley overflows?
It can run sideways under the edge tiles, soaking the battens (small timber rails fixed across the rafters to take the tiles) and the underlay sheet below. In time that leads to rot and to damp patches in the ceiling beneath. It can also spill over the side of the valley and run down the face of the roof in a sheet, instead of gathering in the gutter, so it lands against the wall or soaks the ground at the foot of it.
Moss itself adds weight and holds moisture against the metal lining, which can hasten wear on the valley material. Lead, for example, is long-lived, and MyJobQuote says it can last for over 80 years, but a valley that stays wet and loaded with debris is under more strain than one that drains freely. A survey will show whether the lining is sound, which a quick clear-out cannot tell you.
The gutter at the bottom takes the strain
At the foot of every valley the water has to leave the roof, and it does so through a gutter, a downpipe or both. Debris from the valley travels with the water and lands there, so a blocked valley and a blocked gutter often go hand in hand.
Can moss from the valley block my gutters?
Yes, easily. When moss is cleared or loosens naturally, it slides down the valley in clumps and drops into the gutter below. A gutter that is already half full cannot take it, and the water overflows at the corner where the valley discharges. Our answer on moss in gutters explains how clumps end up there and what they do to the outlets.
Overflow at that corner often shows up as a green streak down the wall, a damp patch on the brickwork or a run of mould under the eaves. Those marks point to the source. If you see one below a valley, the roof deserves a closer look before anyone cleans the gutter alone, because clearing the gutter without clearing the valley simply lets it fill again at the next rain.
Clearing a valley safely
Valleys are among the riskiest places to work. They are narrow, damp and often reached from steep slopes on either side. Metal linings are slippery when wet, and walking in a valley can dent or crack lead.
Should I clear a roof valley myself?
We would leave it. A fall from height is a real risk, and so is damage to a lining that is otherwise sound. Professional-only biocides are off limits to householders under HSE guidance, so spraying a valley with a strong product is no answer either. From the ground you can trim overhanging branches and keep gutters clear, which reduces what reaches the valley.
When we survey, the valley is one of the first parts we look at, and we show you photos of its condition. If it needs only clearing, that is priced as part of the clean. If the lining is worn, the repair is priced with it, in one written total. Send a photo of any mark below a valley and the survey will cover the join and the gutter beneath it.
More on valley moss
What are the signs of a clogged valley on my house?
Use binoculars on a dry day and look for green growth, leaf litter or standing debris in the join between two slopes. From inside, watch for ceiling stains below a valley. Outside, look for overflow marks beneath the gutter. A blocked valley also sounds different in rain, as water splashes instead of running.
Will clearing the valley fix a leak in the ceiling?
It might, if overflow was soaking the timber beside it, but a leak can have other sources such as a cracked tile or worn flashing. Clearing removes one cause and lets you see the lining. If the stain persists after a dry spell, a survey will trace where the water is getting in.
The full picture: Roof cleaning and moss removal in Taunton ยท More short answers