What happens when a roof valley gets blocked?

Picture a back extension tucked against the main house: the inside crease between its roof and the old one is the valley. Rain from both sides runs into one lined channel, which might be lead, a preformed GRP trough (glass-reinforced plastic) or a line of special valley tiles. The channel is narrow yet has to cope with far more water than an equal strip anywhere else.

That is why it clogs. Leaves, twigs, moss, silt washed off old tiles and bird nest material all drift down the slope and settle in the bottom. Once a plug forms, water slows, stands and rises up the side of the channel. Valleys under trees, below flat roofs that shed debris, and beneath overflowing gutters block first.

The first sign indoors is usually a brown ring on a ceiling beneath where the valley runs, often following a spell of heavy rain. In the loft you may see dark marks on the rafters, wet insulation or drips on the underside of the felt. Outside, from the garden, look for green moss or a mat of leaves along the bottom of the valley, or water sheeting over the edge of the lead during a downpour.

Another clue is a sound. A blocked valley sometimes gurgles or trickles at odd points in heavy rain because the water is finding a route. If a leak only appears when it rains hard, not in light drizzle, a blockage in a valley is a likely cause. Our page on finding a roof leak helps you work through other causes systematically.

A valley is a hard place to work. Slates and tiles are fragile and slippery when wet, a lead lining is soft and easily dented, and the spot is often several metres up on a steep pitch. This is the one job on the roof where the debris is exactly where a person does not want to step.

A roofer clears by hand with a trowel and soft brush, working from a ladder, tower or scaffold platform with the right edge protection. The debris goes into a bag, not down the gutter, and the lining is photographed as it appears. Pressure washing is rarely a good idea because it drives water under the tiles beside the valley and can strip lead or lift mortar, and hosing from the ground reaches nothing useful. A garden hose from the ground has no real effect, and reaching in from a ladder with a long tool is how people fall.

Clearing the channel is only half the job. Once the lining is visible, we look for lead that has cracked at the laps, a GRP trough that has lifted or split along an edge, and tiles along both sides that have slipped or lost their mortar bedding. Water that has been standing for a long time often has seeped into the felt and battens under the valley, so we check timber from the loft for softness.

If the lining is sound, emptying the channel and refixing a few tiles is the right answer. If it has failed, relining is the next step, and the cost depends largely on access. MyBuilder's guide, updated in August 2026, puts a minor roof repair at £100 to £300, a moderate one at £500 to £1,000 and a major one at £2,500 or more, with the valley job landing in one of those bands depending on what we find. Our page on tracing a roof leak explains why clearing alone sometimes does not cure the problem.

The best prevention is routine. Checking the valleys when the leaves have dropped, and again once spring arrives, catches most blockages before they cause damage. Trim branches that hang over the roof. Keep gutters at the foot of valleys clear, as a full gutter lets debris back up the slope. A moss treatment applied by a roofer can slow regrowth, though moss that has built up thickly needs removing by hand first.

Roofs with complicated shapes, such as hipped bungalows with multiple valleys and Victorian villas with a rear wing, deserve a check more often than a simple gable. A valley that overflows once usually will again, so a hidden problem such as a lining that sits too low or laps that are too short deserves attention rather than a repeat clear-out.

If you can see a ceiling stain, or if your gutter is overflowing at a valley, call us. We prioritise active leaks, and the first job is to stop water entering. A written quote then follows the survey, with photographs showing the valley as we found it.

If clearing is enough, clearing is all we price. Where the lining has failed, we explain if a patch will hold or if relining is the better spend. Our page on fixing a leak from inside the loft shows what can be done temporarily from below while you wait for a repair. Neighbouring detail such as parapet gutters and abutments block and fail in similar ways. Our 10-year workmanship guarantee is given in writing, and valley work belongs under roof repairs.