Can a Blocked Roof Valley Cause a Leak?

What a roof valley does

Where two roof slopes meet at an internal angle, the valley is the channel that takes the combined run-off and carries it down to the gutter. On a house with a rear return, a dormer or an extension, a single valley can be handling rainwater from a large area of roof.

Because of that concentration, valleys are one of the details most likely to cause a leak when something goes wrong — and one of the least likely to be noticed from the ground.

How blockages build up

Valleys are a natural collection point. Anything that lands on either slope tends to wash into the channel, and once a small amount lodges, the rest builds up behind it.

Typical causes

• Leaves and twigs, particularly where there are trees close to the house
• Moss washing down off the slates or tiles
• Grit and granules shed by older coverings
• Silt building up at a shallow point in the channel
• Birds' nests, or debris carried in by them
• Mortar and rubble left behind after earlier work on the roof

Signs of a valley problem

Most of these can be seen from the ground or from a window, which is where any looking you do should stop.

What to look for

• Visible debris, moss or plant growth sitting in the valley
• Water running or spilling over the edges of the valley during rain
• Damp appearing internally along the line where two roof sections meet
• Staining on a ceiling or wall below a valley, particularly after heavy rain
• Slates or tiles along the valley edge that look uneven, cracked or displaced
• Gutter below the valley overflowing at one spot

Why clearing alone may not be enough

By the time a blockage is noticed, water has often been backing up in that valley for several seasons. Standing water works at the lining, at the laps in it, and at the cut edges of the slates or tiles beside it.

So a valley that has been clogged may also have a corroded or split lining, open laps, loose or broken cut slates, or perished mortar along the edges. Clearing the debris removes the immediate cause; it does not put right damage that has already happened. Our roof valley repairs page covers what that work involves.

Repair or renewal of the valley

Where the lining is sound and only the edges need attention, a localised repair is usually the right answer. Where the lining itself has failed along its length, renewing the valley — and the coverings along it — is generally better value than repeatedly patching it.

Valleys come in several forms and materials, and we identify what is actually there before recommending anything.