Roof Valleys: Why Moss Collects and How It Is Cleared

Types of valley

Some valleys are lined with lead or another metal sheet, with tiles cut to sit either side. Others use preformed plastic or fibreglass trays. Older roofs may have tiled valleys, where specially shaped tiles form the channel. Each type collects debris in a slightly different way.

Why moss builds up

Water moves slowly at the bottom of a valley, especially on shallow pitches, so silt settles there. Cut tile edges either side catch leaves. Once a layer of grit forms, moss roots into it, and the growth then traps more debris. Valleys facing north or shaded by trees can fill in surprisingly quickly.

The risk of a blocked valley

When a valley is choked, water spreads sideways under the tiles instead of running down the channel. Heavy rain is when this shows, often as a damp patch on a bedroom ceiling near an inside corner of the roof. Debris can also overflow where the valley meets the gutter, making that corner spill.

How valleys are cleared

Valleys are cleared by hand, lifting moss and silt out of the channel and from the cut tile edges. We take care not to scrape or puncture lead or trays. Remaining growth on the adjacent tiles is treated at low pressure where suitable, and the gutter at the foot of the valley is cleared at the end.

Pressure washing a valley is particularly risky because it can drive water under the cut tiles and lead.

What we look for while there

A clean valley shows split or thinning lead, cracked mortar bedding on tiled valleys, slipped cut tiles and gaps where the lining has moved. We report anything we see so you can decide what to do.

Keeping valleys clear

Trimming overhanging branches, keeping the roof treated to slow moss and having gutters cleared regularly all reduce how much ends up in a valley. A look from an upstairs window after autumn is a simple check.

Valleys on dormers and extensions

Valleys are not only found on large hipped roofs. Wherever a dormer, porch or extension roof meets the main roof, a small valley or gutter is formed. These little channels are often steeper and narrower, so leaves wedge quickly. They are also hard to see from the ground, which is why they are often found blocked only when a ceiling stain appears.

Snow and ice in valleys

Snow lingers longest in valleys because they are shaded by the slopes either side. As it thaws, water runs into a channel that may still be partly blocked with ice or debris, and backs up under the tiles. A clear valley copes far better with a thaw than one already half full of moss and silt.

Valleys and gutter corners

Where a valley empties into the gutter, a large volume of water arrives at one point. If the gutter there is part blocked, it overflows at the corner. Clearing the valley and the gutter corner together solves both. See overflowing corners for that side of the problem.

Do not climb into valleys yourself

Valleys are at height, often steep and slippery with moss. They are not a safe place for DIY clearing. A view from an upstairs window or binoculars is enough to see whether one is filling up.

Timing

After autumn leaf fall is the best time to check valleys.

Book a valley clear

Valley clearing is usually part of roof cleaning. Get in touch with a photo for a free quote across Hampshire, Surrey and Sussex.