Is Moss Lifting Your Roof Tiles?

It can, and it’s one of the few ways moss does real damage rather than just looking bad.

The short answer

A thin green film on a roof is mostly a cosmetic problem. Thick, cushion-like moss is different. It grows in the joints where one tile overlaps the next, holds water like a sponge and, over a few winters, can wedge tiles apart so they stop sitting flat. Once a tile is lifted, wind-driven rain can get under it. Whether that becomes a leak depends on the state of the felt underneath.

How moss lifts tiles

• It grows in the laps. Moss roots into the grit and dirt that collects where tiles overlap. As the clump thickens it pushes the lower edge of the tile above upwards.
• It holds water. A roof is meant to shed water quickly. Moss keeps the surface and the joints wet for days after rain.
• Frost does the rest. Water held in the moss and in the surface of the tile freezes and expands. Over repeated winters that can flake tile edges and open the gaps further.
• It blocks the drainage channels. Interlocking concrete tiles have side channels that carry water down the roof. Packed with moss, they overflow sideways under the next tile.

How that turns into a leak

Tiles are the first layer and the roofing felt underneath is the second. On a newer roof, a bit of water getting past a lifted tile runs down the felt and into the gutter, and you never know about it. On an older roof the felt is often brittle or torn, especially at the eaves. Water that gets under the tiles then drips onto the insulation or runs down a rafter, and shows up as a stain on an upstairs ceiling.

Moss also causes leaks indirectly by blocking gutters and valleys. See can blocked gutters cause a roof leak?

Signs to look for

From the ground, with binoculars if you have them:

• Tiles that sit proud of the ones around them, or a row with an uneven bottom edge.
• Thick moss cushions sitting in the joints between tiles, not only on the surface.
• Moss packed into valleys or around the chimney and ridge.
• Clumps of moss on the path, patio or in the gutters after wind or frost.

From inside the loft, on a bright day and again after heavy rain:

• Daylight showing where it shouldn’t.
• Dark stains or tide marks on the felt or the rafters.
• Damp or flattened insulation.

What to do about it

• Don’t pull it out from a ladder. Tiles crack underfoot and falls from height cause serious injuries. The HSE has plain guidance on working at height. See also should you scrape moss off your roof yourself?
• Don’t let anyone jet wash it. High pressure forces water under tiles that are already lifted. See soft wash vs jet wash.
• Have the moss removed and the roof treated. The moss is taken off by hand from poles or safe access, then a biocide kills what’s left in the joints so it doesn’t grow straight back. See moss removal.
• Get any damage looked at. Cracked or slipped tiles, loose ridge tiles and torn felt are a roofer’s job, not a cleaner’s. A good specialist will point them out if they see them.

When it isn’t the moss

Plenty of mossy roofs never leak, and plenty of leaks have nothing to do with moss. If water is coming in, a roofer should check the flashing around chimneys, the valleys, the ridge and the felt before anyone assumes a clean will fix it. Cleaning removes a cause of future damage. It doesn’t repair a roof that is already letting water in.

What to do next

Hampshire Roof Care is a matching service: tell us about the roof and our vetted local specialist partner quotes it directly. WhatsApp 023 8129 0232 or send an enquiry with a photo or two. Roof type guides: concrete tile · clay tile · slate. Local pages: Southampton · Portsmouth · Winchester · Eastleigh · Romsey · Petersfield · Alton · Emsworth · Waterlooville · Havant.

Quick answers

Can moss lift roof tiles?

Yes. Thick moss growing in the joints between tiles can wedge them apart and lift the lower edge, so the tiles no longer sit flat on the row below. Wind-driven rain can then get underneath.

Can moss on a roof cause a leak?

It can. Moss holds water against the tiles, slows the roof from draining and can lift tiles or block the channels in interlocking tiles. Water then reaches the felt, and if the felt is old or torn it gets into the loft.

Should I pull the moss out from under lifted tiles myself?

Not from a ladder or by walking on the roof. Tiles crack easily underfoot and falls from height cause serious injuries. Check from the ground and from inside the loft, then get a specialist to quote.