What Moss and Lichen Do to a Tiled Roof

A tiled roof can look perfectly respectable from the front of the house and be quietly going green down the back. Moss, lichen and algae rarely arrive all at once. They creep in along one edge, usually the slope that never gets much sun, and spread out from there.

After more than 30 years in the trade we see moss on roof tiles Melbourne wide, from Cranbourne out to Frankston, and it almost always begins in the same place. The damp side goes first, then the growth works its way across the roof.

Here is what those green and grey patches actually are, why the shaded slope is always the worst of it, what the growth does to your tiles and mortar, and how it comes off without creating a bigger problem.

What moss, lichen and algae actually are

Three different things get lumped together under the word moss, and they behave differently on a roof.

Algae is the flat green or black staining that follows the water. It has no roots. It is a film that sits on the surface of the tile wherever moisture lingers, and it is often the first thing you notice from the ground.

Lichen is the tough one. It is a partnership between a fungus and an algae, and it grows as crusty grey, green or orange discs. Lichen does not sit on the surface of a tile. It bonds to it.

Moss is the soft green cushion. It holds water like a sponge, which is the real trouble with it, and it needs somewhere to anchor. On a roof that means the gaps between tiles, the laps, the valleys and any surface that has worn rough.

All three want the same conditions: moisture that hangs about, shade, and something to grip. An older tile that has lost its surface coating hands them all three at once.

Why the shaded side of the roof is always worst

Stand on the footpath and look at the southern slope of almost any tiled roof around Melbourne’s south east. That side sees the least direct sun through the cooler months, so after rain or a heavy dew it stays damp for hours longer than the northern slope.

Overhanging trees compound it twice over. They cast shade, and they drop leaf litter into the valleys and behind the ridge lines. That litter holds moisture and breaks down into a thin layer of organic matter, which is exactly what a spore needs to take hold.

The other quiet contributor is airflow. A roof tucked behind a high fence line, close to neighbouring walls or under a tree canopy simply does not dry out in the breeze the way an exposed roof does.

You will often see a clean roof beside a green one in the same street. It is rarely the tiles that differ. It is the sun, the trees and the wind.

We see the same pattern on every street we work, whether that is a weatherboard in Cranbourne or one of the brick homes we visit for roof repairs in Narre Warren.

What growth does to tiles and mortar over time

The growth itself is not really what damages a roof. What it does to water is.

Moss holds moisture against the tile long after the rain has stopped. A concrete tile that stays wet takes far longer to dry through, and a tile that never dries properly stays heavier and more porous than it ought to be.

That matters most along the ridge. The mortar bedding and the pointing that hold the ridge caps in place are the first things to suffer from constant damp. Melbourne’s swing from a cold wet night to a warm afternoon works moisture in and out of every hairline crack it can find.

Lichen brings a second problem. Because it bonds to the tile rather than resting on it, lifting established lichen off an old tile can take the weakened surface away with it. That is not a reason to leave it there, but it is a reason to be careful about how it comes off.

Then there is sheer volume. A thick mat of moss along the bottom course of tiles, or sitting in a valley, slows water down and pushes it sideways. Water that cannot run away quickly starts looking for the gaps between tiles instead.

Debris in a valley is held in place by the moss growing through it. The valley stops working as a drain and starts behaving like a garden bed. Our complete guide to roof care and maintenance goes through the rest of what builds up on a tiled roof.

Why home remedies make it worse

The advice that circulates online is usually some combination of a high pressure washer and a household cleaning solution. Both tend to create more work than they save.

High pressure on a tiled roof does two unhelpful things. It strips away whatever is left of the tile’s surface, and it drives water up under the laps and into the roof space, where nobody sees it until a ceiling says something.

Household mixtures usually take only the top off the growth. The lichen gripping the tile stays exactly where it is and greens up again, and the run off ends up in garden beds and stormwater.

The bigger issue is the ladder. The method is genuinely the least dangerous part of cleaning a roof. Wet, mossy tiles are slippery, tiles crack under foot traffic, and a cracked tile turns a cosmetic problem into a leak.

So do your looking from the ground, from an upstairs window, or from inside the house in daylight. A phone camera zoomed in, or a pair of binoculars, shows you more detail than you would pick up standing on the roof anyway. Pick a dry, still day, and stay off the roof.

What you are looking for is simple enough: green or grey patches on the shaded slope, dark staining running down from the ridge, growth packed into the valleys, and any ridge line where the mortar looks cracked or crumbly. That is a job for a proper clean of the tiles rather than a weekend with a hose.

What removal will not fix on an older tile

Cleaning gives you back the colour of the tile you have. It does not give you back the tile you had.

On a concrete roof, once the surface coating has worn away the tile underneath is porous and takes on water. Removing the moss reveals that rather than repairing it. Plenty of roofs look patchy straight after a clean, because the growth was hiding how unevenly the surface had worn.

On terracotta, the glaze is what sheds the water. Where the glaze is intact, a clean does a great deal of good. Where it has gone, cleaning is only the first step.

There is also whatever the moss was sitting on. Cracked tiles, worn valleys and crumbling ridge pointing do not improve because the green has gone. They are simply easier to see.

That is why a clean is the beginning of a restoration rather than a substitute for one. A restoration generally starts with cleaning the tiles, then broken tiles are replaced, ridge caps are rebedded and repointed where they need it, and a concrete roof is sealed and coated. Terracotta is handled differently, because it is glazed rather than coated.

What to do about a green roof

If the shaded side of your roof has gone green, the useful next step is finding out what is underneath the growth before you decide anything. That is a look at the tiles, the valleys and the ridge pointing together, not a guess from the driveway.

As a rough guide on cost: restorations start at $4,500, repointing only jobs start at $1,500, and most complete jobs land between $4,000 and $9,000 depending on size, pitch and access. Full roof restorations carry a 7 year warranty.

Spic and Span is a family owned business based in Cranbourne, working on tiled and terracotta roofs across Melbourne’s south east, residential and commercial, with more than 1,000 roofs restored. You can read more about our roof restoration service before you call.

Quotes are free. Call 0435 104 623 and we will come and look at it. We are open 7 days, 8am to 5pm.

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