You are standing in the backyard, look up, and notice the south side of the roof has gone a patchy grey green. Up close it looks like crusty little rosettes stuck flat to the tiles, with a few soft green cushions of moss tucked into the joints. The question we get asked next is always the same: is lichen on the roof actually doing any harm, or is it just ugly?
The honest answer is that it depends on what your roof is made of, how much growth there is, and where it is sitting. Here is how we size it up on the Peninsula, and what we would and would not do about it.
Lichen and moss are not the same thing
Lichen is a fungus and an alga living together as one organism. It grows flat and crusty, usually grey, pale green or a mustard yellow, and it grips the surface with tiny root-like threads. It grows slowly, which is why a roof can look clean for years and then seem to turn almost overnight.
Moss is a small plant. It grows in soft, raised clumps and needs more moisture than lichen does, so it collects where water and debris sit: in tile joints, along the bottom edge of the roof, in valleys and behind anything that blocks the run of water.
The difference matters because they cause different problems. Lichen works on the surface of the tile. Moss works on the water path.
Shade, trees and sea air are why the Peninsula grows so much of it
Lichen and moss like shade, damp and a surface that stays wet for a while after rain. A lot of Peninsula homes tick all three. The roofs we see with the heaviest growth are usually:
- under or beside big gums, tea tree or moonah, especially around Mount Eliza, Mount Martha and the Red Hill end
- on the south facing side, which gets the least sun in winter
- concrete tile roofs that have lost their original surface coating
- close enough to the bay or Bass Strait that sea mist keeps the roof damp on still mornings
Leaf litter makes it worse. Leaves break down on the roof and in the gutters into a damp, rich layer, and that is exactly what moss wants to root into.
When it is cosmetic and when it is real damage
A light dusting of lichen on a sound terracotta roof is mostly a looks problem. Glazed terracotta in particular is hard and slick, and lichen struggles to get much of a grip.
Concrete tiles are a different story. Once the factory coating wears off, the tile surface is porous and slightly rough. Lichen holds moisture against it and slowly breaks the surface down, so the tile gets more porous, holds more water, and grows more lichen. On older concrete roofs we often find the top layer of the tile has gone sandy where heavy growth has been sitting for years. At that point the roof usually needs a full restoration rather than a clean.
Moss is the bigger worry for leaks. A clump sitting in a joint or along the lower edge of a tile acts like a sponge and a dam at the same time. In heavy rain, water backs up behind it and can track sideways under the tile above. Moss and debris in valleys and gutters do the same thing on a bigger scale, and that is one of the common reasons a roof that was fine for years suddenly drips in a downpour.
On metal roofs, lichen and dirt mostly build up on the sections rain does not wash clean, like under eaves overhangs or behind a solar panel frame. Anything that keeps the metal damp for long periods works against the coating, so those spots deserve attention even if the rest of the roof looks fine.
Signs the growth has moved past cosmetic:
- moss clumps sitting in tile joints or packed along the bottom row
- tiles that look crumbly, sandy or darker than the rest when dry
- water stains on the ceiling under a shaded part of the roof
- gutters full of moss, grit and tile granules
If you are seeing cracked or slipped tiles at the same time, our guide on whether cracked tiles can wait is worth a read before you book anything.
Blasting it off with a pressure washer causes more problems than it solves
The quickest looking fix is a pressure washer, and on an old roof it is the one that causes the most trouble. Too much pressure, or a nozzle held too close, can crack brittle tiles, chew into soft concrete, strip whatever coating is left and drive water up under the laps and into the roof space. On a tile roof it also means someone walking all over the tiles, and every step on a fragile tile is a chance to crack one.
Lichen also does not let go easily. You can wash the visible top off and leave the base attached, and it grows straight back.
We prefer to treat first and clean second. A suitable biocide is applied to kill the growth, then it is either left for the weather to break down over the following months or followed by a careful low pressure clean, depending on the roof and what comes next. If the roof is going to be painted or restored, the clean has to be thorough, because coatings will not bond to a surface with dead growth still on it.
If the roof itself is in good shape and you mainly want the growth gone and the water path cleared, a professional roof cleaning and moss treatment is usually the sensible starting point. While we are up there we also check the joints, the ridge capping and the gutters, since those are where moss has usually done its damage.
Colorbond and Zincalume need gentler treatment
Metal roofs are where well meaning DIY goes wrong most often. Household bleach and strong chlorine products can damage the coating on Colorbond and Zincalume, and harsh scrubbing does the same. Stick to products whose label says they are safe for prepainted steel, and rinse well.
One that surprises people: copper. Lichen will not grow below copper, so some people fix copper strips along the ridge as a natural treatment. On a tile roof that can work. On Zincalume or Colorbond it is a bad idea, because water running off copper onto those steels speeds up corrosion. The same goes for copper pipes or old copper fittings draining onto a metal roof.
Light, airflow and clear gutters stop it coming back
Treatment kills what is there. Keeping it away is mostly about light, airflow and water.
Trimming back branches that hang over the roof is the single biggest change most people can make. More sun and wind means the roof dries faster after rain, and fewer leaves land on it in the first place. Keeping gutters and valleys clear stops moss getting the damp, rich base it needs. On a metal roof, hosing down the areas rain does not reach helps keep the coating healthy.
For concrete tiles that have lost their surface, a restoration with a fresh membrane coating puts a sealed surface back on the tile, which gives lichen much less to hold on to. That is a bigger job, and our breakdown of roof restoration costs on the Peninsula explains what goes into the price.
Get a proper look before you climb up there
Most lichen is not an emergency, but moss in the wrong spot can turn into a leak, and a weathered tile roof is not a safe place to walk around with a sprayer. If your roof has gone green or grey and you are not sure whether it needs a clean, a repair or something bigger, get in touch about our roof cleaning service and we will tell you plainly what we find.
Common questions about lichen on roofs
Is lichen on a roof damaging?
It can be. On glazed terracotta it is mostly cosmetic, but on concrete tiles that have lost their coating it holds moisture against the surface and slowly breaks it down. Moss is the bigger leak risk, because it blocks the path water takes off the roof.
Why does lichen grow on my roof?
Lichen needs shade, moisture and a surface it can grip. Roofs under trees, south facing roof sections and weathered concrete tiles stay damp longest after rain, so they grow the most. Leaf litter and blocked gutters add to the problem.
How do you get rid of lichen on a roof?
Kill it first with a suitable biocide, then let the weather break it down or follow with a gentle clean. Scraping or high pressure washing tends to damage the roof and leaves the base of the lichen behind, so it grows back.
Will chlorine kill lichen on the roof?
Chlorine will kill lichen, but it is a poor choice for most roofs. It can damage the coating on Colorbond and Zincalume, and the runoff can harm gardens and stain surfaces below. A product made for roofs, and safe for your roof type, is the better option.
Can I remove lichen from my roof using Bunnings products?
Hardware store roof treatments can work on light growth if you follow the label and check the product suits your roof material. The bigger issue is safety, since spraying from a ladder or walking on old tiles is risky. Heavy moss in joints and valleys usually has to be cleared by hand, which is a job for someone with the right gear.
Should lichen be removed?
Light lichen on a sound roof can be left if you are happy with the look. Remove it if it is heavy on concrete tiles, if moss is building up in the joints or gutters, or if the roof is about to be painted or restored, because coatings will not stick over it.
