Dark stains on a roof are easy to spot from the road. They usually start as a faint grey shadow, then spread into bands that run down the slope. Many homeowners assume it is soot or dirt, yet it may be neither.
This post explains what causes dark streaks, what the trade bodies say about them, and how to treat them without harming your tiles. We will also cover when a coating makes sense, and when it does not.
What the dark streaks usually are
A common cause is algae. The Asphalt Roofing Manufacturers Association (ARMA) is a US trade body, and its page on algae discolouration of roofs names Gloeocapsa magma as probably the most prevalent of several algae species involved. It says the natural pigments these algae produce can gradually turn a white or light-coloured roof dark brown or black.
ARMA writes about asphalt shingles, so treat its detail with care on UK clay and concrete tiles. Even so, the point about appearance carries over. ARMA says the discolouration usually looks brown to black, and can be mistaken for fungus, soot, dirt, moss or tree droppings.
Lichen can also be dark. The NFRC, the trade body for roofing contractors, says in its guidance note on moss and lichens that lichens are coloured green, orange, grey or black. A dark patch may therefore be algae, lichen or a mix of both.
Why they appear where they do
Growth needs two things: something to hold on to and moisture. For example, the NFRC says the rough surface of a tile filters dirt out of rainwater, and blown spores take root in that dirt. It adds that moss tends to flourish where trees are nearby and conditions are shady and damp.
Slope direction matters as well. The NFRC says north-facing slopes, which stay damp longer, may attract more growth. As a result, one side of a roof can look clean while the other looks stained.
Is it just cosmetic?
Sometimes, but we cannot give a blanket answer, because ARMA’s page treats algae mainly as a discolouration problem. Marley, a major UK tile maker, says low to moderate moss or lichen growth is not normally harmful to tiles, unless it becomes severe and stops water flowing off.
Marley also says that moss, algae and lichen growth can trap moisture and cause tiles to degrade over time. So light staining is mostly about looks, whereas heavy growth deserves attention. A survey tells you which case you have.
Why jet washing is the wrong fix
Blasting the stains off looks like the quick answer. Marley says the use of high powered jet washes is not recommended, because it can damage the surface of the tile and reduce its expected life considerably. The NFRC adds that pressure washing can force water into the roof space, particularly when sprayed upward from the ground.
Similarly, ARMA gives the same warning for asphalt, telling owners not to use a power washer or brush on algae. Our post on whether softwash is safe for roof tiles covers the manufacturers’ views in more detail. We also compare the two methods in our guide to softwash vs pressure washing.
How we treat dark staining
We use a low-pressure biocide, so the treatment kills the living growth rather than scouring it off. Rain then washes the dead material away over the following weeks. The result builds gradually, so the roof keeps improving after we leave.
Every job carries our 5-year regrowth protection guarantee. No method stops growth for ever, though. Wienerberger, a clay tile maker, says anti-moss treatments are not a permanent solution and that ongoing monitoring is still required.
Where roof coating fits
A coating is not a cure for stains. We apply one only after a softwash clean, because growth left underneath can keep spreading.
Our roof coating service suits sound concrete tile roofs where the colour has faded. However, we do not coat clay or slate.
We also do not claim a coating makes a roof last longer. The NFRC notes that painting roof coverings after cleaning may invalidate manufacturers’ guarantees of performance and life expectancy. Check your tile maker’s terms before you commit to any coating.
What you can do from the ground
You do not need to climb up to help. Small steps slow the cycle, and none of them involves the roof itself:
- Trim overhanging branches to reduce shade and falling debris.
- Keep gutters clear, so water leaves the roof freely.
- Check the roof from the ground with binoculars each autumn.
- Treat early, because light staining is the cheapest to deal with.
Where we treat stained roofs
We cover Kent, Surrey, East Sussex and West Sussex. For example, we offer roof cleaning in Tunbridge Wells, roof cleaning in Guildford, roof cleaning in Worthing and roof cleaning in Brighton. The survey is free, and we give a fixed price after it.
Common questions
Will the streaks come back?
They can, particularly on shaded or damp slopes. Treatment slows regrowth rather than ending it, and that is why we back our work with a guarantee.
Can bleach from a bottle do the job?
We would not use household products on a roof. ARMA does describe a bleach solution for asphalt roofs, but that advice does not cover UK tiles. The NFRC says chemical washes can harm plants and animals, so the product, dilution and run-off all need planning.
Do I need a coating as well?
Usually not, because most roofs only need a clean, and we will say so at survey. A coating only suits a sound concrete roof with noticeably faded colour.
Related reading
Cleaning counts as upkeep rather than an insured loss, as we explain when asking whether you can claim roof cleaning on insurance. Lichen is another dark growth, so see our post on lichen on roof tiles. Timing can matter too, so think about roof cleaning around bird nesting season when you book.