The short answer
Green or black growth on uPVC soffits is usually algae feeding on dirt and damp, and warm soapy water with a soft brush removes it. On timber soffits, mould can mean wet or rotting wood behind. If your home dates from before 2000, find out whether the board might contain asbestos before you scrub or spray it.
What the growth actually is
Look up at the underside of the overhang and you are looking at the soffit. It sits in shade, stays damp longer than a wall in the sun, and collects dust and spores blown up from the garden. Algae and mould settle in that film and spread in patches, green when fresh and dark when old.
On uPVC the growth is on the surface only. The plastic does not rot, so the board underneath is rarely harmed. It looks poor and it spreads, but cleaning usually brings the white back. North-facing elevations and eaves under trees stay damp longest, so they tend to go first.
Our page on a black roofline deals with a related question for dark finishes, where staining shows differently.
Cleaning uPVC soffits safely
Use warm water with a little washing-up liquid and a soft brush or cloth, working along the board from a safe position. A long-handled soft brush from a stable platform is far better than leaning out from a rung. Rinse with clean water afterwards, and wipe dry if you can reach.
Avoid abrasive pads, which scratch the surface and give the next growth something to hold on to. Avoid pressure washers aimed upwards, since they force water through the ventilation slots and into the roof space. On an upper storey the sensible choice is to have it done from a scaffold or tower while other roofline work is under way, rather than risk a fall.
Trees overhanging the eaves feed the problem, so trimming them slows the return. So does clearing the gutters, because an overflowing run keeps the underside of the eaves wet.
When mould points to damp behind the board
Timber soffits behave differently. Mould, flaking paint and dark staining on wood can mean water is reaching it from above, perhaps from a leaking gutter, a slipped tile or a blocked downpipe soaking the fascia. Press a screwdriver gently into the board, and if it sinks in, the wood is soft.
Check what sits above the stain. A gutter that overflows onto the soffit, or a roof edge where the underlay does not reach the gutter, will keep the area wet. Fixing the water source matters more than any cleaning. Look in the loft too: damp timber at the eaves, or a musty smell, tells you the trouble has gone past the surface.
Our note on soffit ventilation explains why blocked vents add to the damp, since a roof space that cannot breathe holds moisture against the boards.
Products to avoid, and what works instead
Strong bleach is a common first reach, and it is a poor one. It can streak coloured boards, spoil paint on timber, and run down onto render and plants. A mild wash with detergent does the same job on plastic with none of the risk.
For timber, scrape or brush off loose growth, let the board dry fully, then treat with a fungicidal wash made for the purpose and follow the maker's directions. Painting over damp mould only seals it in. Wear eye protection when you work overhead, and keep ladders off the gutters, which bend and crack under a rung. Work in dry weather so the board can dry out afterwards, and rinse any cleaning water off the wall and ground so it does not mark them.
If the growth returns within weeks of cleaning, treat that as a sign the board is staying wet for a reason, and look for the source.
Take care with older boards
Under HSE guidance, a building from before 2000 should be presumed to contain asbestos somewhere, and extra care is the sensible starting point. One form, asbestos insulating board, turns up in external soffits and looks very like the modern boards that replaced it. If you cannot be sure, do not scrub, drill, sand or pressure wash it, because disturbing it can release fibres.
If your home dates from that period and the soffits are original flat panels, get a professional opinion before touching them. Our page on asbestos soffits explains what is involved. Removing insulating board is mostly work for specialist contractors, so on a survey we explain the next step rather than guess.
Keeping the growth from coming back
Dry boards stay clean. Keep the gutters clear so they stop overflowing onto the underside of the eaves. Cut back branches that shade the overhang and drop leaves into the run. Make sure the soffit vents are open, so air moves through the roof edge and carries the damp away.
A yearly wash in spring, before the growing season, keeps the surface from building up. Dark patches that reappear in the same place each year usually trace back to one leaking joint or one blocked outlet above, and fixing that one spot is far cheaper than washing for ever.
Cleaning or replacing
Replacement beats repeated cleaning when a board is cracked, warped or rotten. Soffits seldom come off alone, since the fascia and gutter go at the same visit, which our guide to how long roofline replacement takes walks through.
A visit from us brings photographs of your own eaves and a written quote afterwards. Sometimes the right answer is a gutter repair and a wash, and then that is all we suggest. The job is backed by a written 10-year workmanship guarantee, and we tidy up before we go. More on our gutters, soffits and fascias work.