The short answer
Green or black soffits are usually algae or mould growing on a surface that stays damp. It is mostly a surface stain, but it points to moisture, from an overflowing gutter, a leak at the eaves or poor airflow. Clean it gently with water and a soft brush. Leave the products sold to trade users alone, and do not jet wash the boards.
The colour is the symptom, not the problem. Find out why the board stays wet and the stain will be easier to keep away.
What the green and black actually is
A soffit is the panel fitted under the roof overhang, spanning from the wall out to the fascia. It faces down and sits in shade, which makes it a cool, still place where moisture lingers.
Is it algae or mould?
Both turn up on soffits. Algae gives a green or green-grey film, and darker spotting is often mould or a different growth. You do not need to tell them apart to deal with them, because the conditions that let them grow are the same: a surface that stays damp for long periods, with a little dirt for food.
On uPVC (rigid plastic) boards the growth sits on the surface and wipes off. On timber soffits it can take hold in the paint and the grain, and the board underneath may be softening. A soft or flaking timber soffit is a different problem from a green plastic one, so poke a screwdriver gently at the worst spot to see whether the wood is firm.
Staining that comes back after cleaning almost always means the moisture source is still there, which is where the next section picks up. Dark patches at the joins between boards, or around vent slots, are worth a closer look too, as damp collects in those corners first and lingers longest.
Why the soffit stays damp
Ask where the water is coming from before you reach for a brush.
What causes damp soffits?
The first suspect is the gutter. A blocked gutter spills over its front lip, and the water curls back under the overhang to wet the fascia and soffit. A gutter joint that leaks above the board does the same. Our answer on overflowing gutters goes through the usual causes.
The second is the eaves. Failed felt at the roof edge lets rainwater run behind the fascia and into the soffit space, as covered in our page on eaves felt rot. The third is condensation: a roof void with no airflow lets warm, damp air from the house settle on cold surfaces. Soffit vents, which let air in at the eaves, help prevent that.
Shade and trees matter as well. A north-facing wall under an overhanging tree stays wet far longer than a sunny one. You cannot move the tree, but you can clear the gutter and keep the roof edge in working order, and that often changes the picture.
Cleaning it safely
For a plastic soffit that is simply dirty, gentle cleaning works.
How do I clean green soffits?
Use warm water, a little mild washing-up liquid and a soft brush or cloth, working from a stable position. A tower or a ladder with a stand-off is better than leaning from a window. Rinse with a garden hose at low pressure, then let the board dry. Wipe the vent slots with a damp cloth rather than flooding them. Do not put your weight on the boards, and keep water out of the vents.
Marley does not recommend jet washing roof tiles or slate, and soffit boards are no tougher, so the same caution applies here. Force drives water into joints, strips paint from timber and loosens vents. Patience with a brush does the job, and it costs nothing but time.
Can I use a biocide?
Not a professional-only one. HSE is clear that a biocidal product made for professionals is off limits to householders, and that choosing a product without approval is an offence. Products sold for home use are approved for it; check the label and follow it exactly. If the label says professional use, it is not for you.
For work at height, or for a long run of boards, call a roofer rather than risk a fall. Never lean out of an upstairs window to reach them.
When it is more than a stain
A one-off clean is fine for a few boards. Some signs say the green is a clue to something bigger.
When should a roofer look at it?
If the soffit is soft or sagging, if stains appear on the ceiling inside, if the fascia above it is dark or peeling, or if the growth is back within a season of cleaning, get the eaves looked at. The cause may be a failed gutter joint, perished felt or blocked ventilation, and those are repairs, not cleaning jobs.
On a survey visit we examine the gutter, fascia, soffit and felt line, and you are shown photos of each. A board that has gone can be replaced with vented panels, so air moves through the roof edge again. A soffit made of old asbestos insulating board is identified first, because removing it can be licensed work, which turns on HSE's risk assessment rules. See our page on asbestos soffits.
The main gutters, soffits and fascias page lists what we do at the roof edge.
More on soffit staining
Will green soffits damage my roof?
The algae itself is mostly cosmetic. What matters is the damp that feeds it, because a board that stays wet can rot if it is timber, and a leak behind it can reach the rafter ends. Treat the stain as a prompt to check the gutter and eaves, not as a cleaning job alone.
How often should soffits be cleaned?
No fixed interval applies. Clean when they look grubby, check the gutter at the same time, and clean again if the growth returns. If it comes back quickly, the soffit is staying damp for a reason, and clearing the cause is better than cleaning more often.
The full picture: Gutters, soffits and fascias in Taunton ยท More short answers