The short answer
Remove moss from a polycarbonate roof gently: soften it with lukewarm soapy water, lift it off with a soft brush on a telescopic handle while you stay on the ground, and rinse on a low setting. Never stand on the sheets or scrape them with a blade. A thick mat, or green growth inside twin-wall sheets, is a job for a roofer.
Moss on a conservatory roof is a nuisance and a slip risk. It comes off with patience and the right tools, not with force.
Why moss grows on polycarbonate
Moss and algae need damp, shade and something to hold on to. A polycarbonate sheet gives them all three where the roof is sheltered by trees, faces north, or has a gap along the edges where dust and leaves collect. Shade is the biggest driver. A roof under a tree gathers leaf litter, droppings and drips that feed the growth, so trimming branches does more good than any cleaner.
Where does the growth start?
Usually along the bars, at the lower edge of each sheet and around the end caps, where water lingers and dirt settles. A thin film of green spreads from those points, and in time it forms a mat that holds more water against the seals. That is when the roof starts to look tired and to feel slippery to anyone who gets close.
The growth itself is not usually a structural fault, but it keeps the roof wet. Over many seasons that wet can shorten the life of the seals, and polycarbonate is not a long-lived covering. HomeBuilding gives a typical lifespan of about 10 years, and The Eco Experts say 10 to 15 years for glass or polycarbonate, so an old roof with a lot of moss may be due a check on its condition.
Removing it without damage
Work from the ground, from a step ladder placed on firm ground at the edge, or from inside. Do not climb on the roof. The sheets are not made to take weight, and moss makes them slick. Wait for a dry spell, then pick a mild day so the soapy water does not freeze or flash-dry on the sheet.
What tools and mix do you use?
A sponge or soft-bristle head on a telescopic handle, warm tap water and just a little detergent. Wet the moss first so it softens, then work down the length of the sheet with light strokes in the direction of the flutes, pausing if the brush catches or the sheet flexes and trying again another day, the channels running down each sheet. Rinse with the hose turned low and keep the run-off pointing away from the house wall.
Do not scrape with a metal blade, a stiff brush or a scouring pad. Any of them scratches the surface, and a scratched sheet collects dirt faster than a smooth one. Cleaning products with solvents or abrasives also cloud the sheet and can damage the rubber seals.
Our answer on how to clean a conservatory roof safely goes through the full routine, including clearing the gutter first.
Why a power hose does more harm than good
A jet looks like the quick answer, and it does the most harm. Marley does not recommend jet washing on slate or tile, since the force can shorten the surface's life. A thin plastic sheet is far less forgiving than either of those.
What can a jet do to the roof?
It can drive water under the seals and inside the end caps, scratch or craze the outer face, and loosen the fixings, so a roof that never leaked begins to. It can also blow moss and grit into places that are hard to clear. The damage tends to show up as drips a few days later, long after the cleaning is forgotten.
Are moss killers safe to use?
Some are meant only for professionals. HSE says householders should keep away from biocides made for professionals, and any label terms must be followed. Read the label and keep run-off off plants and out of drains. Treat moss killer as a job for a roofer rather than an experiment from the shed.
When the green is inside the sheet
Twin-wall polycarbonate has small channels running through it. The ends should be sealed with tape, and the tape is meant to keep water, dirt and insects out. If it fails, algae can grow in the channels, where no brush can reach it.
Can you clean inside a twin-wall sheet?
Not properly. A green or grey streak that runs along the inside of a sheet is a sign that the sheet is damp from within, and re-taping or replacing the sheet is the repair. A roofer can check the end seals, the sheet and the bars, and give you photographs of the result. Replacing one sheet costs far less than replacing the roof. Gutters matter here as well: a gutter full of sludge keeps the lower edge of the roof wet, and moss follows.
If many sheets are marked, the roof may be close to worn out, so price a replacement. See our page on conservatory roofs, or how to replace a conservatory roof panel. The written price follows a survey, and we plan the access so nobody has to stand on the roof.
More on moss and algae
Will moss damage a polycarbonate roof?
On its own it keeps the surface wet and the roof dirty, but that damp works at the seals over time and moss can block the gutter. Removing it gently and keeping the gutter clear is worth doing. Hard scraping does more harm than the moss, so choose a soft brush.
How do I stop moss coming back?
You cannot stop it for good, but you can slow it. Trim back overhanging branches, keep the gutter and the roof edge clear of leaves, and wash the roof when the first film appears. A roof washed little and often needs far less effort than one left for years.
The full picture: Conservatory roofs in Taunton ยท More short answers