The short answer
You can deal with some moss problems yourself, but not by climbing onto the roof. A fall from height is the real danger, wet moss is slippery and tiles can snap underfoot. Biocides are also controlled, and products meant only for professionals must never be used by the public. From the ground you can clear gutters, cut back branches and spot trouble early. Leave the roof itself to a roofer.
Why DIY moss removal on the roof is dangerous, what the biocide rules say about the products you can buy, and which jobs are fine to do from the garden.
Why the roof itself is the wrong place for DIY
Moss looks soft, but it is slippery, especially when damp. Put a boot on a mossy tile and the foot can slide, and a slope that looks gentle from the garden feels very different once you are on it. Even a single-storey extension roof is still a drop onto a patio or a fence.
Tiles are not made to be walked on. Concrete and clay tiles crack under a heel or a knee, and slates snap easily. A cracked tile you cannot see then lets water through next winter, so the cleaning job creates a repair job. Roofers use roof ladders, crawling boards and edge protection to spread the load and stop a slip becoming a fall. A step ladder and a stiff brush offer none of that.
Scraping and brushing also push loosened moss down the slope, where it lands in the gutters and blocks them. Pulling at thick moss can lift the edges of tiles and dislodge the mortar at the ridge. If the roof is more than one storey high or has a steep pitch, do not go near it. Our guide on manual moss removal explains how professionals do it by hand.
What the rules say about moss killers
Moss killers are biocides, and the biocides regulation (article 17(5)) makes a product's authorisation conditions and label instructions binding on whoever applies it. In practice the label is not advice you can ignore: it is the legal condition for using the product.
The Health and Safety Executive goes further: anything made for trade use only is off limits to householders, and some of those products need the user to hold particular certification. So a strong treatment that a roofer uses may be off limits to you, and buying something from an unknown seller does not change that.
A product sold in a garden centre for the public is meant to be applied as described: diluted as directed, in suitable weather and kept away from drains, ponds and plants. Poured over a roof it runs straight into the gutters and the surface water system. For more, see roof cleaning chemicals and the garden pond. Bleach and other household chemicals are not made for roofs and have no roof instructions.
What you can do from the ground
Quite a lot of useful work needs no roof access whatever. Start by looking: a pair of binoculars from the garden shows which slopes are green, whether tiles have slipped and whether the ridge line is straight. Take photos on a clear day so you can compare them later and show them to a roofer. Pictures taken a few months apart show whether the green is spreading or holding still.
Next, tackle the causes. Cut back branches that overhang the roof, because they keep a slope shaded and drop leaves into the overlaps. Keep the gutters flowing, as a full gutter holds the eaves wet and encourages growth. If you clear them, use a ladder that is properly footed, or a tool made to reach from the ground, and do not lean out from the top rung.
Moss in the gutter itself, and fallen debris on a low porch roof you can reach safely from a firm platform, is a fair job for an owner. Anything that means standing on a slope is not. Our answer on the best moss killer for a roof covers what a professional treatment should be.
When to call a roofer instead
Call someone when the moss is thick, when it sits on more than one slope, or when you have seen cracked or slipped tiles. A roofer cleans the roof, but also looks for the damage that moss hides, which a ground-level view will never reveal. That check is often the most valuable part of the visit.
Professional cleaning is priced by the area and by how the roof is reached, so the cost depends on the house, and national price ranges from a cost guide make a handy yardstick when a quote arrives. Our first step is an inspection; you then see pictures of the roof and get one total in writing, so you know what the job includes before it starts.
If you decide to do some of the groundwork yourself, take it step by step: trim, clear and watch. A neglected roof gets worse slowly, so there is rarely a reason to rush, and every reason not to take risks. The overview of this work is on our roof cleaning and moss removal page.
More on this in Roof Cleaning & Moss Removal in Crawley
Two more DIY moss questions
Is it safe to spray moss killer from a ladder?
Not really. Reaching across a slope from a ladder puts you off balance, and a wet spray makes the tiles and the ladder feet slippery. Many labels also limit where a product may go, and the run-off ends up in your gutters. If the moss cannot be reached from firm ground, it needs a roofer with proper access.
Is a pressure washer safe on a roof?
We would not. Marley's guidance is that high-pressure jetting is a poor idea on tiles and slates, since it can mark the surface and wear down their lifespan. Water also gets forced up under the laps, towards the underlay and the loft. A roofer works at lower pressure with a treatment, after the hand work. Holding a lance on a slope is a fall risk too.