The short answer
Moss can be treated from the ground with a long-handled brush and a suitable moss killer, but getting onto the roof to scrape it, or using a pressure washer, is where most damage and most falls happen. Tiles and slates crack underfoot, and a hard jet strips their surface. If the moss is thick, high up or on a brittle covering, it is safer left to a roofer.
What you can reasonably do from the ground
If the moss is light and the roof is low, there is a sensible limit to what an owner can do without leaving the ground. A moss killer sold for roofs and masonry can be applied with a garden sprayer from a safe position, and a few weeks later the dead growth can be brushed or will wash off in the rain. The key is patience: treated moss turns brown and loosens by itself.
Gutters are the other job within reach. Moss that has dropped into the gutter, together with leaves and grit, blocks the outlets and spills water past the edge and onto the eaves timbers. Clearing a gutter from a stable ladder, with someone footing it, is ordinary maintenance, and it is worth doing before a wet spell. Our page on moss blocking gutters goes into what happens when they are left.
What is worth avoiding even from the ground is anything that sends water or chemical up the slope at force. Aiming a jet from below can push water into the overlaps between slates or tiles, and the runoff carries whatever you have sprayed down the wall and into the borders.
Why climbing up is the dangerous part
Falls from roofs and ladders are a serious source of injury in home maintenance, and a mossy roof is among the most treacherous surfaces there is. Moss is slippery when damp, and a slope that looks dry from the garden can be slick in the shade. Even a low-pitched roof is unforgiving, because there is no friction to catch you and nothing but a gutter at the edge.
The covering is at risk too. Old clay tiles, natural slates and stone slates are brittle, and a footstep in the wrong place cracks them. A cracked tile may not show for months, and then a leak does. Roofers are trained to spread their weight, use roof ladders or crawling boards and to choose where to stand. A well-meaning owner with a brush and no roof ladder is likely to do more harm than the moss ever did.
Think also about the people below. Debris and dislodged tiles fall onto paths, cars and neighbouring gardens, and a roof clean leaves a surprising amount of waste. Handling that safely is part of the job, and it is easy to overlook when you are on a ladder with a brush.
There is no shame in leaving this work to someone who does it daily. If you cannot reach the roof safely from a ladder and a firm base, it is not a job for a weekend.
Methods to avoid
The hired pressure washer is the classic DIY error. The moss vanishes in seconds, which feels like success, but the jet can strip the protective surface from concrete tiles, loosen pointing, chip the edges of slate and force water beneath the covering to the underlay. Cost guides for roof cleaning warn of exactly this risk to delicate tiles, and a roof that has been blasted tends to regrow its moss faster, because the roughened face holds water. We explain the alternative in our page on pressure washing a roof.
Scraping with a trowel or wire brush has a similar effect on a smaller scale, and it grinds the moss roots into the surface. Household bleach is another temptation, and it has its own drawbacks, which we cover in should you use bleach on a roof.
Be wary too of any product that promises a clean roof from one spray in an afternoon. Moss killers work slowly, and no honest treatment shows its full result on the day. If a label promises otherwise, treat it with suspicion.
Finally, avoid pulling moss out of valleys and from behind chimneys by hand without checking the lead underneath. Lead is soft and tears easily, and a small tear in a flashing or valley is a leak waiting to happen.
When to hand the job to a roofer
The signs that DIY has run its course are simple. The moss is thick enough to form a cushion. It covers more than a patch or two. It sits on a steep pitch, a second storey or a roof reachable only from a neighbour's land. Or the covering is old, thin or already cracked, and you cannot tell how much it will bear.
A professional clean gets at the moss by hand, applies a treatment across the whole slope and rinses at low pressure, with the right access in place. It is more than an owner can easily copy with a ladder and a bucket, and it is done in a day for an average house, as explained in how long roof cleaning takes.
There is also the question of what lies beneath. Heavy moss can hide slipped tiles, cracked slates or rotted verge pointing, and a roofer will find these as the moss comes off. You would not want to have scrubbed over a fault and then wonder where the damp is coming from.
What our visit adds
We begin with a free survey, taking photographs of the covering so you can judge for yourself how bad the moss is. Look at those pictures for tiles lifted by thick moss along their bottom edge, since that gap lets wind-driven rain in and shows where the clean needs most care.
If the survey shows that the moss is the least of the roof's problems, we will tell you, and we will quote for the repair on its own terms. Our roof cleaning service page sets out the full approach, and the number to ring is 01225 285181.
