A roof is not one surface. A pale grey slate on a Victorian terrace, a handmade clay tile on a Hampshire farmhouse and a concrete interlocking tile on a 1990s estate home behave differently under water, brush and chemical. The growth on them may look the same: green moss in the gaps, black streaks down the slope, grey crusts of lichen. The right way to remove it is not the same. This guide sets out the practical differences, so you can judge what a roof cleaner is proposing and ask the right questions before anyone sets foot on your roof.
Why the material matters
Three properties decide how a roof should be cleaned: how hard the surface is, how much water it absorbs, and what is on top of it.
Hardness decides whether a brush, a lance or a scraper can damage it. Absorption, or porosity, decides how deep growth roots and how long a treatment takes to work. The surface layer decides whether cleaning can remove what you want to remove without taking something you want to keep. Concrete tiles carry a coloured surface layer, and clay tiles may have a sanded or glazed face. Slate has its own natural surface, with a fine grain that is easy to scratch and a layered structure that can split.
The broader process of choosing a method is set out in our comparison of moss removal methods. This page deals only with how the three materials change the choice.
Slate roofs
Natural slate is dense and has very low absorption. Moss and lichen sit on the surface and in the lap between slates, but they do not root into the stone the way they can into a softer tile. That makes slate the most forgiving material for growth and the least forgiving for rough handling.
What goes wrong with slate
The main risk is mechanical. A slate is a thin, brittle layered stone, held by two nails or a hook, and sometimes by nothing more than its neighbours. A boot in the wrong place can crack it. A high-pressure jet can force water up between the laps, drive it behind the slates and into the roof space, and chip the edges. Scraping with a hard tool can scratch the surface, and a scratched slate weathers faster. Older slates may already be delaminating, where the layers lift away, and aggressive cleaning accelerates that. Our page on whether moss damages slate roofs explains why the growth itself is often less of a threat than people fear.
What suits slate
A soft brush or a gentle low-pressure rinse, together with a biocide treatment that kills growth and lets it release over weeks, is usually the right approach. Hand removal of thick moss from the laps comes first. Some slate roofs are not worth walking on at all, and a clean may be done from ladders or a platform. We would sooner leave a lightly lichened old slate roof alone than risk damage, and we say so on the survey. For wider slate care, see whether slate roofs need maintenance.
Clay roofs
Clay tiles are fired from clay and come in many forms: machine-made plain tiles, handmade tiles with a rough, sandy face, pantiles and interlocking types, some glazed, some natural. The common feature is that clay is porous to some degree, and handmade types are the most porous.
What goes wrong with clay
A porous tile holds water, and that is where moss and algae thrive. Roots and the dark pigmented filaments of lichen can anchor into the sanded face. Water that sits in a tile freezes and expands, and this is how tiles spall, with the face flaking away in winter. Strong cleaning that strips the protective outer layer of a handmade tile exposes the softer clay beneath and can make spalling more likely. Pressure washing at close range can also blast out the pointing at the ridge and the bedding of the tile ends.
What suits clay
Low-pressure soft washing with a biocide wash, applied and left to work, and a gentle rinse, suits most clay tiles. On handmade clay we prefer to use the chemical to do the work and keep any mechanical action light. For decorative roofs with hand-cut hips, bonnet hip tiles or ornate ridge tiles, we check the mortar before cleaning and note any area that needs repair first, so the water does not wash straight through failed pointing. Glazed tiles are the easiest to clean, but their surface can be scratched.
Our page on what soft washing is explains the low-pressure approach, and roof biocide treatments explains how the treatment works over time.
Concrete roofs
Concrete tiles are common on houses built from the 1960s onwards, and on many of the newer estates around Basingstoke. They are moulded from sand and cement, and most have a coloured coating or a pigmented surface slurry that gives them their colour and some protection. Under that, the body is a rougher, more absorbent material than the face.
What goes wrong with concrete
Concrete tiles can be harmed by cleaning that strips their face. A hard jet can erode the surface layer, leave the roof patchy, and expose the body to faster weathering and to a rougher surface that grows moss faster than before. Black streaks on concrete tiles are often algae on the surface layer, which looks alarming and responds well to treatment. Heavier moss grows in the interlocking channels where water collects, and in the lap. Concrete tiles are heavy, and a roof with a lot of moss holds extra water, which makes it heavier still.
What suits concrete
The careful method is to remove thick moss by hand from the channels and laps, apply a biocide, and rinse at low pressure. We avoid high pressure on concrete, and our page on whether pressure washing damages roof tiles covers why. On a coated tile, a rinse that leaves the coating intact is the aim. If the colour layer is already worn, no cleaning will bring it back, and a coating decision belongs in the conversation, which we cover in roof coatings on tiled roofs.
| Material | Main weak point | Method that usually suits | Avoid |
|---|---|---|---|
| Natural slate | Brittle, easy to crack and scratch | Hand removal, biocide, gentle rinse | Walking freely, hard jets, scraping tools |
| Handmade clay | Porous face that spalls in frost | Soft wash with biocide, light brushing | Close-range pressure, harsh scrubbing |
| Machine-made clay | Pointing at ridge and hips | Soft wash, rinse after checking mortar | High pressure on joints |
| Concrete | Surface layer can be stripped | Hand clearing of channels, biocide, low rinse | High pressure that strips colour |
What stays the same on every roof
A few rules hold no matter what covers the roof. Growth on a roof is a sign of damp, shade and time, and the material does not change that. A clean works best when the roof is sound, and cleaning a roof with slipped or broken tiles can do more harm than good. Never walk on tiles without supports, because even a strong tile can crack under a boot.
Gutters need attention at the same time. Moss and debris that fall in a clean can block outlets within days, so clearing them is part of the job. Our page on moss, gutters and drainage covers how drainage ties into the roof.
Asbestos cement roofs are another matter altogether. They should not be pressure-washed, because the surface can release fibres, and the HSE explains the position on its asbestos pages. If your roof, garage or outbuilding has fibre cement sheets, do not clean them yourself, and our answer on whether an asbestos cement roof can be cleaned explains the options.
Mixed and awkward roofs
Real houses are not always one material. A main roof in concrete with a clay hip, a slate-covered porch, or a lean-to in an older tile all call for different handling on the same job. Extensions often carry a different covering from the main roof. Reclaimed tiles, patched in over the years, can have very different porosity from the originals beside them. The surveyor on your roof should note each area and its covering before choosing methods.
Roofs in conservation areas, or on listed houses, are a further case. Like-for-like repairs are usually fine, but a clean that changes the look of the material, or a coating, may need consent, so check with your council's conservation officer first.
How we choose on your roof
We look at the covering before we propose a method. On the free survey we identify each material, check for broken, slipped or delaminating tiles, look at the ridge and verge mortar, and photograph the growth and any damage. Often the right answer is a gentle treatment and a wait of a few weeks. Sometimes the right answer is repairs first and cleaning later, or no cleaning at all.
You then get a photographed report and a fixed written quote for the method that suits your roof's material. Our work comes with a ten-year written workmanship guarantee, and waste is cleared when we finish. More about the service is on our roof cleaning in Basingstoke page. When you call 01256 213956, mention whether you have slate, clay or concrete, and we will bring the right brushes.
Timing and aftercare by material
How soon a roof looks clean depends on the method and on the material. A biocide treatment does not blast growth off. It kills it, and the dead moss and algae then release over the following weeks as rain and wind work on them. On smooth slate and glazed clay that release is quick. On rough concrete and handmade clay it can take longer, because the growth has more to hold onto, and you may see streaks fading across a whole season. We tell you what to expect so a roof that still looks patchy a fortnight later does not cause alarm.
Afterwards, leave the roof alone. Do not rake off the remaining growth, and do not hose a roof you have just treated. Keep an eye on the gutters for the first few weeks, because the debris that does come down is carried there. Trimming back overhanging branches and clearing leaves from valleys slows the return of growth on every material, and shaded north slopes will always green up first. If you are interested in how long the effect lasts, our answer on how long before moss grows back gives the honest picture.
