Roof cleaning and coating

Roof cleaning and coating is renovation work on an existing roof covering. It is priced per square metre of roof area and typically includes:

• removing leaves, needles and debris from the roof surface, valleys and gutters,
• mechanically removing moss, lichen and loose parts of the old coating,
• washing the roof with pressurised water or a cleaning agent suited to the type of covering,
• removing rust from metal with a wire brush or abrasive tool,
• degreasing and treating the surface, and depending on the system a biocide against moss,
• a primer or penetrating sealer and a topcoat in one or two layers,
• covering windows, the facade and surroundings against splashes, and cleaning up afterwards.

The work does not include repairs to the covering, replacing damaged tiles, new chimney flashing or new gutters. If cleaning reveals that the metal is rusted through or tiles are cracked, they need to be repaired before coating. A coating cannot replace missing metal or make a leaking roof watertight.

Metal roofs

This is the most common case. Galvanised steel, standing seam roofing or older profiled sheets on garages, sheds, farm buildings and warehouses lose their protective layer after many years and start to rust. If the rust is only on the surface and the sheet is not perforated, a coating will extend its life considerably. Roof coating manufacturers generally recommend two coats of primer and one or two topcoats for an older roof or a full renovation.

Concrete tiles

As concrete tiles age they lose their surface finish, absorb more water and grow moss faster. After cleaning they can be treated with a penetrating sealer and a paint made for concrete roof tiles. This noticeably improves the appearance and reduces water absorption, but a coating will not save a tile that is already crumbling.

Clay tiles

Clay tiles are usually not painted. Manufacturers tend to recommend gentle cleaning and a moss treatment instead. Glazed or engobed tiles do not need a coating at all, and paint adheres poorly to them.

Asbestos cement roofs

Older roofs made of asbestos cement slates or corrugated sheets, often installed before the 1990s, are a special case. Pressure washing, brushing and sanding release asbestos fibres, which cause cancer, and there is no known safe level of exposure. Work on such a roof cannot be treated as ordinary painting, and in many European countries cleaning or coating these roofs is restricted or banned outright. If you have an asbestos cement roof, ask your local authority or a contractor licensed for asbestos work what applies where you live. In the long run, professional removal and a new roof covering is usually the more sensible solution.

Roof coatings need dry, warm weather. Technical data sheets commonly give a minimum air and surface temperature of around 10 to 15 °C, relative humidity up to roughly 70 to 80 percent and several hours without rain or dew after application. In most of Europe that means coating from late spring to early autumn, while in southern climates the main limit is summer heat rather than cold. Be careful with metal sheeting heated by direct summer sun. The surface can be far hotter than the air, and the paint dries too quickly. It is sensible to work in the morning and late afternoon and to avoid days when dew lingers on the roof in the morning.

Cleaning can also be done outside the coating season. If the roof is overgrown with moss, it is worth cleaning it in spring, treating it with a biocide and coating it a few weeks later, once the product has taken effect and the moss has died off.

Inspection and preparation

First the roof is inspected at close range. The contractor looks for rust, perforated areas, loose joints, damaged flashing and the condition of the old coating. Windows, the facade, terraces and cars under the roof are covered. Gutters are cleaned and downpipes protected so that sludge and paint residue do not get into them. Rinse water containing cleaning agents should not run unfiltered into drains or the garden.

Mechanical cleaning

Moss and lichen are scraped off and coarse debris is swept away. On metal, loose rust and flaking old coating are removed with a wire brush or an angle grinder with a brush or abrasive disc. Only a firm, sound surface should remain.

Washing the roof

The roof is washed with pressurised water or a cleaning solution. For metal with an organic coating, manufacturers recommend a soft brush, mild cleaning agents and thorough rinsing, without aggressive solvents or abrasive powders. On concrete tiles the pressure and nozzle distance are chosen so that the surface is not damaged. After washing, the roof must dry completely, which in cooler weather can take a whole day.

Degreasing and treatment

Once dry, the metal is degreased. Coating manufacturers stress that long-term adhesion is impossible without thorough degreasing. New galvanised steel has a surface layer that impairs adhesion, so it is either left to weather or treated according to the coating manufacturer's instructions. A biocide can be applied to tiles.

Primer

An anti-corrosion primer is applied to metal, a penetrating sealer to concrete tiles. It is applied by brush, roller or spray. Joints, screws, areas around chimneys and sheet edges are brushed first, because that is where rust most often starts. The layer thickness and the recoat interval in the data sheet are followed.

Topcoat

When the primer has dried, the topcoat is applied in one or two layers. Acrylic, polyurethane or alkyd paints are used on metal roofs, and acrylic roof paints on concrete tiles. The work proceeds from the ridge down to the eaves so that nobody has to step on fresh paint.

Inspection and clean-up

After drying, the roof is checked, missed spots are touched up and the protective covers are removed. Gutters and the area around the house are cleaned.

• Alkyd paints for metal. The traditional choice for metal roofs. They are easy to apply but dry more slowly and have a stronger smell.
• Water-based acrylic coatings. They dry quickly, have little odour and are easy to work with. They require a thoroughly prepared surface.
• Two-component systems. An epoxy primer with a polyurethane or polyacrylate topcoat. They are tougher and last longer, but the mixing ratio and pot life must be followed exactly.
• 2-in-1 paints. Primer and topcoat in one product. They suit smaller and less exposed roofs. On heavily corroded metal a separate primer is the safer choice.
• Sealers and paints for concrete tiles. They reduce water absorption and restore the colour of the tiles.
• Biocides. For treating moss, algae and lichen before coating or on their own as part of maintenance.

It pays to stick to one system from one manufacturer, meaning primer, topcoat and thinner from the same product line. Mixing random products is a frequent cause of peeling.

• Coating an unwashed roof. Dust, pollen and moss residue act as a separating layer. The paint comes off in strips after the first winter.
• Skipping degreasing of the metal. At first sight everything looks fine, but adhesion is weak and the coating peels.
• Coating a damp surface. Crews sometimes start painting early in the morning while dew is still on the roof, or shortly after washing.
• Painting over rust. If rust is not removed mechanically, it keeps spreading under the coating and shows up as blisters within a year.
• Applying too thick a layer at once. The paint runs, wrinkles and does not dry properly inside.
• Unsuitable weather. Rain or a cold night shortly after application spoils the result even with a high-quality paint.
• Pressure washing asbestos cement. It spreads asbestos fibres around the property and onto neighbouring plots.
• Walking on the roof without protecting the covering. Cracked tiles and dented sheets then have to be repaired afterwards.
• Hiring doorstep crews. Teams that turn up unannounced offering quick, cheap roof cleaning often skip preparation and use unsuitable products.

A small, low metal roof, for example on a garage, shed or summerhouse, can be coated by a skilled homeowner. For the roof of a house at height it is safer to hire a contractor with scaffolding and fall protection. Steps for a small roof:

1. Inspect the roof and mark perforated areas, loose screws and joints.
2. Repair perforated areas and tighten or replace screws with sealing washers.
3. Cover windows, the facade and surroundings with sheeting and clean the gutters.
4. Remove moss, dirt and loose old coating with a scraper and brush.
5. Remove rust with a wire brush or grinder down to a firm surface.
6. Wash the roof with water and a cleaning agent and rinse thoroughly.
7. Let the roof dry completely.
8. Degrease the metal according to the coating manufacturer's instructions.
9. Brush joints, screws, edges and corroded areas first.
10. Apply primer to the whole surface and let it dry as stated in the data sheet.
11. Apply the topcoat in one or two layers, from the ridge to the eaves.
12. Check the roof once dry and touch up any missed spots.

Always use fall protection when working on a roof. Falls from roofs are among the most common serious injuries in DIY work on houses.

Cleaning and coating an existing roof counts as maintenance. Check with your local authority whether changing the roof colour needs approval, for example in a conservation area or on a listed building. In an apartment building the roof is a common part of the property, and the owners decide on coating it together.

A contractor working at height must provide fall protection, such as scaffolding, edge protection, anchor points or personal fall arrest equipment. Ask for references and proof of liability insurance, and check that the firm is properly registered for building or roofing work in your country. Asbestos cement roofs fall under occupational health rules on asbestos, and work on them should only be done by a firm authorised for asbestos work.

• the coating is continuous, without missed spots, runs or blisters,
• joints, screws, areas around chimneys, edges and eaves are coated as carefully as the open surfaces,
• no rust shows through the coating and the old coating is not flaking anywhere,
• the colour is even, without patches or marked differences in gloss,
• gutters and downpipes are clean and water drains freely,
• windows, the facade and surroundings are not splashed with paint,
• tiles and sheets are not cracked or dented after the work,
• you have been told which system was used, how many coats were applied and what warranty covers the coating.

You can check adhesion easily once the coating has fully cured. Stick a strong piece of adhesive tape onto an inconspicuous spot and pull it off sharply. No paint should come away on the tape.

The labour price in the NUMIQ price list is about €12 per m² for roof cleaning and coating. It includes small consumables but not the coating products themselves, which are charged separately according to the chosen system. Prices vary by country, region and city, and are usually higher in large cities than in rural areas. A higher-grade two-component system or additional coats will make the material more expensive.

The price is mainly affected by:

• the pitch and height of the roof and any scaffolding or lifting platform needed,
• how complex the roof is, with valleys, chimneys, dormers and roof windows,
• the condition of the covering, meaning how much rust and old coating has to be removed,
• the type of coating system and the number of coats,
• repairs needed before coating, such as replacing tiles or sheets.

Related work can be priced separately, for example installing guttering and gutter brackets or laying clay or concrete tile roofing if the roof can no longer be saved. You will find more items under sheet metal works.