A roof is one of the most exposed surfaces on any building. It is constantly subjected to UV radiation, rain, wind, temperature changes, dust, pollution and biological growth. Over time, these conditions can cause roof coatings to fade, chalk, peel, crack or lose adhesion. Concrete roof tiles can become friable, while on metal roofs, weathering may expose the substrate and lead to corrosion.
Repainting a weathered roof is therefore not only a cosmetic project. It is a technical maintenance process that helps restore protection, improve water resistance, reduce substrate deterioration and extend the service life of the roof coating system.
For long-lasting results, the correct coating system must be selected according to the roof substrate and the condition of the existing coating.
Weathered Roof: Why Roof Identification Matters
Before any roof is repainted, the substrate must be identified. Cement roof tiles, fibre cement sheeting, galvanised metal, mild steel and timber each require different preparation and priming systems. Applying a roof coating directly to the wrong surface, or over an unstable old coating, can lead to premature peeling, poor adhesion, rust breakthrough or coating failure.
Universal Roof Acrylic is a water-based, pure acrylic, long-life roof coating designed for cement tiles, fibre cement, sheeting, primed metal, gutters, palisades and wooden or concrete substrates. It is durable, UV resistant, fully water resistant, low VOC and has a 10-year life expectancy when applied as part of the correct system. It is not recommended for clay tiles.
Cement Tile Roofs
Cement roof tiles become weathered over time as the surface is exposed to sun, rain and changing temperatures. Older tiles may become porous, faded or chalky, and shaded roof areas may develop fungal or algae growth.
Before painting, the tiles must be clean, dry, sound and free from contaminants. Remove dust, loose particles, old flaking coating, foreign material, fungal growth and algae. Where biological growth is present, treat the affected areas with Fungal Wash before priming. Damaged or broken tiles should be repaired or replaced before coating begins.
For new cement tiles, fibre cement and sheeting, the Roof Acrylic system specifies priming with Acrylic Sealer or Bonding Liquid, allowing 4–6 hours drying time before overcoating. Acrylic Sealer is a water-based binder used as a base coat, while Bonding Liquid is a solvent-based binder used as a base coat before applying acrylic topcoats. On roof tiles, these products help seal pores and provide good bonding and adhesion before the final roof coating is applied.
Previously coated cement tile roofs must be assessed carefully. If the existing coating is in poor condition, remove the old coating with a high-pressure washer, followed by scraping and sanding. If the coating is sound, remove all loose and flaking paint, sand the existing coating and perform a cross-cut or hatch adhesion test. The Roof Acrylic TDS specifies a 90% pass rate according to ISO 2409:2013. Bare and repaired areas should be spot primed with Acrylic Sealer and allowed to dry for 4–6 hours.
Once preparation and priming are complete, apply two coats of Roof Acrylic, allowing 4–6 hours drying time between coats.
Fibre Cement and Roof Sheeting
Fibre cement and roof sheeting can become weathered, chalky and porous after long-term exposure. As with cement tiles, surface stability is critical. A coating will only perform if the substrate is clean, dry, sound and properly primed.
Remove dust, loose particles, chalked residue, flaking paint, fungal growth and algae. Treat visible fungal or algae contamination with Fungal Wash. Where the previous coating is in poor condition, remove the coating with a high-pressure washer, followed by scraping and sanding. Where the coating remains sound, sand the existing finish and test adhesion before recoating.
Acrylic Sealer or Bonding Liquid is recommended as the base coat for cement tiles, fibre cement and sheeting before applying Roof Acrylic as a topcoat. It should be applied as one coat and allowed to dry for 4–6 hours before applying the roof coating.
Apply two coats of Roof Acrylic, allowing 4–6 hours drying time between coats. Maintain the recommended film build and do not over-thin. For brush or roller application, Roof Acrylic should not be thinned. For spray application, the first coat may be thinned by 10% if required.
Galvanised Steel and Metal Roofs
Metal roofs require a different system because adhesion and corrosion resistance are key performance factors. New galvanised steel must be thoroughly cleaned and degreased before priming. Manufacturing oils, grease and surface contamination can prevent primer adhesion if they are not removed properly.
Use Zinc Cleaner to degrease galvanised steel and rust-free mild steel. Apply it by cloth, mop or brush, scrub with a bristle brush or broom, and rinse with copious amounts of water until a water-break-free surface is achieved. Repeat if necessary and allow the surface to dry thoroughly.
Previously coated galvanised steel must be clean, dry, sound and free from contaminants. Remove loose and flaking paint, sand the existing coating and test adhesion using a cross-cut or hatch test with a 90% pass rate according to ISO 2409:2013. Rusted areas must be cleaned back to bright metal and spot primed with Galvanised Primer, or Red Oxide Quick-Dry Primer where appropriate, allowing 4–6 hours drying time.
After priming, apply two coats of Roof Acrylic, allowing 4–6 hours drying time between coats.
Wooden Roof-Related Substrates, Gutters and Palisades
Universal Roof Acrylic may also be used on wooden substrates, gutters and palisades where the surface has been correctly prepared and primed.
Previously coated wooden surfaces should be sanded, and all loose and flaking paint must be removed. Rusted metal areas must be cleaned back to bright metal and spot primed with Galvanised Primer or Red Oxide Quick-Dry Primer. Bare or repaired wooden areas should be spot primed before applying the topcoat.
Treating Fungal and Algae Growth
Fungal and algae growth must be treated before the coating system is applied. Painting over biological growth can compromise adhesion and may allow contamination to continue beneath the new film.
Fungal Wash is used on dirty or weathered cementitious substrates where fungal growth is present. Apply generously to affected areas and allow it to absorb into the substrate. After 12 hours, scrub the area using copious amounts of water and allow it to dry completely. If residue is still visible, reapply the treatment. Once the substrate is fully dry, continue with the specified primer and topcoat system.
When Additional Waterproofing Is Required
A roof coating is not a substitute for repairing structural defects, but certain roof areas may require additional waterproofing support before the final roof coating is applied.
Acrylproof is a water-based acrylic waterproofing compound used with a geo-fabric membrane. It is flexible, durable, fully water resistant and must be overcoated. This makes it suitable for waterproofing details where a reinforced membrane system is required.
Uniproof is a water-based acrylic waterproofing compound with a reinforcing fibre membrane incorporated, meaning no separate membrane is required. It is fully water resistant, flexible, fibre reinforced and must also be overcoated.
Flexi Rubber is a water-based, pure acrylic, rubberised waterproofing coating with excellent stretchability and elasticity. It is suitable for interior and exterior use on a variety of substrates and helps seal out moisture, waterproof surfaces and seal vertical and horizontal hairline cracks. Flexi Rubber is UV resistant, quick drying, low VOC and has a 10-year life expectancy when correctly applied. Apply two coats of Flexi Rubber, allowing 6–8 hours drying time between coats, and overcoat with two coats of Universal Paints Roof or Decorative Paint where required.
These waterproofing products should be used where the roof detail requires waterproofing treatment, such as joints, laps, penetrations, flashings or repaired areas, and then overcoated according to the relevant TDS. They should not be used as a shortcut for replacing damaged sheets, fixing loose fasteners or correcting structural roof defects.
Application Conditions
Roof coatings should be applied only under suitable site conditions. The TDS application conditions are based on a surface temperature of 23°C and relative humidity below 50%. Drying and curing times may be extended by cold, damp, humid or poorly ventilated conditions.
Avoid painting when rain, mist, fog or condensation is expected before sufficient drying has occurred. Do not paint if substrate temperatures are below the recommended minimum or if relative humidity is too high. Although the coating may appear touch dry, full curing can take several days depending on film thickness and environmental conditions.
Do not wash or wet painted surfaces for 7 days after application.
Film Build, Spreading Rate and Coverage
Correct film build is essential to roof coating performance. Roof Acrylic has a recommended wet film thickness of 50–86 microns and dry film thickness of 35–50 microns. Its theoretical spreading rate is 13.6 m² per litre at 42.5 microns DFT, while the practical spreading rate is approximately 8.6 m² per litre at 42.5 microns DFT.
Coverage will vary depending on roof profile, porosity, texture, substrate condition, application method and applicator technique. Weathered, porous or profiled roofs will usually require more product than smooth, sound surfaces.
For best results, apply the complete system: correct cleaning and preparation, the specified primer or sealer, and two full coats of Roof Acrylic.
Common Causes of Roof Coating Failure
Premature roof coating failure is usually caused by poor preparation or the wrong coating system. Common causes include:
- Painting over dust, chalking, moss, algae or loose paint
- Not degreasing galvanised steel before priming
- Not removing rust back to bright metal
- Skipping primer on bare or repaired metal areas
- Using the wrong primer for the roof substrate
- Painting over damp surfaces
- Painting too close to rain, mist or condensation
- Applying paint too thinly
- Not allowing the correct drying time between coats
- Using incompatible third-party primers or coatings
- Recoating a failed coating without removing unstable layers
The Roof Acrylic TDS notes that compatibility is tested only with recommended primers, undercoats and coating systems supplied or approved by the manufacturer. Where third-party primers, undercoats or existing coatings are used, compatibility cannot be guaranteed unless prior testing and written approval have been obtained.
Restore and Protect with Universal Paints
A properly repainted roof can dramatically improve the appearance of a home or commercial property while restoring essential protection against UV, rain and weathering.
By following the correct preparation and coating system, you can improve adhesion, extend coating life and help protect the roof against South African weather conditions.
Visit your nearest Universal Paints direct outlet for expert advice on the correct preparation products, primers, waterproofing products and roof coatings for your next roof repainting project.
Always refer back to the Technical Data Sheets for the correct application.
