Four things to confirm first
There is no universal roof product. These four conditions determine which materials are viable, and they determine the price.
- Substrate and condition — reinforced concrete, an existing PU or bituminous membrane, or metal sheeting? Is the existing layer chalking, blistering or delaminating?
- Drainage — is there a fall to the outlets, and how quickly does water clear? Where water stands for hours, most water-based acrylic products are unsuitable.
- Foot traffic — access for maintenance only is very different from regular use, equipment or a drying area.
- Programme and weather — water-based materials typically need 8–24 hours between coats. A three-day dry window rules out a five-step system.
Standard build-up for flat concrete roofs
Where the substrate is sound and drainage normal, the standard build-up is three layers:
Layer 1｜Primer — seals and consolidates the substrate. Coverage around 0.05–0.1 kg/m². Use a high-penetration grade on friable or chalked surfaces.
Layer 2｜Compound membrane — normally two coats at 1.0–1.6 kg/m² total, forming a continuous elastic film. Elongation ranges from 400% to 1500% depending on type.
Layer 3｜Topcoat — the layer most often dropped, and the one that blocks UV. Without it the membrane below ages noticeably faster. Where roof temperature is also a concern, a reflective thermal topcoat (65%+ reflectance) at around 0.8 kg/m² in two coats serves both purposes.
Where there are existing leaks or cracks
The sequence reverses — stop the water first, then build the membrane.
Structural cracks should be opened out, cleaned and filled with sealant or rapid-set plugging mortar. The plugging mortar sets in 3–5 minutes, does not shrink on curing, and stops ingress immediately.
Diffuse seepage is treated with a crystalline penetrating material, which reacts with the concrete to form crystals that seal pores and micro-cracks. It can be applied to damp surfaces, so there is no need to wait for the slab to dry.
For wider cracks and construction joints, embed a reinforcing mesh (commonly 300 g/m² glass fibre) within the membrane so stress is distributed rather than concentrated along one line.
Ponding water or greater film thickness
Two options: a heavy-build flexible cement achieving up to 3 mm in a single application without delamination, at around 1.2 kg/m² per 1 mm on horizontal surfaces; or a solvent-based PU system at 2.6–3.9 kg/m², with better water resistance and mechanical strength for trafficked or abraded roofs.
Metal roofs and corrugated sheeting
On metal roofs the leaks are almost always at fastener penetrations, overlaps and corrosion perforations rather than the sheet face. Treat the rust and seal the joints first, then coat.
Across the sheet face, heat is usually the greater problem. A reflective thermal topcoat measurably lowers internal temperature while also providing waterproofing and mould resistance. Where the structure moves significantly, seal joints with a non-sag sealant before coating.
Common causes of failure
- Applying over a substrate still above 10% moisture content; new concrete needs 28 days of curing
- Failing to remove chalking and failed coatings — new material bonds to the loose layer and lifts in sheets
- Omitting primer or topcoat — the saving is repaid three to five years later as a full rework
- Insufficient coverage — a thin film will not reach the published elongation or weathering figures
- Poor detailing at parapets, outlets and pipe penetrations — where leaks nearly always begin