"What should we use to waterproof this roof?" is the question we are asked most often, and the hardest to answer in one sentence. Material is only part of it. The rest is substrate condition, drainage design and site constraints. The same product applied to a dry, level new roof and to an ageing roof that ponds water will give completely different results.
Confirm four things before choosing a material
These four answers determine which materials are viable, and they determine the price. Without them, any material recommendation is guesswork.
- What the substrate is and what condition it is in. Reinforced concrete, an existing PU membrane, bituminous felt, or metal sheeting? Is the existing coating chalking, blistering or delaminating? The substrate dictates the primer, and whether the old layer has to come off.
- How well it drains. Is there a fall to the outlets? How long does water take to clear after rain? If water stands for hours, most water-based acrylic systems are unsuitable and a more water-resistant system is needed.
- Whether people will walk on it. Access for maintenance only is very different from regular foot traffic, equipment or a drying area. Trafficable roofs generally need a protective screed or a high mechanical strength PU system.
- Schedule and weather. Water-based materials need drying time, typically 8–24 hours between coats. If you only have a three-day dry window, a five-step system is the wrong choice.
Flat concrete roofs: the most common case
Most townhouses and apartment blocks in Taiwan have flat reinforced concrete roofs, and this is the largest share of waterproofing work.
For a sound substrate with normal drainage, the standard build-up is three layers: a water-based primer to seal the substrate and improve adhesion, a compound membrane as the main waterproofing layer (usually two coats, 1.0–1.6 kg/m² total), then a topcoat. The topcoat is the layer most often dropped to save money, but it is what blocks UV — without it the membrane below ages noticeably faster.
Where there are cracks or active leaks, the order reverses: stop the water first. Fill cracks with a rapid-set plugging mortar, and treat seeping areas with a crystalline penetrating material that reacts with the concrete to seal pores. These can be applied to damp surfaces, so you do not have to wait for the slab to dry. Once the substrate is stable, build up the three layers above.
Where the roof ponds water or needs greater film thickness, consider a heavy-build flexible cement (up to 3 mm in a single application), or move to a solvent-based PU system at 2.6–3.9 kg/m², which offers better water resistance and mechanical strength.
Metal roofs: the issue is heat and corrosion, not the sheet
On metal roofs, leaks almost always start at fastener penetrations, overlaps and corrosion perforations rather than the sheet face. So the sequence is: treat the rust, seal the joints, then coat.
Across the sheet face, heat is usually the bigger problem. A reflective thermal topcoat (65%+ reflectance) measurably lowers internal temperature while also providing waterproofing, at around 0.8 kg/m² in two coats. Where the structure moves significantly, seal the joints with a non-sag sealant before coating the whole surface.
Three common selection mistakes
- Buying only the main membrane and skipping primer and topcoat. The most common and most expensive false economy. The primer determines adhesion, the topcoat determines service life. The saving is usually repaid three to five years later as a full rework.
- Mixing products from different systems. If the primer and membrane come from incompatible resin families, blistering and delamination follow. Products within one system are designed to work together.
- Trading coverage rate for cost. Applying a 1.2 kg/m² product at 0.6 kg/m² leaves the film too thin to reach its stated elongation and weathering performance. Half the material can mean a third of the service life.
If in doubt, send us the site conditions
The above are general principles; every project is different. Send us drawings, site photographs, or simply the approximate area and location, and we will come back with a full material specification and quantity estimate.