Ask three contractors to quote a leaking flat roof and you may receive three completely different systems — each presented as the only sensible choice. Usually each contractor is simply selling what he stocks and knows. The truth is that all three mainstream systems work well in the right place and fail predictably in the wrong one.
Torch-on bituminous membrane
Sheets of modified bitumen, typically 3–4mm thick, welded to the slab and to each other with a gas torch.
Where it belongs
- Exposed flat roofs and RC gutters — the mineral-finished grades take direct sun and foot traffic for maintenance
- Large, open areas where sheet material goes down fast and laps can be properly staggered
Where it struggles
- Cluttered roofs crowded with aircon plant and piping — every obstacle multiplies the detailing work, and details are where torch-on fails
- Occupied buildings where hot works are restricted by the management office
Liquid-applied PU membranes
Polyurethane coatings rolled or brushed on in two or three coats, curing into a seamless rubber-like skin.
Where it belongs
- Complicated roofs with many penetrations — liquid flows around pipe collars and brackets that defeat sheet systems
- Balconies and small exposed areas where mobilising a torch crew is uneconomic
Where it struggles
- Ponding areas — most economical PU grades soften under permanent standing water; falls must be corrected first
- Damp substrates: apply PU over a slab that has not fully dried and blisters follow within months
Cementitious systems
Polymer-modified cement slurries, applied in two coats, often with a mesh at corners and joints.
Where it belongs
- Bathrooms, kitchens and any wet area going under tiles — tile adhesive bonds to it beautifully
- Water tanks and planter boxes, in potable-safe grades
Where it struggles
- Anywhere exposed to sun and thermal movement — rigid cement coatings crack when the slab moves
- Structural cracks: a cementitious coat over a live crack simply cracks along the same line
And PU injection?
Injection is not a membrane but a repair technique: expanding resin pumped into a crack to chase out active water. It is the right first move for inter-floor condo leaks and basement walls, because it stops water from inside without hacking — but it treats the crack it is injected into, not the whole surface. Pair it with a proper membrane when the wet area above is due for renewal.
Three questions that reveal a good contractor
- “What preparation is included?” — grinding, crack routing and priming should be itemised. Preparation is most of the job.
- “How will you test it?” — wet areas should be flood-tested 24–48 hours before tiling. No test, no confidence.
- “Why this system for my surface?” — the answer should mention your substrate, exposure and drainage, not just the product brochure.
If you want those answers for your own roof, bathroom or balcony, our waterproofing crew surveys before specifying — and will happily tell you when the cheaper system is the right one.