Ask three contractors what to waterproof your roof with and you'll get three confident, contradictory answers — usually matching whatever each keeps in the store. Here's the comparison we give clients, with the trade-offs left in.
Torch-on bituminous membrane
What it is: factory-made bitumen sheets, typically 3–4mm thick, heat-welded onto a primed concrete surface with overlapped seams.
Where it wins: exposed flat roofs, RC gutters and large open decks. Consistent factory thickness means no thin spots, it tolerates ponding better than coatings, and a properly detailed installation runs 10–15 years. It's the system we put our longest warranty behind.
Where it loses: needs open flame, so it's out for enclosed spaces and near gas lines. Complex roofs with dozens of penetrations get expensive because every detail is manual work. Quality depends heavily on the applicator — a rushed seam is a guaranteed future leak.
Rough cost: mid-range per square metre; cheapest per year of service life on big simple roofs.
PU (polyurethane) systems — coatings and injection
What they are: two different tools sharing one chemistry. Liquid PU coatings brush or roll on to form a seamless rubbery skin. PU injection pumps expanding resin into cracks to chase and seal active leaks from inside.
Where they win: coatings excel on complicated surfaces — balconies with railings, roofs crowded with pipe penetrations — because liquid flows around details that sheets must be cut to fit. Injection is unbeatable when you can't hack: sealing a condo bathroom leak from the ceiling below without touching the upstairs unit's tiles.
Where they lose: coating thickness depends on the applicator's discipline; two thin coats fail years before the datasheet says. UV exposure degrades cheaper grades. Injection stops the leak path it finds but doesn't waterproof the whole slab — it's a repair, not a system.
Rough cost: coatings similar to torch-on; injection priced per point, economical for isolated leaks and poor value for widespread failure.
Cementitious slurry
What it is: polymer-modified cement, brushed on in two or three coats, bonding chemically with concrete.
Where it wins: water tanks, lift pits, basements and wet areas that will be tiled over. Handles constant immersion, is vapour-permeable so trapped moisture doesn't blister it, and it's the most economical system per square metre.
Where it loses: it's rigid. Any structure that moves — exposed roofs cycling through heat, decks with live load — will crack it at the joints. Never the right answer for an exposed flat roof, whatever the quote says.
Rough cost: lowest of the three; the saving disappears fast if it's specified where flexibility was needed.
The honest summary
- Big exposed flat roof: torch-on, almost every time.
- Balcony or detail-heavy deck: liquid PU coating.
- Active leak you can't hack open: PU injection.
- Tank, basement or wet area under tiles: cementitious.
The system matters less than the substrate prep and the detailing — a cheap system installed carefully outlasts a premium one slapped on. That's the uncomfortable truth of this trade. Our waterproofing page explains how we prepare and test, and if you send photos through the contact form we'll tell you which system fits your case — including when the answer is the cheaper one.