How many coats of liquid rubber do you actually need?

Three coats. That is the manufacturer's specification and it is what the guarantee assumes, and it is not a padding exercise to sell more drums.

The membrane only does its job at a certain cured thickness, roughly 3mm. Below that it is a film: it still looks black and shiny and waterproof on the day, but it has less rubber to stretch when the roof moves, less material bridging a hairline crack, and far less to lose to eight summers of Cape Town ultraviolet before it is down to nothing over the high points.

Notice how far apart the ends of that table are. The same drum does three times the area on a smooth slab that it does on slate, because a profiled or rough surface has far more actual surface than its floor area suggests. This is why anybody who quotes you a coverage figure without asking what the surface is has not thought about your roof.

1. 1Measure the area. On a pitched roof, measure the roof planes, not the floor plan.
2. 2Find your surface in the table and take the coverage per drum.
3. 3Divide area by coverage, then round up to the next whole drum. Nobody sells two thirds of a drum.
4. 4Add primer at roughly 6 m² a litre, and round that up too.
5. 5Add a little for detail work at bolts, laps and cracks, which drink more than flat field area does.

A worked example: 100 m² of Harvey tile at about 17 m² a drum is 5.9 drums, so six drums at R1,990, plus roughly 17 litres of Bonding Liquid Super+. That is materials of roughly R14,000 to R15,000 including VAT, and now you can judge any quote you are given against it.

Nothing, for a while. That is the trap. A two-coat roof is dry through the first winter and often the second. Then the thin spots, which are always the high points of the profile and the sun-facing slope, wear through first. You get pinholing rather than an obvious hole, so it does not present as a leak: it presents as damp that comes back and a homeowner who now believes liquid rubber does not work.