Why Most People Get This Decision Wrong
Homeowners ring around for a price per square metre. Three contractors quote, the cheapest wins, and the same roof leaks again in four winters.
The problem is that a price per square metre only makes sense once the system is settled, and the system is settled by the roof. A flat concrete deck holding water for two days after rain has nothing in common with a tiled pitch shedding it in minutes. Applying the same product to both means one of them is wrong.
Before anything else, the roof needs looking at. What it is made of, where the water goes, what has already failed and why.
Which System for Which Roof
| Roof | Usual answer | What decides it |
|---|---|---|
| Concrete flat roof or deck | Torch-on, often two layers | Foot traffic, falls to the outlets, and the condition of the slab |
| Flat roof crowded with pipes, plinths and junctions | Liquid membrane | Seamless detailing around penetrations, applied at the right thickness |
| Tiled pitched roof | Underlay, ridge and valley work, flashings | Tiles shed water. The waterproofing is underneath them, not painted on top |
| Metal roof, IBR, corrugated or Chromadek | Fixings, laps and flashings, then a flexible coating if warranted | The sheeting usually outlasts the screws that hold it down |
| Balcony, terrace or podium deck | Membrane under the tiles, carried up at the threshold | Falls away from the door, and a finished level below the internal floor |
| A sound roof that needs to run cooler | Reflective coating | Reflectivity fades unless the surface is washed |
Concrete decks and flat roofs hold water, so they need a membrane that tolerates standing water and a substrate that falls to an outlet.
Tiled pitched roofs are drainage systems, not sealed ones. Coating a tiled roof to waterproof it is the wrong instinct and usually traps moisture.
Metal roofs rarely fail at the sheeting. The fixings go first, then the laps and flashings, and any coating has to move with thermal expansion.
Get the roof type right and the product choice follows. Get it wrong and no product saves you.
Torch-On, Liquid Rubber and Reflective Coatings
Torch-on is a bitumen membrane bonded to the deck with a gas torch, in one or two layers. It is tough, takes foot traffic and tolerates standing water better than most systems. It fails at laps and upstands, where the bond depends on the person holding the torch.
Liquid rubber and liquid membranes go on cold and cure into a seamless skin. With no laps, they suit roofs crowded with pipes, plinths and awkward junctions. They fail when they go on thin, which is why thickness is measured wet during application.
Reflective coatings are light-coloured elastomeric coatings that reflect heat. They are worth it on metal and concrete roofs in full sun, applied over a roof that is already watertight. Dust and salt grey the surface, so they need washing to keep working.
Roof paint is none of these. It colours and protects the surface. It does not waterproof the roof.
How Long Roof Waterproofing Lasts
A correctly specified and correctly detailed system should give you well over a decade. The product brand matters less than three things.
Detailing. Upstands, outlets, laps and penetrations are where systems fail. A roof detailed properly outlasts the same membrane laid carelessly by years.
Drainage. Every system ages faster under standing water. A roof that ponds shortens the life of whatever is on it.
Maintenance. Outlets cleared before winter, coatings washed, and small failures repaired while they are small. We check every roof we waterproof after its first winter, because that is when a weak detail shows itself.
Signs Your Roof Waterproofing Is Failing
- Blisters and bubbles in the membrane, especially ones that have split
- Laps lifting or edges peeling, often after a summer of south-easter
- Membrane pulling away at the top of upstands and parapets
- Water standing 48 hours after rain, which means the falls or outlets have gone
- Cracking or crazing across a liquid membrane or coating
- Ceiling stains that appear during fronts and dry out between them
A failing system is not always a full replacement. A sound membrane with local failures gets repaired. A membrane that is brittle and failing everywhere gets replaced, and we tell you which yours is.
Comparing Roof Waterproofing Quotes
A price per square metre only compares like with like. Before comparing quotes, check that each one states:
- The system, and how many layers
- Primer and substrate preparation, and who repairs failed screed or spalling concrete
- How upstands, outlets and penetrations are detailed
- Whether old layers are stripped off or overlaid
- Falls correction, if the roof ponds
- The guarantee, in writing, and what it covers
The cheapest quote is often the one that left one of those out.
What Actually Fails
Almost never the middle of the sheet or the middle of the coating. Failures cluster at:
- Upstands and parapets, where the membrane meets a vertical and was terminated short
- Outlets, which carry the most water for the longest
- Laps and joints, where the bond was partial
- Penetrations, every pipe, vent, aerial mount and plant plinth
- The substrate, where a sound membrane sits on failing concrete
This is why we quote preparation separately. It is the part that determines whether the waterproofing lasts, and the part most quotes hide.
The Cape Town Rain Window
Roughly 70% of Cape Town’s annual rainfall falls between June and September, and it arrives on north-westerly cold fronts at gales of 50 to 65kph. That is rain moving sideways.
Sideways rain tests verticals. Parapets, upstands, flashings and terminations take water that vertical rain never reaches, which is why detailing on the vertical surfaces matters more here than the flat field does.
The summer south-easter is dry and does a different kind of damage. It lifts anything poorly bonded and abrades exposed edges. A partially bonded lap survives winter and peels in December.
Most waterproofing systems need a dry substrate, so March and April are the right months to book work, not the middle of a June storm.
