What Torch-On Actually Is
Torch-on is a bitumen membrane that arrives as a roll and is bonded to the roof with a gas torch. The installer melts the underside of the sheet as it unrolls, and the molten bitumen fuses to the primed substrate beneath.
Two grades matter. SBS-modified membrane stays flexible in the cold and handles movement. APP-modified membrane holds up better under UV and heat. Most Cape Town roofs run a 3mm base layer with a 4mm mineral-finish cap sheet over it, though the specification follows what the roof does and what sits underneath.
The material is rarely the problem. Torch-on has been used on flat and low-pitch roofs for decades and it works. What separates a roof that lasts from one that leaks in three winters is almost entirely the detailing.
It Never Fails in the Middle
Walk any failed torch-on roof and the sheet field is usually intact. The water is coming in somewhere else.
Laps. Where two sheets overlap, the bond has to be continuous. Side laps need roughly 100mm of overlap and end laps closer to 150mm, staggered so four corners never meet at a point. A properly torched lap shows a small bead of bitumen squeezed out along the seam. No bead means the installer moved too fast and the bond is partial. That seam will open.
Upstands. Every vertical the membrane meets, a parapet, a wall, a plant plinth, needs the sheet carried at least 150mm above finished roof level and terminated properly, either chased into the wall or held with a termination bar and sealed. Membrane stopped short and smeared with mastic is the most common failure we find.
Outlets. Water leaves at the outlets, so they carry the most water for the longest. The membrane has to be dressed into the outlet, not just up to it.
Trapped moisture. Torch-on cannot be laid over a damp substrate. Water in the screed turns to vapour under a black membrane on the first hot day, and the sheet blisters. Blisters split, and then you have a leak with no obvious source.
There is also a fire consideration. An open gas torch does not belong against timber, insulation board or anything combustible. Those junctions need a self-adhesive or cold-applied detail.
When Torch-On Is the Right Choice
Torch-on suits flat and low-pitch roofs, concrete decks, and any roof where the membrane will be walked on or covered. It is tough, it takes traffic, and a mineral cap sheet handles UV without a separate coating.
It is less suited to complex roof shapes with many penetrations, where a seamless liquid system means fewer joints to detail. It is also the wrong call on a slab that is spalling, because a membrane over failing concrete only delays the real repair.
We will say so when that is the case, even when it means a smaller invoice.
Torch-On and the Cape Town Winter
Cape Town takes roughly 70% of its rain between June and September, arriving on north-westerly cold fronts at 50 to 65kph. Sideways rain finds upstands and terminations that vertical rain never touches, which is why the verticals matter more here than the flat field.
The summer south-easter is dry, so it will not put water through the roof, but it lifts anything not properly bonded and abrades exposed edges. A partially bonded lap survives winter and peels in December.
The rain window also drives scheduling. Torch-on needs a dry substrate, so March and April are the months to book.
