Torch-on or liquid membrane: which fits your roof?
These are the two systems most Johannesburg contractors discuss for a flat or low-slope roof. Neither is better in the abstract; each fits a different roof, and the difference comes down to shape, detail and how the deck behaves.
How torch-on works
A torch-on membrane is a reinforced bitumen sheet, supplied in rolls, that is heat-bonded to the deck with a torch and overlapped at the seams. The heat melts the underside of the sheet and the surface of the layer below, so the two fuse into a continuous skin. The laps are the critical part: a lap that is not properly fused is a future leak.
Torch-on suits a sound, simple deck. On a large flat concrete roof with few penetrations it is fast, robust and proven. It does not like complicated shapes, because every corner and every pipe needs a detail cut by hand, and it does not tolerate a deck that moves or holds water.
How a liquid membrane works
A liquid membrane - often a liquid rubber or polyurethane - is poured or rolled onto the deck and cures into a seamless skin. It has no laps, because it is applied as a liquid, so the common failure point of a sheet system is designed out. It can follow a shape that a roll cannot, and it wraps around penetrations and upstands as one continuous surface.
A liquid membrane suits a complicated roof: many pipes, awkward junctions, a parapet with a difficult detail, or a shape that is not a simple rectangle. It usually needs more coats and longer curing time than a torch-on sheet, and it wants a dry, clean, primed surface to bond to.
The trade-offs, in plain terms
Torch-on is quicker on a big simple deck and gives a thick, tough layer. It relies on the laps and on a sound deck, and it needs a flame on the roof, which has to be managed around anything flammable. A liquid membrane has no laps, handles detail well and is easier to apply around obstacles, but it takes longer and is more sensitive to the surface and the weather during application.
Cost differs between the two, and so does the finished thickness and how the roof can be used afterwards. A flat roof that people walk on needs protection over either system, or a paved path, because traffic wears through a membrane at the corners.
How to decide
The decision is not about which product is better; it is about your roof. A large, simple, sound concrete deck with few details leans towards torch-on. A smaller roof with many penetrations, or a shape a roll cannot follow, leans towards a liquid membrane. A deck that is cracked or hollow needs repair before either system goes down.
Ask the contractor which system they propose and why, and ask them to explain it against the roof you have. A good answer will mention the deck, the details and the drainage, not only the product name.
The question to ask your contractor
The useful question is not which system is better, but which system suits this roof and why. Ask the contractor to walk you through the deck, the drainage, the number of penetrations and the shape, and to say how each of those points to one system over the other.
A reply that names the product and stops there is not a reason. A reply that talks about the surface, the details and how water leaves the roof is. If a roof could take either system, a good contractor will say so and explain the trade-off in time, thickness and cost rather than pretend only one answer exists.
Frequently asked questions
Can I use torch-on on a tiled roof?
Not usually. Torch-on is for flat or low-slope decks. A pitched tiled roof is normally coated or has individual tiles and flashings repaired.
Does a liquid membrane last longer than torch-on?
Neither lasts longer in the abstract. Life depends on the system, the deck and the maintenance. A well-laid system on a sound deck outlasts a good system on a failing one.
Which is more expensive?
They are priced differently and it varies by roof. Ask for the scope of each, because the cheaper one on paper may exclude the preparation your roof needs.