What Is Torch-On Waterproofing?
Torch-on waterproofing uses modified bitumen membrane sheets — rolls of polymer-modified bitumen with a heat-activated underside — applied using a propane torch. As the torch is passed along the sheet, the underside melts and fuses to the substrate below, creating a seamless, continuous waterproof layer. Where sheets overlap, the laps are also heat-sealed, eliminating any gaps. The result is a monolithic membrane with no cold joints and no fasteners that could allow water entry. Torch-on waterproofing in Pretoria is the most widely used flat roof waterproofing method in South Africa and performs particularly well in the local climate.
Why Torch-On Is the Best Choice for Flat Roofs in Pretoria
- Durability: 15–20 year lifespan when correctly applied to a prepared surface
- Seamless bond: no joints or gaps in the membrane itself — only controlled laps which are also heat-fused
- UV and heat resistance: bitumen is naturally resistant to Pretoria's intense UV radiation
- Thermal flexibility: modified bitumen accommodates the expansion and contraction caused by Pretoria's temperature swings
- Proven in SA conditions: torch-on is the most widely used flat roof waterproofing system in South Africa
Where Torch-On Waterproofing Is Used
Torch-on is appropriate for concrete flat roofs, screed decks, internal gutters, parapet wall upstands, and balcony slabs — anywhere with a hard, non-combustible substrate that can accept a heat-fused membrane. It's less suitable for surfaces with significant combustible elements directly below, for substrates that can't withstand torch heat, or for intricate shapes with many penetrations where a liquid-applied system would be more practical. Where timber elements or insulation boards are present near the application area, cold-applied liquid systems are the safer and equally effective alternative.
The Torch-On Application Process
Surface Prep
All cracks repaired, surface cleaned and profiled. The most critical step.
Primer
Bitumen primer applied to improve adhesion to the substrate.
Base Layer
First membrane layer heat-fused to the primed surface with overlapping laps sealed.
Cap Layer
Second membrane layer torched over the base, offset from first layer laps.
Detail Work
All upstands, drains, and penetrations properly flashed and sealed.
Inspection
Full quality check before sign-off and client confirmation.
Torch-On vs Liquid Waterproofing
Torch-on excels on larger, simpler concrete flat roofs where its durability advantage — 15–20 years versus 10–15 for liquid rubber — makes the most impact. Liquid rubber is the better choice for surfaces with many penetrations, irregular geometry, or where open-flame application creates safety concerns. For a full comparison, see the torch-on vs liquid waterproofing guide. The right choice depends on the specific roof — an inspection confirms which is appropriate.
Read: Torch-On vs Liquid Waterproofing — Which Is Right for Your Roof? →Frequently Asked Questions
Torch-on waterproofing is a method where modified bitumen membrane sheets are heat-fused to the roof surface using a propane torch. The heat melts the underside of each sheet and bonds it directly to the substrate, creating a seamless waterproof layer with no cold joints. Laps between sheets are also torch-sealed. It is the most common flat roof waterproofing method in South Africa.
When correctly applied to a clean, sound, primed surface with adequate drainage, torch-on bitumen membranes last 15–20 years in South African conditions. The critical factors are surface preparation quality, correct drainage design, and proper detailing at penetrations and upstands. A poorly prepped surface or inadequate drainage will shorten the lifespan significantly regardless of the membrane quality.
Sometimes. If the existing membrane is well-adhered, sound, and compatible, a new layer can be applied over it. Where the existing membrane is blistered, delaminated, or contaminated, it must be stripped first — applying over a failed membrane produces a result that looks fine initially but fails quickly. An inspection determines which approach is appropriate for the specific roof.
Trained applicators manage open-flame risks carefully on every job. Where timber elements, insulation boards, or other combustibles are close to the application area, cold-applied liquid systems are used instead — they deliver the same waterproofing performance without the flame risk and are the recommended alternative for those situations.