Torch-On Membrane Roof Installation Steps, from Primer to Flood Test

A bituminous membrane rarely fails because the roll itself is poor. It fails because one step was rushed: primer over a dusty slab, a lap welded in a hurry, or an upstand that was not carried high enough. This guide lays out the work in the order a disciplined site follows, and it points out what you should watch for as the owner.

What must be ready before the first roll is unrolled?

The work starts days before the rolls arrive. Sika's application guide for torch-on membranes requires a sound, smooth substrate free of sharp protrusions, clean, dry, and free of oil, dust and loose particles. For new concrete, the guide gives a curing period of 8 days to 3 weeks before application.

The torch-on membrane installation steps in detail

1. 1

   Final surface cleaning

   Sweep and vacuum the dust immediately before priming. A thin film of dust is enough to weaken adhesion.

2. 2

   Apply the bituminous primer

   Apply it at the rate in the product data sheet for your substrate and let it dry before welding, as Sika's guide states. Wet primer is flammable and weakens the bond.

3. 3

   Set out the roll direction

   Unroll the sheets dry first to fix their alignment. Start at the lowest point, at the drains, and work uphill so that water runs over the laps rather than into them.

4. 4

   Torch-weld the sheet

   The flame heats the underside of the roll and the substrate together until the bottom film begins to melt, then the roll is unrolled and pressed down. The guide warns against overheating because the polyester reinforcement melts at about 260 °C.

5. 5

   Set the laps

   Follow the lap width in the data sheet. For example, the SikaShield P34 S data sheet gives 100 mm for side laps and 150 mm for end laps. A bead of melted bitumen about 1 cm wide should appear along the whole seam.

6. 6

   Stagger the end joints

   End joints in neighbouring rows are offset by at least one metre so that several joints never meet at one point, and the corner of the sheet end is trimmed at 45°.

7. 7

   Upstands and details

   The sheet is turned up the walls and parapet and fixed at the top edge, while drains and penetrations get extra pieces that are welded in carefully.

8. 8

   Second layer where specified

   In a two-layer system, the second layer is laid so that it sits over the laps of the first, never directly on top of them, and is fully bonded.

Sika's guide also says not to apply the membrane below 5 °C, or during rain or high humidity. In a Saudi summer it is wise to work in the early morning, because the SikaShield P34 S data sheet sets a maximum application temperature of 40 °C.

Details that are often neglected: drains and pipe penetrations

Most leaks on new roofs start at the details, not in the open field. A drain outlet needs an extra reinforcing piece welded around it before the main layer, and the main sheet is then cut precisely and pressed into the drain throat. AC and plumbing pipes that pass through the slab are wrapped with a collar cut from membrane, lifted up the pipe and sealed at the top. Filling the gap with mastic alone is not enough.

It also helps to agree with the contractor, before work starts, where any new equipment will sit on the roof. Every base that is bolted down after waterproofing means holes in a layer that has just passed its test.

How is the flood test carried out after installation?

The flood test is the moment that exposes workmanship before it disappears under tiles. The ASTM D5957 (external source via the Consumer Safety Center) guide describes flooding the waterproofed area to a depth of at least 1 inch (about 25 mm) at the lowest point and no more than 4 inches (about 100 mm), for a period of no less than 24 hours and no more than 72 hours.

• Drains are plugged with tight temporary stoppers, and the plugs are watched.
• The ceiling below is checked for damp patches or dripping.
• The water level on the roof is monitored; a clear drop that evaporation cannot explain signals a leak.
• The roof is never loaded with more water depth than allowed, because the weight rises with depth.

If a leak appears, the water is drained, the point is located and repaired, and the test is repeated. Do not accept covering the roof before the test passes. If you need to trace an existing leak on an older roof, a leak detection survey is more reliable than guessing.

What happens after the test passes?

Smooth or sanded rolls are not left exposed. The SikaShield P34 S data sheet states that it is not suitable for permanent exposure to sunlight and needs a protection layer. So after the test comes a felt or separation layer, then protective screed or thermal insulation, then tiles, depending on the system chosen for your roof insulation.

If the design leaves the membrane exposed, the top layer must be a granule-surfaced product from the start. We explain the differences between roll types in APP vs SBS membranes, and layer thicknesses in the membrane thickness guide.

What should the owner watch during the work?

If you want disciplined execution with a documented flood test, Azl provides membrane insulation from surface inspection through to handover after the test.