Torch-on bitumen membrane on vertical surfaces, single layer

Vertical bitumen waterproofing has one main job: to stop water and moisture from the soil getting into the masonry. On a roof, it stops water running across the roof surface from getting behind the waterproofing at a wall or parapet. Typical locations include:

• outside walls of cellars and basements before backfilling,
• strip footings and the plinth of a house, where the vertical membrane joins the horizontal damp-proof course under the wall,
• the inner face of a flat roof parapet, where the roof membrane is turned up,
• the junction of a terrace or balcony with the house wall,
• retaining walls, wells, inspection chambers and tanks from the outside.

Why use a torch at all? A torched membrane bonds to the substrate over its whole area. No voids form behind it that water could travel along, and the bitumen in the laps fuses into one piece. When the work is done properly, separate sheets become one continuous layer with no weak points at the joints.

When one layer is enough

A single layer of torch-on membrane is normally used against soil moisture and against water that does not build up pressure, meaning water that runs down and is carried away by a land drain or free-draining backfill. A single layer is also common for the upstand of a roof membrane on a parapet, as long as it matches the roof build-up. If the house sits on clay where rainwater stands around the foundations, or the water table reaches the basement floor, you are dealing with water under pressure. In that case the design usually calls for two layers, a different system or watertight concrete. That decision belongs to the designer, based on the ground and water conditions on your plot, not to the tradesperson on the day. If you are not sure what kind of water acts on your house, ask before anyone lights a torch.

The sequence varies a little between a basement wall in an excavation and a parapet on a roof, but the principle stays the same.

1. Check the substrate: it must be sound, dry and even, free of sharp projections, mortar snots and dust. External corners get rounded or chamfered.
2. Form a fillet, a sloped or rounded transition in the internal corner between the horizontal and vertical surface, from mortar or a preformed cant strip. That way the membrane never has to bend through a sharp 90 degree angle.
3. Coat the substrate with bitumen primer and let it dry as the manufacturer specifies, usually a few hours.
4. Cut the sheets to the height of the wall, plus an allowance for the lap onto the horizontal surface and for the top termination.
5. Roll the sheet up, set it against the bottom of the wall, heat the underside of the sheet and the substrate, and unroll it gradually upwards. Press it down with a roller or trowel so it bonds over the whole area.
6. On taller walls, fix the top of the sheet mechanically, temporarily or permanently, so it cannot slide down while it is still warm.
7. Torch the side laps so that a thin continuous bead of bitumen, usually about 5 mm wide, flows out along the edge.
8. Detail internal and external corners, pipe penetrations and anchors with extra patches cut from the same membrane.
9. Finish the top edge with a termination bar, fix it and seal the gap above it with a permanently flexible sealant suitable for bitumen.
10. Check all laps and photograph the finished surface before any protection layer covers it.

The junction with the horizontal waterproofing

The critical part of any vertical membrane is the bottom, where it meets the waterproofing under the foundation or on the roof. Usually the horizontal sheet is turned up the wall by at least a few centimetres and the vertical sheet laps over it, or the other way round, depending on the order of work. The lap needs to be wide enough, roughly 10 to 15 cm, and torched without any gaps. On basements it often turns out that the horizontal membrane sticks out only a little from under the slab and is covered in mud. That edge has to be cleaned, dried and primed. Otherwise the vertical sheet sticks to dirt, not to the membrane.

Many problems with vertical waterproofing start before anyone lights the torch. Before the crew turns up, the following should be in place:

• the wall is finished and the concrete or render has had time to cure and dry as the manufacturer requires, since fresh concrete holds a lot of water and the membrane will not stick to it,
• for a basement, the excavation is wide enough to stand safely at the wall and handle a torch, and the sides are battered back or shored against collapse,
• for a parapet, scaffolding or safe access with edge protection is available,
• combustible materials have been cleared away, especially polystyrene, timber, plastic sheeting and packaging,
• pipes, drains and cable ducts passing through the wall are installed beforehand, because drilling through finished waterproofing always creates a weak point,
• everyone knows how the membrane will be protected once it is finished and who will do it.

Weather matters a great deal. Torching is not possible in rain, fog or on a damp substrate covered in dew: water behind the sheet turns to steam and forms blisters. In cold weather bitumen stiffens and the sheet cracks when it is bent, which is why manufacturers state a minimum application temperature. SBS modified membranes can usually be laid in cooler weather than ordinary oxidised ones, but the figure on the data sheet still applies.

For vertical surfaces, contractors today almost always use modified bitumen membranes designed for torching, typically around 4 mm thick. The differences lie mainly in the type of bitumen, the reinforcement and the surface finish.

Type of bitumen

• SBS modified bitumen (elastomeric) is flexible, copes well with substrate movement and low temperatures, and is the usual choice for walls and details,
• APP modified bitumen (plastomeric) handles heat and sunlight well, but it is stiffer and less suited to corners and uneven walls,
• oxidised bitumen is the budget option with little flexibility, and on vertical surfaces it is now used only in exceptional cases.

Reinforcement

The reinforcement determines how strong the sheet is and how far it can stretch. A polyester mat is flexible and resists tearing, so it is the preferred option for basements and anywhere the building may move. A glass fibre mat or scrim holds its dimensions better but tends to crack if the substrate moves. A glass-reinforced sheet makes sense where the substrate is stable and settlement is not expected.

Surface finish

A sheet with a fine sand finish or a thin film on top suits use below ground and under insulation. On a parapet left exposed to the sun, contractors use a sheet with a coarse mineral (slate) finish that shields the bitumen from UV, or cover the membrane with metal flashing. If an unprotected sheet stays in the sun, the bitumen ages and cracks over time.

When not to use a torch

Wherever there is polystyrene insulation, timber or plastic window and door frames nearby, an open flame is a risk. Polystyrene melts with heat and can catch fire. In such places, self-adhesive bitumen membranes or cold-applied bitumen coatings are used instead, or the sequence of work is changed so that torching happens before the insulation goes on. This comes up often at the plinth of an externally insulated house and should be settled before work starts.

The price list has a separate item for torching a single layer of bitumen membrane onto a horizontal surface. The material is often identical, but work on a wall is different and slower. In brief:

• on a horizontal surface the whole roll is unrolled in one go and pressed down underfoot or with a roller, while on a wall the crew works with cut lengths and heats and presses each one separately,
• on a wall gravity acts on the sheet, so warm membrane tends to slip and peel at the top edge, which is why it needs mechanical fixing and a termination bar,
• a wall has more laps, corners and penetrations per square metre, and each detail takes time and skill,
• the operative works in an awkward position, in a trench or on scaffolding, and finds it harder to see whether the bitumen has flowed properly in the lap,
• on a horizontal surface water can stand, so the laps carry the highest load, whereas on a wall water runs off, so the correct direction of laps and a sound top termination matter most,
• vertical waterproofing needs protection straight away against damage during backfilling, while horizontal waterproofing is usually covered by concrete, screed or the next layer of the build-up.

In practice, waterproofing a basement means the estimate contains both items: the horizontal surface under or on the slab, and the vertical surface around the walls. On a flat roof most of the area is horizontal and the vertical part is only a strip along parapets and walls, yet that strip often decides whether the roof leaks. Measure the vertical area over the full developed height, including the lap onto the horizontal part and the termination, not just up to ground level.

The effort on a vertical surface does not depend only on the number of square metres. These factors make the biggest difference:

• wall height, because taller walls need joined or fixed sheets and work from scaffolding,
• the number of corners, recesses, piers and penetrations, since a long straight wall is far simpler than an irregular footprint,
• the condition of the substrate, because old stone or brick walls without a levelling coat need rendering or a skim first,
• removing old waterproofing, coating residues and soil in renovation work,
• access, as a narrow trench next to a neighbour's fence or a parapet without scaffolding slows the job considerably,
• nearby combustible materials and windows that need protecting or a different method,
• the weather, because damp and cold conditions interrupt the work.

When renovating an older house, digging out, cleaning and levelling the wall often takes the most time, and the torching itself goes faster. On a new build with a straight concrete or blockwork wall it is the other way round.

Vertical waterproofing sits in the middle of a chain of work, and its quality depends on how well the trades coordinate.

• with bricklayers and concrete workers: an even substrate, rounded corners, a fillet in the internal corner and properly cured concrete,
• with plumbers and electricians: all penetrations installed beforehand, ideally with a sealing collar or flange the membrane can bond to,
• with whoever insulates the basement: the adhesive for perimeter insulation boards must be compatible with bitumen, because solvent-based adhesives attack it,
• with whoever backfills: no backfill until the waterproofing has been inspected, no stones directly against the wall, and filling in layers,
• on a roof, with the roofer or sheet metal worker: the parapet coping must cover the top termination so water cannot get behind the membrane.

In a block of flats, for example when waterproofing the basement or roof, the work has to be agreed with the building management or owners' association, who make the decision. If the waterproofing is part of a larger building project, check with your local building authority what local building regulations require. Do not assume that no approval is needed.

Faults in vertical waterproofing are treacherous because nobody sees them once the trench is filled. They show up as damp patches, salt efflorescence or mould in the basement, sometimes years later.

• unprimed or dusty substrate, so the sheet only grips the surface and peels off over time,
• too little heat, so the sheet bonds only in spots and channels for water form behind it,
• too much heat, which burns the reinforcement, makes the bitumen run and weakens the sheet at that point,
• no mechanical fixing at the top, so the sheet peels away, folds over and water runs behind it,
• top edge below ground level, so water from the plinth gets behind it, when the membrane should normally finish above ground,
• a sharp internal corner without a fillet, where the sheet does not sit tight and tears if the building settles,
• laps that are too short or run the wrong way, letting water into the joint,
• penetrations wrapped only in sealant, which hardens and comes away with time,
• backfill containing rubble and stones dumped straight onto the membrane without a protection layer,
• a single layer where water under pressure acts on the wall.

Protecting the finished membrane is a separate but essential step. Below ground, contractors usually fit a dimpled drainage membrane with the studs facing the soil, perimeter insulation boards or another protection layer. Backfill goes in layers and is compacted carefully so the waterproofing does not shift or get punctured.

The torched membrane itself is finished as soon as it cools, usually within minutes. The overall duration depends on preparation. Primer needs anything from a few hours to roughly a day to dry, according to the manufacturer, and longer in cold, damp weather. Fresh render or a levelling coat must dry before priming. New concrete walls need time for excess moisture to escape, usually several weeks.

On a wall the crew covers fewer square metres per day than on a horizontal surface, because there are more details. A basement with a simple footprint and straight walls is often waterproofed within a few days, while an irregular footprint and old masonry take longer. Do not backfill straight after the crew finishes: have the waterproofing inspected and photographed first.

On safety: whenever an open flame is in use, a fire extinguisher must be on site, and someone should check the area for a while after work ends in case any material is smouldering. Propane cylinders must not stay in the trench, because the heavier-than-air gas collects at the bottom. If you smell gas or a fire breaks out, leave the area and call 112 or your local emergency number.

Leave the torching itself to professional waterproofers. Working with a torch in a trench or on scaffolding is risky, and mistakes are hard to spot. You can, however, help the crew a lot by preparing the substrate, and you also make sure the membrane has something sound to bond to.

1. Dig away the soil along the wall to the agreed depth only as far as it is safe, and leave deeper excavation to a contractor who can shore it.
2. Clean soil, old coatings and loose material off the wall with a wire brush and let it dry.
3. Knock off mortar projections and round off sharp edges.
4. Fill larger holes and open joints with repair mortar and let it cure as the manufacturer specifies.
5. Sweep or vacuum dust off the wall face and the horizontal edge of the footing.
6. Apply bitumen primer with a brush or roller in an even thin coat, but only after agreeing the type of primer with the waterproofer.
7. Let the primer dry as the data sheet says and protect it from rain and dust.
8. Clear polystyrene, timber and other combustible materials from the area.

Inspect the work before anything covers it. Walk the full length of the wall and gently test the lap edges with a blunt scraper: you should not be able to lift them. At handover, check the following:

• The membrane is bonded over its whole area with no blisters, creases or lifting edges
• A continuous bead of bitumen is visible along the side laps
• Laps are wide enough and run so that water flows over them, not into them
• There is a fillet in the internal corner and the membrane sits tight in it with no void
• Corners and penetrations have extra patches and are sealed tightly
• The bottom lap onto the horizontal waterproofing is continuous and fully torched
• The top edge has a termination bar, is fixed and sealed with sealant
• The membrane finishes above future ground level or under the parapet coping
• There are no burnt spots or mechanical damage on the membrane
• You have photographs of the whole surface and the details, plus the product details of the membrane used