Torch-applied Bitumen Felt - Horizontal Surface (Single Layer)

On a horizontal surface, gravity helps. The roll is laid out, torched and settles onto the base on its own. Typical examples are a ground slab, a concrete deck under a terrace or a flat roof. On roofs and terraces the base needs a gentle fall, usually at least roughly 1.5 to 2 %, so water does not sit in one place.

On a vertical surface, the installer has to hold the membrane in place while torching it, keep reheating it and press it on. The work is slower and needs more experience. It often happens in a narrow trench around the house. On tall basement walls the membrane is cut into shorter lengths and sometimes fixed mechanically at the top. Otherwise a long strip would pull the still-soft bitumen down under its own weight and the layer would thin out at the top. Once the trench is backfilled, the vertical waterproofing is practically out of reach, so any mistake is very hard to put right later.

The two layers must join up. If the wall membrane is not sealed watertight to the slab waterproofing, moisture gets round the corner and rises into the masonry. In practice, a damp basement is often caused not by the membrane itself but by a poorly made joint at this point.

• SBS-modified bitumen membrane stays flexible in frost and bridges small cracks in the base. It is a common, sensible choice for foundations, basement walls and house roofs
• Oxidised bitumen membrane is simpler and less flexible. It turns brittle in the cold and can crack, so it suits underlayers or less demanding areas
• Mineral-surfaced membrane is used as the top layer on a roof exposed to the sun. The granules protect the bitumen from UV light
• Membrane with an aluminium foil core is specified where radon is a concern. In that case the designer sets out the build-up based on a radon assessment of the site

Membranes for below-ground waterproofing are covered by the European standard EN 13969. When choosing, look at the label on the roll and the technical data sheet: bitumen type, reinforcement and thickness. A trade name tells you nothing about quality.

A single-layer system is meant for ground moisture in free-draining soil with a working drain, or for surfaces where water runs off quickly. With clay soil, a high water table, a slope with water running towards the house or a basement used as living space, a two-layer build-up is usually needed. That decision belongs to the designer, not to a guess on site.

1. Clean the base of dust, laitance, release oil residue and loose material. Fill holes and voids with repair mortar.
2. Round off internal corners and edges with a mortar fillet. A membrane cannot be laid over a sharp right angle without being under tension.
3. Check that the concrete has cured and is dry right through, not only on the surface.
4. Prime the whole area with bitumen primer and let it dry as the manufacturer specifies. A properly primed surface is evenly dark, with no shiny wet patches.
5. Lay the rolls out dry and line them up so the end laps of neighbouring strips do not meet at one point. Make the laps as the manufacturer specifies, usually about 10 cm along the sides and 15 cm at the ends.
6. Torch the membrane across its full width while warming the base at the same time. A small wave of molten bitumen in front of the roll shows the temperature is right. On a wall, always work from the bottom up so the upper strip overlaps the lower one in the direction water runs.
7. Press the membrane down with a roller. A thin, continuous bead of bitumen should squeeze out along every lap.
8. Take the wall waterproofing up above ground level, usually at least roughly 30 cm, and protect it with dimpled membrane or XPS boards before backfilling.

Pipe and duct penetrations should be fitted before the waterproofing goes on and sealed with a system collar or a collar torched from membrane offcuts. Drilling through the finished layer afterwards cannot be repaired reliably.

Working with an open flame is a job for a professional. Torching is done only in dry weather, without strong wind and at temperatures the manufacturer allows, usually from about +5 °C for modified membranes. Flammable materials nearby, such as timber, polystyrene or plastic sheeting, must be covered or kept at a safe distance, and a fire extinguisher should be at hand. Where the membrane meets insulation or timber, a self-adhesive membrane is used instead of torching.

The work goes fastest on a large, flat, clean area with free access. More time and care are needed for:

• Vertical walls, especially tall basement walls where the membrane has to be cut into lengths and fixed
• A narrow trench where there is no room to move around the wall. A trench roughly 60 to 80 cm wide makes the work much easier
• Lots of corners, penetrations, light wells and lowered courtyards
• An uneven or damaged base that needs repairing first
• Work outside the main season, when the weather forces breaks in the work

When renovating a flat roof, the old layer sometimes does not need stripping. It must, however, be sound, dry and free of blisters. On a block of flats, agree the work with the building management or owners' association first. If the renovation changes the roof build-up or adds insulation, check with your local building authority what approval you need.

• Missing or wet primer: the membrane bonds only in spots and peels off, often during backfilling
• Damp base: the water turns to steam when heated, lifts the membrane and leaves a void underneath
• Overheating: on walls it is tempting to heat for longer, but this burns the reinforcement and the membrane later tears under load
• Short laps or joints without a bitumen bead: water gets under the waterproofing and spreads across the area
• Working top-down on a wall: every joint then channels water into the structure
• Sharp corners without a fillet: the membrane stays under tension and cracks as the building settles
• Waterproofing stopped at ground level: splash water and melting snow get behind the membrane
• Backfilling with stony soil or rubble without a protective layer: the membrane gets punctured

You can only check the wall waterproofing before the trench is backfilled, so ask for photos of the corners, penetrations and the joint to the slab waterproofing. At handover, look for:

• A continuous bead of bitumen along the full length of every lap
• Laps that cannot be lifted by hand or with a scraper
• A surface without bubbles, ripples or shiny scorched patches
• Rounded corners with no membrane stretched taut
• Sealed pipe penetrations
• A watertight joint between the wall and the slab waterproofing
• A neat finish above ground level under a plinth trim or flashing
• A protective layer before backfilling and a working drain
• On roofs and terraces, no standing water after rain