Bituminous Waterproofing Membrane — APP and SBS for Roofs and Basements

What it is

A bituminous waterproofing membrane is a factory-made sheet of polymer-modified bitumen carried on a reinforcing mat, rolled out and bonded to a roof slab, terrace, podium, basement wall or wet area. The polymer modifier gives bitumen properties it lacks on its own: APP (atactic polypropylene) makes it resist heat; SBS (styrene butadiene styrene) makes it elastic.

What the choice turns on commercially is where the water goes when it fails. A membrane is buried under screed, tiles, soil or a basement backfill, so a leak is found from below, months later, as stained ceilings, damaged fit-out or a wet electrical room. Repair means breaking out the finishes to find a defect that is usually at a lap or an edge. The sheet costs little relative to the cost of getting into it again.

APP vs SBS

Reinforcement and thickness

The reinforcement mat governs strength and how the sheet behaves at laps and over cracks.

• Polyester reinforcement stretches, tears less easily and copes with movement and handling, and is the usual choice for roofs and basements
• Glass fibre reinforcement is dimensionally stable but brittle; it suits base layers and situations with little movement

Thickness follows from use and exposure. A thicker sheet resists puncture from aggregate, reinforcement bar ends and site traffic better. Two-layer systems are used where the consequence of a leak is high, such as a basement under a water table or a podium over a car park. The specification should name sheet type, thickness, reinforcement, surface finish and number of layers. European product characteristics for roof sheets are set out in EN 13707, and for damp-proofing and basement tanking sheets in EN 13969.

Detailing: laps, upstands and penetrations

The sheet is continuous; the details are not. That is where workmanship decides the outcome.

• Laps are fully bonded to the width the manufacturer states, staggered so end laps do not line up, and checked by probing along the edge. A lap that lifts with a trowel is a leak waiting for rain.
• Upstands run up parapets, walls and kerbs above the finished floor level, with a fillet at the corner so the sheet is not bent through a right angle, and a proper termination (a chase, a termination bar or a cover flashing) so water cannot run behind the top edge.
• Penetrations for pipes, railing posts, HVAC stands and drains get purpose-made collars or sleeves bonded into the membrane, ideally fixed before the sheet is laid rather than drilled through it afterwards.
• Outlets are set low, with the membrane dressed into the outlet body, and the slab or screed laid to falls so water reaches them. A membrane cannot fix ponding caused by a flat slab.

Indian codes of practice describe the same sequence for bitumen-felt systems: IS 1346 for roofs and IS 1609 for damp-proofing below ground. On basements, the membrane is normally detailed with the RCC design, so the structure and foundation works and the waterproofing are sequenced together rather than as an afterthought.

Protection and exposure

Unless the sheet has a mineral-granule or other UV-protective top surface, it should not be left exposed. Indian sun oxidises bitumen quickly. Most terraces receive a protection screed laid to falls, then tiles or another finish; basements receive a protection board before backfilling. The protection also saves the membrane from later trades, which is where many roofs are damaged.

For large single-ply roofs where heat reflectivity and long exposed life matter more, a TPO roof membrane is the usual alternative to bitumen.

Common mistakes

• Laid on a damp or dusty slab without primer. The sheet never bonds and water tracks underneath to the nearest defect.
• Upstands cut short at door thresholds and parapets to suit a finish level decided later.
• No flood test before screeding. The leak is found after tiling.
• Penetrations added after waterproofing. Railing posts and HVAC supports drilled straight through a finished membrane.
• A flat slab with no falls. Ponding water finds every weak lap.
• Drains at the wrong level. Outlets set above the membrane leave a permanent pond; coordinate with plumbing and drainage before the slab is cast.

What to ask your contractor or supplier

• Is the sheet APP or SBS, what reinforcement, what thickness, and how many layers?
• Which product standard does the sheet conform to?
• What primer and surface preparation are proposed, and how is slab dryness checked?
• How are upstands terminated, and to what height above finished level?
• How are penetrations and outlets detailed?
• When is the flood test done, for how long, and who witnesses it?

Standards referenced

APP-modified sheets with polyester reinforcement under ASTM D6222/D6222M, and SBS-modified sheets under ASTM D6164/D6164M. Reinforced bitumen sheets for roofing under EN 13707, and bitumen damp-proof and basement tanking sheets under EN 13969. Codes of practice for bitumen-felt waterproofing of roofs under IS 1346 and damp-proofing under IS 1609. The waterproofing system, number of layers and detailing for a specific building must be confirmed by the project's designer and engineer.