# Bituminous Waterproofing Membrane — APP and SBS for Roofs and Basements

> A bituminous waterproofing membrane is a factory-made sheet of polymer-modified bitumen on a polyester or glass reinforcement, bonded to a roof, terrace or basement by torch or self-adhesive. APP-modified sheets tolerate heat well; SBS-modified sheets stay flexible and accommodate movement better. Most leaks occur at laps, upstands and penetrations, not through the sheet itself.

## 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

| Property | APP-modified | SBS-modified |
| --- | --- | --- |
| Modifier | Atactic polypropylene (plastomer) | Styrene butadiene styrene (elastomer) |
| Behaviour | Stiffer; resists flow at high temperature | Elastic; stretches and recovers |
| Low-temperature flexibility | Lower | Higher |
| Application | Mostly torch-applied | Torch-applied, self-adhered or hot-applied |
| Strengths | Heat resistance on sun-exposed roofs | Movement, cracking substrates, complex details |
| Typical uses in India | Terraces, roofs, sunken areas | Podiums, basements, roofs with movement |
| Standard (polyester reinforced) | ASTM D6222/D6222M | ASTM D6164/D6164M |

## 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](/services/construction/rcc-structure-and-foundations)
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](/glossary/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](/materials/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](/services/mep/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.

## Frequently asked questions

### Which is better for an Indian terrace, APP or SBS?

Neither is better in general; they suit different conditions. APP sheets are stiffer and tolerate high surface temperatures well, which is why they are common on Indian roofs. SBS sheets stay flexible at lower temperatures and stretch further, so they cope better with structural movement, cracking substrates and complex details. On a covered or protected terrace, either works if the laps and upstands are done properly.

### What thickness of membrane should be used?

It follows from the use and from the manufacturer's system. Thicker sheets resist puncture and site damage better, and exposed roofs or two-layer systems are specified differently from a protected basement or a sunken toilet. The specification should name the sheet type, thickness, reinforcement and number of layers, because 'bitumen membrane' alone lets the thinnest sheet be quoted.

### Torch-applied or self-adhered: which should I choose?

Torch-applied sheets are melted onto a primed surface with a gas flame and give a strong, fully bonded lap when done well. Self-adhered sheets use a pressure-sensitive adhesive layer and avoid open flame, which matters near timber, insulation, occupied buildings or fuel stores. Self-adhered sheets need a clean, dry, primed surface and are less forgiving of dust and moisture at laps.

### Can a bitumen membrane be left exposed to sun?

Only if the sheet is made for exposure, usually with a mineral-granule or other UV-protective top surface. A smooth sheet left in Indian sun oxidises, crazes and becomes brittle. Most terraces in India cover the membrane with a protection screed and tiles or a finish, which also protects it from foot traffic and later trades.

### How do I test whether the waterproofing works?

On flat roofs, terraces and wet areas, a flood or ponding test is the usual check: outlets are plugged, the surface is flooded for a period agreed in the specification, and the soffit and walls below are inspected for damp. Test before the protection screed and finishes go on, because a leak found after tiling means removing them.

### Why do new membrane roofs still leak?

Because the sheet is rarely the problem. Leaks come from poorly sealed laps, upstands that stop short or are not terminated, unsealed pipe and bolt penetrations, ponding where there is no slope to the outlets, and damage by later trades such as railing and HVAC installers drilling or dragging equipment across the roof.

## Sources

- [ASTM D6222/D6222M](https://www.astm.org/products-services/standards-and-publications.html) — Atactic polypropylene (APP) modified bituminous sheet materials using polyester reinforcements
- [ASTM D6164/D6164M](https://www.astm.org/products-services/standards-and-publications.html) — Styrene butadiene styrene (SBS) modified bituminous sheet materials using polyester reinforcements
- [EN 13707](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Flexible sheets for waterproofing — reinforced bitumen sheets for roof waterproofing
- [EN 13969](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Flexible sheets for waterproofing — bitumen damp proof sheets including bitumen basement tanking sheets
- [IS 1346](https://standards.bis.gov.in/website/published-standards/department-wise) — Waterproofing of roofs with bitumen felts — code of practice
- [IS 1609](https://standards.bis.gov.in/website/published-standards/department-wise) — Damp-proofing treatment using bitumen felts — code of practice

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Source: https://hagerstone.com/materials/bituminous-waterproofing-membrane