APP vs SBS membrane: the difference and which suits your roof in Saudi Arabia

When a contractor offers you a "4 mm roll", that is only half the information. The other half is the type of modified bitumen inside the roll, which determines how the layer will behave under the July sun in Riyadh, or in the cold winter nights of Tabuk and Abha. This article explains the difference between APP and SBS membranes in practical terms, without complicated chemistry.

What do APP and SBS mean in waterproofing rolls?

Ordinary bitumen softens in heat and turns brittle in the cold, so manufacturers add a polymer to widen the temperature range in which it works. APP stands for atactic polypropylene, a plastic polymer that raises the softening point and makes the blend stiffer. SBS is a synthetic rubber (styrene-butadiene-styrene) that gives the blend rubber-like behaviour, so it stretches and recovers.

American standards treat the two types in separate documents: ASTM D6222 (external source via the Consumer Safety Center) for polyester-reinforced APP sheets, and ASTM D6164 (external source via the Consumer Safety Center) for polyester-reinforced SBS sheets. A separate specification for each type is a sign that the difference is not a marketing one but in the performance itself.

APP and SBS membranes side by side

What do the data sheets say, in numbers?

The data sheet for SikaShield P24 MG (external source via the Consumer Safety Center), a 4 mm APP roll with a granular surface, states a softening point of not less than 145 C and no flow at 120 C for two hours, while its cold flexibility is at -2 C. The SikaShield P34 S data sheet (APP with a sand surface) gives cold flexibility at -5 C and elongation of roughly 35-40%. These figures show APP's strength, heat, and its relative weakness, cold.

SBS rolls are the opposite: they are designed to stay flexible at sub-zero temperatures and to tolerate greater elongation, which is why they are widespread in cold countries. In the Kingdom, winter cold in most cities does not approach the limits at which APP rolls crack, whereas roof surface temperature in summer can exceed air temperature by a wide margin.

Which suits the Saudi climate better?

The practical rule: the hotter the site and the less the roof moves, the more the balance tips towards APP. The more the roof moves, or the colder the nights, the more SBS's advantage shows. But some details change the decision:

• Riyadh, Qassim and the Eastern Province: high surface temperatures over a long season, and granular APP, or APP under tiles, is the common choice.
• Abha, Khamis Mushait, Al-Baha and Tabuk in winter: cold nights and rain, where SBS flexibility becomes a real advantage, especially on metal or light roofs.
• Jeddah and the coast: heat and humidity together; here the dryness of the surface at the time of welding matters more than the polymer type.
• Roofs over expansion joints or recent additions: concrete movement makes SBS elongation, or a dedicated joint detail, necessary.

How do you choose between the two, step by step?

1. 1

   Decide whether the roof will stay exposed

   If it is exposed, choose a granular-surface roll first, then look at the polymer type. If it will be covered with tiles or a protective layer, a sand or smooth surface is enough.

2. 2

   Assess heat and cold at your site

   Hot, dry regions lean towards APP, and highlands with cold nights benefit from the flexibility of SBS.

3. 3

   Check roof movement

   Wide roofs, roofs with expansion joints, or roofs on a metal structure need a material that absorbs movement or special joint details.

4. 4

   Ask for the technical data sheet

   Compare softening point, cold flexibility, tensile strength and elongation, and make sure the roll conforms to the standard named in the offer.

5. 5

   Ask about the installation method

   The welding method and the width of the laps determine the life of the waterproofing as much as the polymer type does, as we explain in the steps for installing membrane waterproofing on a roof.

Common mistakes when comparing APP and SBS

The commonest mistake we see is judging a roll by its thickness and colour alone. Two 4 mm rolls can differ a great deal in the type of reinforcement (polyester or glass fibre) and in polymer content. The second mistake is leaving a smooth roll exposed to the sun on the grounds that "APP tolerates heat"; tolerating heat is one thing, and resisting ultraviolet radiation is another.

The third mistake is mixing the two types in the layers of a system without consulting the manufacturer. Some systems allow a lower layer of one type and an upper of another, and some do not guarantee compatibility. Here the decision belongs to the data sheet and not to guesswork. For the general properties of bituminous rolls, see our guide to bituminous membrane.

And remember that a good roll does not make up for poor workmanship: an excellent APP roll with incomplete laps will leak before an average SBS roll welded with care. So ask about the installation method and the flood test with the same weight as you ask about the polymer type.

When is neither the best solution?

On roofs crowded with bases, pipes and air-conditioning units, each opening becomes an extra welded detail, and liquid systems may be easier to execute. We discuss this in our comparison of membrane rolls versus liquid waterproofing. On roofs that also need thermal insulation, the membrane is often built into a complete system in the roof insulation service.

If you want a specialist to assess your roof and suggest the roll type and number of layers, the membrane waterproofing service at Azl begins with a roof inspection and moisture measurement before any material is chosen. You can also read how thickness affects performance in our membrane thickness guide.