Bituminous Membrane Rolls: SBS vs APP, Thickness and Laps

What are membrane rolls and how do they work?

A bituminous membrane is a factory-made roll of polymer-modified bitumen with a reinforcement core and a polyethylene film, sand or mineral granules on its faces. Once the rolls are torch-welded to each other it acts as a continuous water barrier, turning the roof into a single sheet that sheds water as long as the laps are sound. The common roll size is 1 m wide by 10 m long according to Sika data sheets.

The three components that define a roll

US specifications separate types by modifier and reinforcement: ASTM D6164 (external source via the Consumer Safety Center) for SBS sheets with polyester reinforcement, ASTM D6163 for SBS sheets with glass fibre reinforcement, and ASTM D6222 for APP sheets with polyester reinforcement, all distinguishing grade G (granule surfaced) from grade S (smooth).

3 mm or 4 mm?

Greater thickness means more bitumen above and below the reinforcement, giving a wider margin for welding laps and higher puncture resistance. That is why 4 mm rolls are typically used in single-ply systems or as the cap sheet, while 3 mm is usually the base sheet in two-ply systems or used under full protection such as tiles. On exposed roofs, a 4 mm mineral-surfaced roll is preferred.

Recommended uses

The most common uses in Saudi Arabia are villa roofs beneath a protection screed and tiles, exposed roofs with mineral-surfaced rolls, and basement walls and foundations exposed to groundwater, protected before backfilling. Details are on the waterproofing page, and it is compared with other systems in the roof and building insulation guide.

Advantages and technical properties

• Factory-controlled thickness that does not depend on applicator skill as liquid systems do.
• High tensile and puncture resistance thanks to the reinforcement.
• Wide market experience with readily available materials and installers.
• Compatibility with bitumen primers and foundation coatings.

Disadvantages and cautions

• Laps are the first weak point: a few centimetres of incomplete weld are enough for a leak that is hard to trace beneath tiles.
• Fire risk: torch application requires removing combustibles, having an extinguisher at hand and a fire watch after work ends. See safety procedures.
• Blisters: welding onto a damp or uneven surface traps vapour that expands with heat.
• Ultraviolet: smooth and sanded rolls must not be left exposed; they need protection or a mineral cap sheet.
• Heat and cold: exposed SBS softens in extreme heat and APP is less flexible in cold, so the type is chosen by region and exposure.
• Many penetrations multiply the details and weld points.

Standard application steps

1. 1

   Surface and slope preparation

   Remove projections and dust, correct falls towards the drains, form rounded fillets where the roof meets the parapets, and confirm the surface is dry.

2. 2

   Primer

   Apply a bitumen primer (ASTM D41 type or as the data sheet recommends) and let it dry completely before welding.

3. 3

   Details first

   Weld reinforcing strips around drains, pipes and corners before the main field.

4. 4

   Laying and welding the rolls

   Start from the lowest point and work upwards so the laps shed water, directing the flame 50–150 mm ahead of the roll where it meets the substrate and keeping the torch moving.

5. 5

   Laps

   Side laps of 80–100 mm and end laps of 150 mm per the data sheet (reference manuals give minimums of 75 and 150 mm), with bitumen bleed-out of no more than about 6 mm and staggered end laps. On mineral rolls, granules in the lap area are embedded with a hot trowel before welding.

6. 6

   Inspection

   Run the trowel tip along every lap to find any opening, check for blisters, then flood test with the drains temporarily plugged for a duration set by the supervising engineer.

7. 7

   Protection and handover

   A protection layer and tiles, or the mineral cap left exposed, then a handover report with a written warranty whose term depends on the insulation type and roof condition under the warranty policy.

Climate suitability by Saudi region

For local work: waterproofing in Riyadh, waterproofing in Jeddah, roof insulation in Dammam and roof insulation in Al-Baha.

Estimated cost

Membrane waterproofing typically costs SAR 18–40 per square metre depending on thickness, roll type and number of plies, and a complete roof system (waterproofing and thermal insulation with protection) SAR 45–85 per square metre. These ranges are indicative, exclude VAT and are confirmed after a site inspection. Cost drivers: mineral or smooth roll, one or two plies, removal of old insulation, and the number of details and penetrations. To choose the system, request an inspection.