A raft slab waterproofing membrane has one job that nobody can redo later. It must keep groundwater out of the basement for the life of the building. Once the concrete is poured and the soil is back, nobody can reach the membrane again. So the choice of roll, primer and details matters more here than anywhere else. This guide covers double-layer, torch-applied APP systems and how Target installs them below the water table.
Contents
- When groundwater calls for a double-layer raft slab waterproofing membrane
- Choosing the roll: Alpha-P, Beta-P or Insuextra P-E
- Primer: the bond starts here
- Laps, turn-ups and the raft-to-wall junction
- Protection board before backfilling
- Execution steps for a raft slab waterproofing membrane
- Common mistakes to avoid
- FAQ
- Conclusion
When groundwater calls for a double-layer raft slab waterproofing membrane
Cold-applied bitumen coatings work well for foundation waterproofing where soil is only damp, which is classic damp proofing. However, a raft that sits in groundwater faces constant water pressure from below and from the sides. In that case, a single coat or a single roll leaves no margin for a faulty lap.
Two torch-applied layers give that margin. That is why a double-layer raft slab waterproofing membrane suits rafts below the water table. In fact, the Bitumode data sheets from Modern Waterproofing list a “double layer system” for basements, tunnels and underground structures. They also list retaining walls and under-raft foundations as uses.
APP stands for atactic polypropylene, a plastomer added to the bitumen. It raises the softening point, so the roll stays stable in Egypt’s summer heat during installation.
Choosing the roll: Alpha-P, Beta-P or Insuextra P-E
The main question is how much water pressure the raft slab waterproofing membrane will face. The structural or geotechnical designer usually defines the design water level. After that, we match the membrane to it.
Bitumode Alpha-P for high water pressure
Bitumode Alpha-P uses a composite polyester carrier: a non-woven polyester mat armoured with glass fibre. Its sheet names it for underground structures “subject to high water pressure”. It carries the EOS quality mark of the Egyptian government and refers to ASTM D6222/2016.
Bitumode Beta-P for moderate water pressure
Bitumode Beta-P uses a non-woven polyester carrier. Its sheet lists the same double-layer system, but for “moderate water pressure”. It also lists retaining walls and under-raft foundations. Both Bitumode products come from Modern Waterproofing, which the sheets describe as “A Sika Company”.
INSUTECH Insuextra P-E
Insuextra P-E is an APP membrane on a non-woven polyester carrier. INSUTECH lists foundations, underground structures and slab on grade among its uses. Its sheet also gives a hydrostatic-head resistance of 20 m (ASTM D5385).
| Property (4 mm, from data sheets) | Bitumode Alpha-P | Bitumode Beta-P | Insuextra P-E |
|---|---|---|---|
| Reinforcement | Composite polyester + glass fibre | Non-woven polyester | Non-woven polyester |
| Stated water pressure use | High | Moderate | Hydrostatic head 20 m |
| Tensile, long./trans. (N/5 cm) | 900 / 600 | 800 / 600 | 1000 / 800 |
| Cold flexibility (EN 1109) | −10 to −15 °C | −6 to −10 °C | −10 to −15 °C |
| Softening point | ≥ 148 °C | > 147 °C | ≥ 150 °C |
| Roll size | 10 m × 1 m | 10 m × 1 m | 10 m × 1 m |
You can compare these with other rolls on our bitumen membranes page.
Primer: the bond starts here
A torch-applied membrane is only as good as its bond to the concrete. Therefore, every Bitumode sheet asks for a primer coat first, using Nirol W, WM, WB, R or S.
Nirol-W is a cold-applied bitumen emulsion from the same maker. Its sheet lists foundation footings, retaining walls and basements among its uses. It also states that it resists chlorides and sulphates likely to be present in groundwater. The product complies with ASTM D1227 Type III Class I and DIN 18195 Part 2.
As a primer, you dilute Nirol-W with 10–20% water and apply one coat at 200–250 g/m². For INSUTECH membranes, the sheet gives an INSUPRIMER rate of 0.2–0.3 litres/m². In both cases, let the primer dry before torching.
Laps, turn-ups and the raft-to-wall junction
Most leaks in a raft slab waterproofing membrane start at a lap or a corner. So the details deserve as much care as the field area.
- Side and end laps. Bitumode sheets ask for 10 cm side laps and 15 cm staggered end laps. The INSUTECH sheet gives 8–10 cm side laps and 12–15 cm end laps.
- Second layer. Lay it fully bonded over the first, with its laps offset from the laps below.
- Corners. At every 90° internal corner, form a 5 × 5 cm sand-cement cant strip. This stops the roll from over-bending.
- Turn-ups. Carry both layers up the raft edge, then leave a protected lap for the retaining-wall membrane.
The raft-to-wall junction is the most sensitive detail. The horizontal membrane goes down before the raft is poured, while the wall membrane follows weeks later. So we leave a generous tail of both layers, protect it during steel fixing, and then torch the wall layers onto it with a full lap.
Torch the seams from above while pressing the top sheet with a trowel. At the same time, keep the flame moving so that one spot does not overheat.
Protection board before backfilling
Backfill stones and compaction equipment can puncture a membrane in seconds. For this reason, the vertical membrane on retaining walls needs a protection layer before any soil goes back.
Fox Cartotech G400 is a 3 mm polypropylene board, supplied in 1 × 2 m sheets. Its technical table gives a puncture resistance of 1100 N (BS EN ISO 12236). The maker also states that acids, bases and salts in contaminated water tables do not affect the board.
To fix it, lightly torch the membrane in separate spots until the surface turns sticky and shiny. Then press the board on. The sheet recommends an overlap of 3–5 cm between boards. In addition, its table allows about one month of sun exposure, so plan the backfill soon after.
Execution steps for a raft slab waterproofing membrane
At Target, we work in this order:
- Confirm the design water level and choose the roll grade with the consultant.
- Prepare a smooth, clean, dry blinding concrete and form the cant strips.
- Apply the primer and allow it to dry.
- Torch the first layer fully bonded, with the stated side and end laps.
- Torch the second layer with offset laps.
- Inspect every lap with a trowel tip and repair any open edge.
- Protect the horizontal membrane before reinforcement work starts.
- After the walls are cast, continue the two layers up the wall and fix the protection board.
Our bituminous sheets page lists the other torch-applied products we install.
Common mistakes to avoid
- Choosing by price, not by water pressure. A moderate-pressure roll may be under-specified for a deep raft.
- Torching onto wet primer. The sheet requires a dry primer first.
- Aligning laps in both layers. Offset them so a weak spot never stacks.
- Skipping cant strips. Sharp 90° bends stress the membrane.
- Leaving the board in the sun for weeks. Backfill within the period the sheet allows.
- Driving or dumping stones on unprotected membrane.
FAQ
Is a single layer enough under a raft?
The Bitumode sheets name the double-layer system for underground structures under water pressure. A single layer leaves no backup if one lap fails.
Can I use a cold bitumen coating instead?
Cold coatings suit damp soil without water pressure. For a raft in groundwater, a raft slab waterproofing membrane in two torch-applied layers is the safer choice.
How long does the primer need?
The Nirol-W sheet gives about 2 hours to bone dry and 2–3 days for complete hardening. Torch only when it is dry.
What is the practical difference between Alpha-P and Beta-P?
Both are 4 mm APP rolls, 10 × 1 m. However, Alpha-P has a composite polyester and glass-fibre carrier, and its sheet names it for high water pressure. Beta-P has a non-woven polyester carrier and is listed for moderate pressure.
Do I need protection on the horizontal membrane?
Yes, steel fixing and concreting can damage it. Protect it before the reinforcement crews start.
Conclusion
A raft slab waterproofing membrane works when three things line up. You need the right roll for the water pressure, a dry primed substrate, and careful laps and turn-ups. Then a protection board must cover the walls before backfilling.
Target supplies and installs double-layer APP systems for rafts and retaining walls across Egypt. To review your foundation design or ask for a site visit, contact our team.
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