An under slab vapor barrier is the polyethylene sheet laid on the compacted ground before a ground-bearing slab is cast. On Egyptian sites, people simply call it “nylon under the concrete”. It looks like a minor item. However, it often decides whether a floor finish stays bonded or starts to fail within months.
Contents
- Why Ground Slabs Need an Under Slab Vapor Barrier
- What an Under Slab Vapor Barrier Actually Does
- Choosing Thickness and Roll Size
- Practical Execution Steps
- When a Full Membrane Is Needed Instead
- Common Mistakes on Site
- FAQ
- Conclusion
Why Ground Slabs Need an Under Slab Vapor Barrier
Soil always holds some water. That moisture rises by capillary action and as vapor into the concrete above it. As a result, the slab never fully dries from below.
The symptoms appear later and far from the cause. For example, epoxy and vinyl floors blister, parquet swells, and tile adhesive loses grip. Salts can also crystallise at the surface and leave white stains.
In Egypt, this risk is common on ground floors of villas, warehouses and factories. It is especially relevant in the Delta and in coastal areas, where soils can be damp and saline. A cheap sheet placed at the right time prevents an expensive repair later.
What an Under Slab Vapor Barrier Actually Does
The sheet has three jobs. First, it slows vapor moving up from the soil. Second, it acts as a damp-proof layer against capillary moisture. Third, it stops the fresh concrete losing water and fines into dry, absorbent fill.
The Poliane data sheet describes its polyethylene film in exactly these terms. It is a vapor barrier and damp-proof membrane for concrete slabs and floors. According to the sheet, it resists alkalis, salts, acids and chemicals found in sub-soils.
That said, a plain sheet is not a waterproofing system. It does not resist groundwater under pressure. Instead, it controls moisture in soils that are damp but not flooded.
Under the Blinding or Under the Slab?
On Egyptian sites, the sheet often goes down before the plain concrete blinding under footings and ground beams. It is also laid directly under a ground slab cast on fill. In both cases, the goal is the same: to separate concrete from soil moisture.
However, the position changes with the floor build-up. Insulation boards or a waterproofing layer over the blinding alter the layer order. For that reason, we follow the designer’s drawings and confirm the details before work starts.
Choosing Thickness and Roll Size
A thicker under slab vapor barrier resists tearing better during reinforcement fixing and concreting. Thin film, on the other hand, punctures easily under boots, chairs and wheelbarrows.
The table below summarises the supply data for Poliane and two related products. Your project specification should set the final thickness.
| Item | Poliane PE sheet | Insucoat WB (bitumen coating) | Bitumode Delta-PT (APP membrane) |
|---|---|---|---|
| Role under or around slabs | Vapor barrier and damp-proof membrane | Damp-proofing coating, above the water table | Membrane listed for slab on grade |
| Thickness | 100 to 1200 gauge (25 to 300 microns) | 125 microns dry film at 4 m²/kg/coat | 4 mm |
| Roll / pack | Lengths up to 25 m, widths up to 4 m | Pails from 10 kg to 200 kg drums | 10 m × 1 m rolls |
| Key note | Resists alkalis, salts and acids in sub-soil | Protect with a 150-micron PE sheet during backfilling | Torch-applied on a primed surface |
Wide rolls reduce the number of laps. Therefore, a 4 m width suits large warehouse floors. Narrow rolls are easier to handle in small rooms and around foundations.
For guidance on vapor retarder classes, engineers often refer to ASTM E1745. Installation and inspection practice is covered by ASTM E1643. We do not quote class values here, because they depend on the chosen product and its own test certificate.
Practical Execution Steps
At Target, we treat the under slab vapor barrier as part of the slab, not as an afterthought. The sequence below reflects that approach.
- Compact the sub-base and remove sharp stones, rebar offcuts and debris.
- Add a thin sand blinding where the fill is coarse, so the film rests on a smooth bed.
- Unroll the sheet in the direction of concreting, keeping it flat and free of folds.
- Overlap adjacent sheets generously and tape the laps, following the project specification.
- Turn the sheet up at walls and ground beams so the edge sits above the slab level.
- Seal around pipes and drains with cut patches and tape, never with loose pieces.
- Place reinforcement on spacers that do not pierce the film.
- Inspect the whole area, patch any tears, and then pour.
Coordination matters here. In addition, the concreting crew must know that the film is a finished layer. Our concrete flooring works team plans walkways and pump lines before the pour for this reason.
Protecting Bitumen Coatings with the Same Sheet
Polyethylene also protects coatings on below-ground concrete. For instance, the INSUTECH sheet for Insucoat WB asks for a 150-micron polyethylene sheet during backfilling. Without it, stones and site traffic scratch the bitumen film.
Similarly, Insuguard RB60 is a rubberised bitumen emulsion for damp-proofing foundations in contact with groundwater. Its sheet gives 2 to 4 m²/kg per coat. It also states that the product acts as a concrete curing compound under ASTM C309.
When a Full Membrane Is Needed Instead
An under slab vapor barrier controls vapor. It does not hold back water pressure. So, if the water table can reach the slab, you need a real waterproofing layer.
Bitumode Delta-PT is one example from Modern Waterproofing. Its data sheet lists “slab on grade” among its uses. It is a 4 mm APP membrane on a non-woven polyester carrier. The sheet requires a Nirol primer and laps of 10 cm on the sides and 15 cm at the ends.
Wet rooms on the ground floor also deserve more than a sheet. In those cases, the membrane or a cementitious system carries the waterproofing role. The sheet below the slab remains useful, but it becomes a secondary layer.
Common Mistakes on Site
- Using thin, recycled film that tears before the pour.
- Laying the sheet over sharp fill without sand blinding.
- Leaving laps untaped, so moisture finds the gaps.
- Cutting the film around pipes and leaving open holes.
- Driving wheelbarrows directly on the sheet.
- Stopping the sheet short of walls, which leaves the slab edge exposed.
- Treating an under slab vapor barrier as protection against groundwater pressure.
Each of these mistakes is easy to avoid. Even so, they are the usual cause of damp patches near walls and drains.
FAQ
Is “nylon” the same as polyethylene?
On Egyptian sites, “nylon” is the everyday word for the clear or black sheet. Technically, the product is polyethylene film, such as Poliane.
Does an under slab vapor barrier replace a waterproofing membrane?
No. A sheet slows vapor and capillary moisture. Where water pressure is possible, a membrane such as Bitumode Delta-PT is required.
Should the sheet go under or over the insulation?
That depends on the floor design. The Poliane sheet lists use over insulation boards as well as under slabs. Your designer should fix the position on the drawings.
Can I use the same sheet to protect bitumen coatings?
Yes, where the coating data sheet asks for it. Insucoat WB, for example, calls for a 150-micron polyethylene sheet during backfilling.
What if the sheet tears during rebar fixing?
Patch it with a new piece that extends well past the tear on every side, then tape the edges. If a large area is damaged, replace that section before the pour.
Conclusion
An under slab vapor barrier is cheap, quick and easy to get wrong. Good results come from the right thickness, careful laps and a protected surface until the pour. Where water pressure exists, a proper membrane must take over.
Target supplies polyethylene sheets and installs them with slab, screed and membrane works across Egypt. If your project also needs a level base for finishes, see our screed works. Contact Target to review your ground-floor build-up before the pour.
For background, the vapor barrier article on Wikipedia gives a useful general overview.
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