Construction of an external waterproof basement envelope

Solutions & Applications · Basement & Foundation

The waterproof basement envelope ‘encases’ the lower part of the building in a single, watertight shell. However, it has a distinctive feature that no other waterproofing system possesses: the majority of it will never be seen again. Once the floor slab has been concreted, any mistakes made underneath remain mistakes forever.

First, the water condition

The waterproof basement envelope is not designed in the same way for every basement. The first decision is a geotechnical one:

Condition	What does this mean?	Consistency in design
Percolating water	Rainwater that runs off and is drained away	Waterproofing and drainage — pressure is relieved
Seasonal aquifer	Rising water levels during periods of heavy rain	Designing to the highest standard, not the average
Permanent hydrostatic pressure	The basement is below water level	Complete waterproof basement envelope, joint protection, control of buoyancy in the floor slab

The third scenario is not merely a matter of waterproofing: the water pushes the floor slab upwards, and the structural analysis must take this into account. Waterproofing keeps the water out; it does not hold the building down.

Stage 1 — Substrate preparation & separation

Before the concrete is laid, a separation geotextile separates the ground from the structure: it prevents the mixing of materials, water loss from the fresh concrete and the ingress of fine particles.

Stage 2 — Horizontal waterproofing beneath the screed

This is the most critical stage, because once the concrete has been poured, access is impossible. Two technologies:

	Synthetic membrane	Bituminous membrane
Associations	Heat-sealing of seams	Thermal bonding with a blowtorch
Substrate	Can be laid freely	Requires an asphalt bonding primer
Advantage	Long service life, sealed seams	Widespread technical expertise amongst contractor crews
Please note	Chemical compatibility with adjacent materials	Flexibility suitable for a location not exposed to the elements

The membrane should extend at least 50 cm beyond the edges of the floor slab, so that it can later be turned up onto the walls.

Protection prior to reinforcement

On top of the waterproofing come steel bars, spacers, labourers and concrete. Without a protective layer, every spacer is a point of pressure and every piece of steel bar is a potential puncture. Protection is provided by a cement mortar or by a heavy geotextile for mechanical protection — which has the advantage of being installed immediately, without having to wait for it to cure.

Stage 3 — Vertical waterproofing & joints

Once the walls have been erected, the horizontal waterproofing is turned and joined to the vertical waterproofing with an overlap — and the basin is sealed.

For walls, cementitious waterproofing mortars are recommended, which are incorporated into the concrete. The critical point here is the interface: two different materials meet, and their compatibility and overlap must be specified prior to installation, not on site.

The floor-to-wall joint

The waterproof basement envelope is the first line of defence. However, the construction joint between the floor slab and the wall is the point through which water will enter if the membrane fails locally. A second line of defence is provided there: a water-swelling cord or waterstop tape, installed during concreting — as it cannot be fitted afterwards.

Penetrations

Every pipe passing through the roof — drainage, water supply, electrical conduits — constitutes a hole in the roof membrane. It is sealed individually, using a water-swelling ring or a specialised fitting, attached to the membrane. Never with foam.

Stage 4 — Drainage & wall protection

The dimpled drainage membrane protects the waterproofing during backfilling and relieves water pressure. At the base, a perforated pipe collects the water and drains it away. Where the backfill consists of fine-grained material — that is, when excavated material is reused — the membrane must have a built-in filter, otherwise the drainage channel will become clogged without warning within a few years.

The best joint is the one that is never put to the test: proper drainage means that no column of water forms on the wall, so the basin is not subjected to hydrostatic pressure.

Stage 5 — Thermal insulation

In habitable basements, the waterproof basement envelope is combined with external thermal insulation to prevent condensation. XPS is used – the only insulating material that can withstand permanent contact with damp ground without losing its thermal properties – and is installed on the outside of the waterproofing.

Below ground level, the slabs are bonded only — without dowels. Any mechanical fixing would involve drilling a hole through the waterproofing that has just been installed.

The scope of works

1. Geotextile for separation in the excavation.
2. Clean, smooth concrete, free from sharp edges.
3. Horizontal waterproofing with an overlap of at least 50 cm beyond the floor slab — and protection of the overlap from day one.
4. Protective layer over the waterproofing.
5. Waterstop at the junction between the floor slab and the wall.
6. Reinforcement and concreting of floor slabs.
7. Concreting of walls and sealing of penetrations.
8. Vertical waterproofing and connection to the horizontal — after cleaning the joint.
9. Adhered thermal insulation, with a separator where required.
10. Dimpled drainage membrane with a filter and a perforated pipe at the base.
11. Backfilling in layers, without large stones in contact.

Separation & Protection

Horizontal & Vertical Waterproofing

Joints & Penetrations

Drainage & Thermal Insulation

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