Waterproofing Foundations and Basements

Protecting foundations and basements from ground moisture and groundwater: site investigation, external and internal waterproofing, drainage and junction details.

Damp in a basement is one of the hardest problems to fix, because the outer face of the foundation is buried and reaching it after construction means excavation. So foundation waterproofing does not tolerate a "we'll see later" approach: the choice must be made early, before the excavation around the foundation is backfilled.

First, get to know the site

The waterproofing system depends on how water will act. To establish this, geological and hydrogeological investigations are carried out: soil type, groundwater level and its seasonal variation are studied. Sandy soil drains water quickly, while clay holds it and can create a "bathtub" effect around the foundation.

The results fall into three basic situations:

• groundwater is well below the foundation, and only soil moisture acts on it;
• groundwater rises seasonally, or rainwater collects in the ground;
• groundwater is permanently at foundation level and acts under pressure.

In the first case a simple system may be enough; in the third, a multi-layer, pressure-resistant solution — and often drainage — is essential.

External waterproofing: the main line of defence

The most reliable approach is to stop water from the outside before it reaches the structure. A waterproofing layer is applied on the blinding concrete beneath the foundation slab and on the outer face of the walls. Polymer-modified bitumen sheets, synthetic membranes, bentonite mats or thick polymer-modified bitumen coatings are used here.

Key conditions:

• the waterproofing under the slab and on the walls must join without a break — that transition creates a sealed "tank";
• the membrane is covered with protection board or a dimpled membrane so it is not damaged during backfilling;
• backfill is free of large stones and construction debris and is compacted in layers;
• the waterproofing is carried up above ground level into the plinth zone.

If the basement will be heated living space, moisture-resistant insulation boards are also placed over the waterproofing. These boards protect the membrane at the same time.

Drainage: relieving water pressure

If the soil drains poorly or groundwater rises seasonally, a drainage system is installed around the foundation. Perforated pipes are laid along the edge of the foundation to a fall, in a gravel bed wrapped in geotextile. Collected water is led via inspection chambers to a point away from the house, to a drainage network or to a soakaway or sump.

Drainage does not replace waterproofing; it takes load off it. The pipe fall, the position of inspection chambers and keeping the geotextile from clogging with soil are decisive for the system working for years.

Surface water must be considered too. If roof downpipes discharge at the base of the foundation, even the best waterproofing is kept under constant load. Rainwater should be carried away from the house in separate pipes, and the ground and paving around the building should fall away from the walls. An impermeable apron around the building that sheds water outwards considerably reduces the amount of water reaching the foundation directly.

Joints and penetrations: where water most often gets in

Most foundation leaks come not through the main surface but through details:

• the construction joint between slab and wall — hydrophilic swelling strips or special joint profiles are installed here;
• where water, drainage pipes and electrical cables pass through the wall — these need dedicated sealed sleeves and collars;
• corners and projections — the membrane is reinforced with an extra layer;
• movement joints — detailed with an elastic system that accommodates movement.

These details should be shown in the design and put in place during concreting. A penetration added afterwards is always weaker.

Internal waterproofing for existing buildings

If a basement in an existing building is damp and external excavation is not possible, the work is done from the inside. Penetrating products on concrete walls, injection into cracks and joints, and cement-based waterproof render on the surface can be used. This approach stops water within the structure, but the wall itself may remain damp, so ventilation and humidity control in the basement are also needed.

Before internal work, the cause of the damp must be pinned down. Sometimes the problem is not the foundation at all but roof water discharging at the base of the building, a broken pipe or condensation. Waterproofing carried out without removing the cause gives only a temporary result.

Checklist

• Is geological and hydrogeological data available and included in the design?
• Do the slab and wall waterproofing join without a break?
• Have sealed details been chosen for pipe and cable penetrations?
• If drainage is needed, is it clear where the water will be discharged?
• Are photos and records of the concealed work being kept?

In private house and villa construction Zaferoğlu İnşaat plans foundation waterproofing to suit site conditions and checks concealed work stage by stage.