Basements below ground level are at risk of groundwater penetration through the walls. There is also a risk of rainwater penetration through any part of the basement wall that is above ground level. Basement design solutions must be submitted for consent as an alternative method – they are not covered by E2/AS1.
Leaks through basement walls below ground level are extremely difficult to repair, and it is often hard to identify the location of the leak. In some situations, there are also part-height basement walls that support the concrete slab of an upper floor, and the backs of these walls are impossible to access for repair once the floor slab has been poured.
Ensure that basement walls are well dampproofed, as below-ground walls are often required to manage large amounts of groundwater.
The majority of leaks in basements occur in the following locations:
- At the joint between the basement floor slab and the walls.
- Through the floor slab.
- Through the dampproof system that is used on below-ground walls.
- At the top of the wall dampproofing system where it terminates above ground level and is exposed to rainwater.
- At the face of a basement wall that is above ground, between the top of the dampproofing system and the bottom of the upper wall cladding.
Designers and builders will need to be aware of all requirements for weathertight basement construction. They should ensure that a suitable dampproofing membrane is accurately installed by a specialist installer and is protected on all below-ground walls and that adequate drainage ensures that subsoil water is effectively drained.
Basement floors are formed by steel-reinforced concrete poured in situ.
All concrete floors must incorporate a damp-proof membrane (DPM) to ensure that moisture from within the ground does not penetrate through the finished slab. This can be formed by polythene sheet that is taped at the laps and laid over a compacted hardfill and sand blinding. Alternatively, a proprietary system can be specified.
The DPM must be installed under the full extent of the concrete slab and under any internal or perimeter foundations.
At the external edges of the slab, the DPM must be carried up and over the junction of the floor slab and the basement walls. The dampproofing system that is used on the walls must be lapped and sealed over the upstand of the slab DPM to ensure moisture does not penetrate the floor-to-wall junction.
The dampproof membrane specified must be compatible with the DPM to ensure that an effective seal can be achieved.
The finish of the floor-to-wall junction is critical to the dampproof performance of the completed system.
Basement walls that are below ground are structural retaining walls (as they need to retain the adjoining ground), and these are generally constructed in concrete block masonry. Some walls are also built with reinforced concrete poured in situ or precast concrete.
Any part of these walls that is below ground must be made waterproof by the application of a dampproofing membrane so that ground moisture can’t penetrate.
Below-ground dampproof membranes come in the following types (see the table below for characteristics and installation requirements):
- Liquid coatings (bitumen or tar emulsions, epoxies, polyurethanes and modified cement or bitumen).
- Modified bitumen-based sheet (self-adhesive and torch-on).
Each type has specific performance characteristics, and designers need to specify a system that is relevant to the particular application. Check that any dampproofing system specified is fit for purpose and is applied by a manufacturer-approved specialist.
|Characteristics and installation requirements of common below-ground damproofing systems|
|Liquid coatings||Applied to a dry surface by brush or spray (depends on coating type).
Conformable but does not bridge cracks well.
Requires a cure time before backfilling.
Bitumen-based sheet (self-adhesive)
|Pliable reinforced rubber/bitumen.||Applied to dry, clean surface.
Self-bonding at laps.
Some types require a primer.
|Bitumen-based sheet (torch-on)||Fully or spot bonded to a clean, dry surface by blow torch applied heat.
Once the selected dampproofing membrane has been applied to the basement walls, any membrane that is below ground needs to be protected before the wall is backfilled. Any damage caused while backfilling (such as backfill drainage metal puncturing the membrane) can potentially lead to water leaks.
Membrane can be protected with:
- proprietary drainage mat/protection sheet (made from PVC or polyethylene sheet)
- rigid sheeting (plywood, corflute or extruded polystyrene).
The use of sacrificial sheet protection (MDF, fibre-cement or plasterboard sheet) that may break down over time is not sufficient protection long term, as potential ground and building movement can lead to damage of the membrane once the protective board has deteriorated.
Before backfilling commences, ensure that any in-ground dampproofing membrane has been well installed by a specialist installer and that it is adequately protected.
Any membrane that is above ground level also needs to be protected. The exposed edge at the termination of a dampproofing membrane on a wall above ground level also needs to be sealed. This can be achieved with sealant to the joint or an overflashing to protect the exposed edge of the membrane. In all situations, the joint must be finished to ensure that water can’t penetrate behind the dampproofing membrane where it could potentially leak through the basement wall or cause the membrane to detach from the wall.
Groundwater can accumulate against in-ground walls. Subsoil drainage must be incorporated to ensure that water does not accumulate (where the increase in hydrostatic pressure may overload the membrane and cause it to leak) and that the water is drained effectively.
To do this, ensure that the building consent documentation is followed. Drainage systems should incorporate :
- a minimum 100 mm diameter perforated subsoil drainage pipe installed at the base of the wall on free-draining metal at a level above the bottom of the footing – ensure the invert level of the drain is a minimum of 200 mm below the finished floor level
- the pipe protected from clogging by a water-permeable fabric
- the full extent of the wall backfilled with free draining metal.
All subsoil drainage pipes at the base of in-ground walls need to have sufficient fall to ensure that effective disposal occurs and must be connected to a stormwater system. Pipe outlets need to be accessible so that they can be cleaned as required.
Most below-ground dampproofed basement walls fail because the dampproof system has been put under too great a load as the result of groundwater accumulation from poor subsoil drainage.
Updated: 23 February 2023