November 2006

Lift pits are perhaps the most vulnerable part of a sub-structure in any building and they are often overlooked as a critical element of the waterproofing strategy.

Generally, lift pits are located beneath the level of the lowest floor slab and often below the level of the proposed land drainage. Subsequently, pressure from ground water will be at its highest around a lift pit.

Careful consideration should be given to the form of construction and waterproofing of lift pits for a number of reasons, principally because:

• Lift pits are notorious for continual or periodic leaks that cannot be rectified easily once the lift installation is complete and;
• Any leading lift manufacturer’s guarantee may become invalid if water and/or high levels of dampness are present in the lift pit.

Therefore, when waterproofing a lift pit the design considerations should be similar if not the same as when waterproofing a basement. In essence, the waterproofing strategy for a lift pit should comply fully with the design guidance outlined in BS8102:1990 Code of Practice for protection of structures against water from the ground.

So what are the important design considerations that should be taken into account when designing your lift pit waterproofing strategy?

The design team and the client should firstly agree upon on the acceptable levels of water-tightness as this will influence the chosen form of construction and selected waterproofing system.

The information gathered in the site investigation should then be considered to assess the surrounding ground conditions as these will need to be considered when selecting the appropriate form of construction. This will be principally influenced by the height of the water table and characteristics of the soil. If the soil type cannot be categorised as free draining it would be good practice to incorporate external sub-soil drainage that will assist in lowering the water table around the sub-structure.

In addition, a cuspated drainage board, such as RIW Double Drain applied vertically against the external face of the sub-structure will channel ground water to the sub-soil drainage and enhance the performance of the waterproofing system as shown in drawing SD / TYPICAL / 008.

The form of construction can also be considered part of the waterproofing strategy as it can be designed to have some integral waterproofing qualities. For instance, it would be advisable to construct the lift in water-resistant concrete (BS8110 Structural use of concrete. Part 1: Code of Practice for design and construction) in ground vulnerable to a high or variable water table or areas susceptible to flooding. In these vulnerable locations it would also be advisable to combine watertight concrete with an externally applied waterproof membrane as shown in SD / TYPICAL / 009.

More common forms of construction, as shown in SD / TYPICAL / 008 are perfectly acceptable provided consideration is given to the quality and suitability of the proposed waterproofing system.

All waterproofing systems have ‘attributes & limitations’ which need to be fully understood by the design team prior to specifying, especially the products’ limitations. The design team should ascertain that the proposed system is fit for purpose and then consideration should be given to how and under what conditions the product(s) will be applied on site.

Incorrectly specified products,inadequate surface preparation and poor workmanship are generally the main contributors to the failure of a lift pit waterproofing strategy.

Therefore, it is advisable to contact a leading manufacturer of waterproofing systems at an early stage of the design process to discuss and assist in the design of the most appropriate waterproofing strategy incorporating lift pits.