November 2007

When is simply using traditional and cost effective Damp Proof Membranes (DPM) such as polythene below a concrete slab at ground level not acceptable and under what circumstances would a more robust tanking solution be required?

Building Regulations Approved Document C (Site preparation and resistance to contaminants and moisture) states; ‘floors next the ground should resist the passage of moisture to the upper surface of the floor’.

NHBC Standards and Technical Requirements Document states; ‘Where habitable rooms are wholly or partly below ground level, they require waterproofing, usually in the form of tanking. Polythene or Visqueen will not be acceptable to NHBC as a tanking material’.

BS8102:1990 (Code of practice for protection of structures against water from the ground) states; ‘Almost all structures are likely to be subjected to water pressure at some point of their life’.

The above statements show that serious consideration must be given to the design considerations outlined below before choosing the most suitable waterproofing strategy / products. The overall costs of installing the waterproofing can be less than 1% of the overall contract value, however it could cost as much as 10 to 12% of the overall contract value to rectify any failure that may occur.

The Building Regulations Approved Document C outlines a number of design considerations that should be taken in to account before you build your waterproofing strategy;

1. the presence of contaminants and gases in the ground,
2. the level of the water table
3. sub-soil drainage, and
4. the resistance to moisture

Each of these will have a significant influence on the form and selection of the waterproofing.

1. The presence of contaminants and gases in the ground

A wide range of solid, liquid and gaseous contaminants can arise on site and will be identified in the soil investigation. Armed with this information the potential risk can then be evaluated. Some membranes will provide an effective barrier to contaminants and gases such as Radon, Methane and Carbon Dioxide however, the importance of the membrane maintaining its integrity is often over looked.

CIRIA guidance publication 659 (Assessing risks posed by hazardous gases to buildings) states that ‘Membranes should be based on their performance characteristics and ability to survive the construction phase’. It is widely accepted that some membranes such as 1200g Polythene, the minimum thickness for a gas resistant membrane on low risk sites, is unlikely to achieve the required level of robustness.

2. The level of the water table

The severity of the risk falls in to three categories;

Permanently low – No significant risk of percolating water building up to create a hydrostatic head of water against the structure (free draining),

Permanently high – Water table is close to or above the lowest level of the concrete floor slab creating a high risk of hydrostatic head of water against the structure, and,

Variable or perched – Water table may become temporarily high seasonally or during periods of heavy rainfall thus creating a risk of hydrostatic head of water against the structure (the severity of risk will depend on how high and for how long the water table remains perched).

Therefore, it could be argued that where a risk of a hydrostatic head of water pressure being applied to the DPM exists, then a suitable ‘tanking system’ should be considered in preference to a DPM.

3. Sub-soil drainage

Sub-soil drainage should be provided where (i) there is a risk that groundwater beneath or around the building could adversely affect the stability and properties of the ground and (ii) contaminants are present in the ground, the sub-soil drainage will help prevent the risk of water-borne contaminants being transported in to the building or its foundations.

4. Resistance to moisture

It is essential that the structural elements, whether wholly / partly below or at ground level, resist moisture from surrounding ground. This is generally achieved with the inclusion of a waterproof membrane and by providing a means of drainage, where necessary, that will allow moisture to disperse from the structure.

Selection of the waterproof system

Selecting an appropriate membrane has to be more than a cost engineering exercise, and all of the above design considerations must be taken into account to ensure the right system is selected.

It is important to understand the attributes of each product, but it is essential to understand the product's limitations.

For instance, polythene which is commonly used below a concrete slab as a DPM, is cost effective and easily installed but is extremely susceptible to damage, un-bonded and not considered suitable as a tanking system. It is also suggested in CP102:1973 that polythene is not suitable below a concrete slab to be covered with sensitive finishes such as Vinyl, Lino and timber.

Therefore, as suggested in the Building Regulations and the NHBC technical standards, polythene should only be used as a DPM in low risk free draining ground conditions and not for habitable rooms wholly or partially below ground.

There are many other generic waterproofing systems available for use as a DPM in the more high risk ground conditions and where the habitable areas are wholly or partially below ground, such as; Bitumen coatings, self adhesive sheet membranes, Geo-composite bentonite membranes, Epoxy coatings, Flexible polyurethane coatings.

As with polythene, all of these systems have ‘attributes and limitations’ and should be considered individually to assess the products suitability for specific forms of floor and wall construction, for instance;

Ground bearing slab

Consider placing the DPM on top of the slab, as the membrane;
• is less likely to be subjected to significant hydrostatic pressure
• can be applied later in the project in a controlled ‘dry’ environment
• is less susceptible to damage during the floor construction, thus retaining it’s integrity
• protects sensitive floor finishes from moisture in the concrete slab

Suspended slab in contact with the ground

Susceptible to the ground falling away from beneath the slab, the membrane;
• can be applied on top of the slab, as above, or
• must have the ability to mechanically or chemically bond to the underside of the slab
• must be capable of retaining its integrity and have sufficient toughness to remain undamaged during the floor construction phase (CP102:1973 & CIRIA 659).
• should be sufficiently versatile to be installed around piles, pile caps and ring beams without the need for complex and unnecessarily complicated detailing

Concrete slab wholly or partly below ground

Where the structure has one of more vertical retaining walls, the membranes;
• should be recognised as ‘tanking’ systems’ (polythene is not acceptable)
• continuity must be maintained between the horizontal and vertical membranes
• performance can be enhanced with the provision of effective sub-soil drainage

To conclude, a polythene DPM may be a cost effective damp-proofing solution in low risk ground conditions, however, should it prove ineffective, the cost implications to rectify the problem can be significant.

Therefore, it is essential to understand that Building Regulations Approved Document C and other widely used technical guidance notes / standards (NHBC, CIRIA, BS8102, CP102) outline that there are many situations where a more high performance and robust ‘tanking membrane’ is required to protect the structure against moisture and contaminates from the ground especially if;

• Contaminates and gases are present in the ground
• The water table is permanently high, variable or perched
• The surrounding ground conditions are not considered free draining
• The habitable room is wholly or partly below ground level

The RIW Technical Department can be contacted on 01344 397777 to advise on the selection of the most appropriate waterproofing strategy for your structure, or just follow the links provided in this technical newsletter.

Please note: CIRIA 659 is currently under review.