Basement conversions are increasingly popular and a complete service is available through Kilrot's sister company, Cellar-tec Basement Converters. The key to success in this field is to make absolutely sure everything is waterproof and new basement systems are designed to deal with defects should they occur in the future.

All this means that once-neglected, mouldy and damp basements can be transformed into wonderful cosy living areas. 

The following points need to be taken into consideration when 
waterproofing a cellar

Penetration of Water from the Ground

Water can penetrate a structure below ground level in two ways:

1. Where the ground is not saturated, it will move through the capillaries of the ground and structure by capillary suction. When this form of moisture comes to the internal surface of the building, surface dampness is evident but there will be no free flow (flooding) of water. Capillary moisture can enter a structure laterally, such as when a wall is earth retaining, and \ or it can rise up from the ground through floors or free standing walls, appearing internally as rising damp.

2. Where the ground is saturated and hydrostatic pressure occurs, water will be paused through the capillaries of the ground and structure. Where the water comes to the surface, the pressure behind it will force it into the property in the form of liquid water and flooding will occur.

Where a wall is earth retaining, penetration will be lateral through the retained soil. If the water table is high (such as when water is trapped in a clay sump created when the foundation are excavated) or if rainwater is percolating down through the soil, then this water will be under hydrostatic pressure and could flood the property internally. If there is no water pressure present, then penetration will be though capillary action, and the property will not floor internally.

Producing a Dry Basement

Nothing is absolutely dry. Water and water vapour will always exist bound up in the building and in the air. However, a building will always be perceived as being dry if the water present does not pose a problem for the inhabitants or contents of that building, or cause damage to decorations.

Producing a dry basement requires two distinct and separate steps:

1. Firstly, penetration of water from the ground must be stopped.

In existing properties, this is commonly done by applying waterproof membranes to the internal face of the structure, or by fitting a drainage membrane, which directs ingress water to a suitable collection and disposal point.

2. Secondly, the internal environment needs to be heated to the appropriate temperature. Furthermore, excess humidity, created by the normal activities of the occupants, must be removed. This is normally doe by ensuring there is adequate ventilation, (natural or forced) in the area although, in extreme cases, air conditioning or dehumidification may prove to be necessary.

Cementitious Membranes

Cementitious membranes are modified renders and slurries applied to the internal face of the structure.

Cementitious membranes can be easily applied onto curved and intricate surfaces. However, because they stop water at the point of entry, pressure can build up behind the membrane and it is important that the substrate is capable of accepting the induced stresses that result.

Cavity Drain Membranes 

Cavity drain membranes are polyethylene sheets with dimples moulded into them, creating a cavity between the membrane and the substrate. Ingress water will enter the structure and run inside the cavity to a convenient point for removal by drainage or pumping.

Cavity drain membranes do not stop water at the point of entry, and so do not increase the stresses n the substrate as can happen with cementitious systems. However, they are not as versatile as cementitious systems in that it is difficult to apply them to curved and intricate surfaces. Furthermore, it is essential that hydrostatic pressure is not allowed to build up in the cavity. If it does, leakage is almost certain to occur.

Code of Practice for Basements

Clause 3.4 (b) of BS 8102, Code of Practice for the Protection of Structures Against Water from the Ground, states that in basements less than 4m deep we have to assume a hydrostatic head of 3\4 the depth of the basement, or 1m, whichever is the greater.

Clearly, on existing structures where the design detail is not known, great care needs to be exercised when applying certain waterproofing systems. If the water is stopped at the point of entry, significant additional stresses are likely to be induced as a result of increased hydrostatic pressure, however short lived the pressure may be.

Cementitious systems stop water at the point of entry, and hydrostatic pressure is likely to induce additional tensile and flexural stresses in the substrate. The existing structure must, therefore, be carefully evaluated to ensure it is capable of accepting those induced stresses.

Very often, this evaluation reveals that the substrate (usually the floor) would not be able to accept the additional stresses, and significant re-medial measure would need to be taken. However, where basements have been in existence for many year and have never experienced flooding of any sort, it is acknowledged that the risk of flooding in the future is small. In these circumstances, the cost of undertaking additional works needs to be weighed against the potential cost of damage should flooding ever occur.

It is not a statutory requirement to design in accordance with BS 8102. As long as the potential risks are considered, the decision to ignore the threat of hydrostatic pressure is acceptable and it is perfectly acceptable to provide a damp proofing solution only (as opposed to a structural waterproofing solution).

However, although the evidence would suggest that the likelihood of water tables rising – and causing flooding internally in the basement – is remote, it is my view that it can never be discarded entirely and some provision should be made for dealing with the event.

Ventilation

When you are dealing with confined basement areas with poor or restricted ventilation, then you may consider the risk of condensation and the introduction of mechanical extraction systems with humidistat sensors. This is imperative of a Grade 4 totally dry environment is required.

Methods of drainage for Newton 500 to the floor.

The drainage must effectively remove all water from below the membrane and taken to a point of discharge such as a sump chamber or a form of natural drainage. Standing water can block the membrane with silt or lime scale so it is important for water to flow uninterrupted to the drainage point.

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