New Build Basements - Grade 2 Waterproofing
Everything you need to know about grade 2 waterproofing, based on the BS8102 standard.
Grade 2 waterproofing is permissible for your basement according to BS8102.
But, before commissioning grade 2 waterproofing, it is critical that the design team take a very clear written brief from the client, having advised them on the implications of damp and condensation with stored items and plant. The advice must be given in non-technical terms or to a client’s suitably qualified representative.
First, establish the proposed use of the basement and discuss the required relative humidity, the risk of damp to stored items such as paper or wood (risk of decay), metal, tools, plant and equipment (risk of corrosion), agree any wall finishes and aesthetics, and agree the potential change of use in the future.
If your client does not accept that damp stains and high humidity are acceptable for the proposed use then you should revert to grade 3.
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Waterpoofing according to BS8102
It is a requirement of BS8102 that you should consider two forms of waterproofing when the risk of a leak is high and if the consequence of a leak is not acceptable, which, of course, it wouldn’t be in your new basement.
If you reduce the waterproofing because it is Grade 2 then you increase the risk of a leak. If it leaks then it is wet, not damp, and so it will not comply as Grade 2.
If you have a high water table you must not build your basement out of blockwork or stepoc, as it will not comply with BS8102. So, you must build out of reinforced concrete.
If you are building out of blockwork , ICF or Stepoc you must consider that you have to stop water penetrating into the structure by using external membranes and drainage.
It is a precedence set in law (see the Outwing Case) that you cannot install an external sheet membrane system defect free. So, if you are applying it to a structure built of block or stepoc which is porous you are risking leaks or flooding at some time.
If you cannot drain the outside of the basement from a point below the internal slab to a soakaway or suitable drainage then you will have to install pumped drainage and keep it maintained. A pumped system in a permanently high water table is not practical.
So you must have two forms of waterproofing to keep the inside dry and to comply with British Standards, legal precedence and all other codes of practice.
A cavity drain system is the best solution
You will have to have a cavity drain internally as it is the only system that can be guaranteed on a blockwork basement. Any other system will not comply with British Standards.
If you have a cavity drain system you should always have an external system to stop lime, calcium carbonate coming in from the ground and calcium hydroxide coming from the cement in the new structure. Lime entering a cavity drain system will cause blockages and will need regular servicing and cleaning.
The options for the second form of waterproofing are external coatings, sheet membranes, cement tanking. Additives in block infill do not work.
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What are the options for the second form of waterproofing?
Concrete additive in a concrete block cavity cannot and will not work. It is not possible to effectively fill the voids or to achieve the necessary compaction. Specifiers of concrete block systems will claim that a well grouted block wall will be watertight. It isn’t.
Cement, epoxy coatings do not work on concrete block construction as it is prone to some movement.
Sheet membranes are the most suitable for concrete block basements but have to be installed with a cavity drain system inside as they cannot be installed defect free.
What else do you need to think about when waterproofing a basement?