Basement Conversion: Does a dry space need waterproofing? Good Question

I just answered this question and thought I'd post it on our blog. The question:

Hello there
I think your website and the advice you give is simply the best that I have ever seen.
I am in a bit of a position in that my other half is a (very good) builder but not versed in basement conversions – not his bag nor does he want it to be HOWEVER in true builder style he wants to do the work on our cellar but I am wary of the methods that he wishes to employ as he feels it is a ’dry’ cellar and I know from reading countless online forums that there isn’t really any such thing!
He has already, some time ago, dug down to increase head height and then insulated and concreted the base of the cellar. Other than that, no work has yet been carried out…
I am unable to bring in anybody else for fear of upsetting the apple cart but neither do I want a job done that will only have to be re-done at a later stage.
Any advices would be gratefully received.
Many thanks in advance,

Our answer:


 There is a British Standard, ‘8102 (2009) Code of practice for the protection of below ground structures against water from the ground’.  As an approved code of practice this sets the minimum standard required for waterproofing in the UK.  If we installed systems and there were issues with those systems, our design/installation would be compared against the advice detailed within it.  If we didn’t comply and the client took us to task, we would probably be found liable.

There is guidance within the standard that as a designer, you should consider the risk that any below ground space will come under groundwater pressure at some point in its lifetime, even when site examination indicates dry conditions.  The standard advises that we should consider things like the effects of climate change (increasing rainfall) and burst water mains / sewers.

So yes, it may well be dry now, but may not always be that way.

It may also not be that ‘wet’ but the retained ground externally will always receive rainfall, and while it might not ever become saturated leading to liquid water penetration, the ground will always be damp and the probability is that the retaining walls hold capillary dampness as a result of the damp ground which bears against them.  At present this dampness may evaporate readily from the wall surface.  If such walls/floors are lined out with plasterboard, timber, carpets etc. and other materials sensitive to moisture, they may well deteriorate in time, even where liquid water penetration does not occur.  Worst case, liquid water penetration does occur and the space floods.

In conclusion, if you convert it without giving sufficient consideration to waterproofing, the risk will be that deterioration will occur and you will have to strip out, waterproof and then fit it out again, with the investment in time and funds on the original fit out, strip out and second fit out being lost.

The space should comply with the Building Regulations and should go through a building regs application with the local authority so that you obtain a completion certificate recognising the change of use to habitable space.

If you do not waterproof correctly, and do not go through building regulations, this might cause complication in the event that you then look to sell the property.  This because I, and others would typically not recommend purchasing or paying full value for,  converted, but not waterproofed / approved by the LABC space.  In other words I would advise prospective purchasers not to treat it as a finished habitable space, and to accept that if they have future issues, they will have to foot the cost of stripping out, waterproofing  and all the works that follow to get the ‘habitable’ space back.  You may therefore lose the investment in conversion even if problems do not occur.

Building regs. also includes means of escape in the event of fire, which on a safety basis is more important than the waterproofing.

A few years ago we remedied a residential converted basement in Altrincham, where a couple had purchased a property with the basement already converted.  One Christmas water started to penetrate through the floor and we were asked to investigate.  There was no waterproofing and the assumption had obviously been made that it was a dry space.  They had a very nice Italian rubber floor tile in there that matched the kitchen flooring over.  It was fully stripped out, with cavity drainage waterproofing being installed by Trace, before it was fit out again.  The project cost was circa £20k from memory.

 Hope that helps.

These are photos from the Altrincham project mentioned above:

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