We are building a new house in the back garden, and we are interested in using block and beam foundations.

Question 1 = Will a 150mm void under the block and beam be ok? Or does it need to be greater?

We are thinking of digging the void down into the ground a little to avoid a big ramp up to the front door for Part M disabled access (using vents down through the foundations of course), but the void may suffer from surface water during heavy rain.

Question 2 = Should we install some sort of water barrier at the base of the void to keep it the water out? Is this allowed and is there a specific product we should use?

Question 3 = Do we need to install a Radon barrier as standard practice? How do we find out whether we need one?

Thanks very much

1. 150mm is the min recommended if you are using clay to backfill the founds.  It can't have organic material in it.  Your looking this gap to accommodate any swell in your fill.  You can fill it will stone but this costs more. 

2. Are you on a site that's liable to flood or have a high water table?? 

3. The radon barrier is dependant on your location.  Check where you are. https://www.ukradon.org/information/ukmaps

Its not expensive to buy any way. £100 would cover most people's  build.

First question should always be - do I need to use B&B or can I use a ground bearing slab. 

 

The issue you may face is not having the DPC high enough above ground level to satisfy the BCO. Do you have a largely sloping site..?

Thanks guys. The site is nearly flat (very slight slope from back to front). It doesn't really suffer from surface water, just gets a little wet during heavy downpours. The DPC should not be an issue. Do you think a slab would be better (I'm really in two minds)? I'm thinking with block and beam I can do much of the labour myself? Is block and beam much more expensive?