Suspended Timber Floor

[padstar]

I am not sure if this is best in here or the general building section but here goes.

I want to install a suspended timber floor in the small front extension area that will sit "within the house". The area is 3m x 1.5m. The ground level sits approximately 450mm bellow the top of the proposed joist.

Assuming i use 6x2's for the joists (This appears to be more than enough for a 1.5m span but will it give any bounce?) this leaves circa 300mm between the underside of the joist and the ground level.

From what i can make out i am then required to provide 150mm of free void space to allow venting to prevent the timbers being compromised by any dampness.

I think the above is correct and in line with what the BCO expects but it is the next step that confuses me. what is required to "cap off" the ground. In existing rooms there appears to be 50mm+ concrete oversite and thats it but the house is 100 years old. What do i need to do?

Can i just lay 50-100mm Type 1 and compact. Am i right in thinking this layer is not to provide damp proofing as you have the vented void and just to stop growth of vegetation?

Help and clarification much appreciated.

 

[tony1851]

In the old days, it was 100mm of concrete..
Now, we have the option of 50mm 'inert material' eg sand or gravel, laid on a dpm of 1200g polythene. This is probably just to stop weeds growing, so you still need good airflow.
BTW, 6" joists are a bit OTT for 1.5m span - 125 x 50s would be fine.

 

[frutbunn]

Depending on spacing the joists will span at least 2.5m. (Eurocode 5 Span Tables - TRADA)
The DPM reduces any excessive damp.
In my part of the world the traditional method was to just to remove the topsoil and spread lime over the ground for a sub floor. I normally lay a DPM directly over the lime for floor replacements, any moisture just condenses on the underside.