Oversite concrete and timber floor?

[xdrive]

Hi. I'm building single story rear kitchen extension. Just sorting the floor out.

Will leave 6 inch air gap under joists. Span is 3.7 metres. Will be Using 8x2 @ 400mm centres.

Will use ledger plate on existing house wall and wall on new extension.

1) Are m10 bolts sufficient every 400mm? Or m12? Can I use lightening bolts? (no plugs required)

2) is it best to put down a DPM before concrete?

3) do I have to be 150mm below Dpc or does that only apply to external wall?
Was thinking 50mm below dpc which gives 6 inches below joists.


4) does the dpm have to join up to the internal dpc? How will that be done.

Cheers

 

[tony1851]

Google 'Approved Document C', pages 26 - 27 have the details for suspended timber floors.

 

[^woody^]

No DPM, it will retain water in the slab.

 

[xdrive]

No DPM, it will retain water in the slab.

I have natural ground water (spring) running 1 metre below. Would that make a difference?

 

[^woody^]

I have natural ground water (spring) running 1 metre below. Would that make a difference?

It might to the foundations.

If the ground is saturated, or there is a high water table, then you should consider whether a suspended timber floor is the best option, or whether additional measures are required along with a timber floor.

With oversite, if you try and seal ground water out, then you need to be sure that no wafer gets in above it (via the walls) and that the void is adequately ventilated to stop it filling up like a pond.

 

[xdrive]

@^woody^

The ground doesn't saturate. Ideally should have gone the concrete floor route. However I'm confident once it's sealed the water will remain below and follow its natural course as before.

 

[^woody^]

That's fine, but you just need to be aware of the principles of the oversite being allowed to get damp and this being dealt with by natural (or additional) drainage and ventilation to keep the humidity down - that is the principle of this type of floor construction.

So with any additional damp proofing, you may think it's helping, but you need to be sure it's not in fact hindering this principle.

 

[xdrive]

That's fine, but you just need to be aware of the principles of the oversite being allowed to get damp and this being dealt with by natural (or additional) drainage and ventilation to keep the humidity down - that is the principle of this type of floor construction.

So with any additional damp proofing, you may think it's helping, but you need to be sure it's not in fact hindering this principle.

I see. I'll just pour 3 to 4 inches of concrete with out dpm. Ive levelled it out with 8 inches of hardcore.

 

[frutbunn]

A DPM is perfectly acceptable providing the sub floor is above the external ground level or the sub floor is drained. By using a DPM you can also reduce the sub floor concrete to 50mm thickness rather than 100mm. Personally I would install one if it was for myself.