Suspended floor construction in side extension

[Jaxta1]

I am starting to plan a 3m x 5m side extension which I hope to build myself. A question I have is about the floor. As it's on a slope, and to match existing floor structure, I intend to use a suspended timber floor.

At the top end the ground level is so high that ill struggle to get the bottom of the joists (5 inch joists) 150mm above the ground level, but ill just about manage by digging it out.

My question is, can the DCP go immediately under the level of the joists, otherwise ill struggle to get the DCP 150mm above the ground.

Also, do I need vented bricks on the 3m wall at the top end? if so could these go between the joist rather than under? that's the only way I think I could fit them in.

On another point how regular a distance do you need sleeper walls if im using joist hangers at either end?

Ive added a pic to illustrate my questions.

 

[noseall]

Why a suspended floor? Are ground conditions dictating that you can not do otherwise?

 

[Jaxta1]

Yeah I assumed it would need to be a suspended floor as the ground slopes down from one end of extension to the other?

Basically I couldn't find any pics, guides on internet of a slab being used on a slope!

 

[freddiemercurystwin]

If the void is bigger than 600mm then a ground bearing concrete slab is unsuitable anyway.

Anyway I don’t really understand your proposal? And your diagram is too small so can’t read the writing etc.

 

[Jaxta1]

I'll try and rephrase my question!

At the top end of the house, and top end of proposed extension, where ground is at the top of the slope-

From what I can see the ground level is only 180mm below the internal floor level.

The current floor is constructed with 130mm joists

This means that the bottom of the joists must only be 50mm above the outside ground level.

How can I fit the DPC in this also if this has to be below the joists but 150 above the ground level

Also how could vents be fitted in such a short space?

 

[oldbutnotdead]

Think you're getting a tad confused with DPC and DPM.

Inside under your floor you need a minimum 150mm air gap between the underside of the joists and your oversite (suggested is 50mm concrete on top of 1000g/sq m DPM). So with ground level at 180mm below floor level you're going to struggle.

In your wall the DPC should be 150mm above ground level. DPC and DPM don't have to meet in a suspended floor, as long as DPM is BELOW DPC you'll be fine.

Air bricks can't really go between the joists since they won't work blocked with the insulation you'll be putting between the joists.

How feasible is it to dig out (internally and externally) so you've got 300mm as your minimum void depth under the floor? If you can do this then you can use periscope air bricks (so outside they're 150mm AGL, inside they're at ground level), you'll have enough gap under the joists for air circulation, you can have DPC below your joists and all will be good. Oh yes, just below joists is fine for DPC.

Another thing to think about- if you do get any water under your floor you want it to be able to escape so consider air bricks or some form of drainage at or near ground level at the deep end of the void. Part C of building regulations has lots of useful diagrams for you.....