Timber floor

I'm installing a timber floor in my garage conversion.

I need to raise the floor 200mm to existing, so im going to lay 175mm joists straight onto floor with DPM and 22mm chipboard with 100mm celotex between joists.

When I thought it was originally going to be suspended, I was going to fit wall plates with thunderbolts and joist hangers. As they now will be sitting directly on floor, will it be suitable to install the wall plates with a few less thunderbolts and just spike the joists to it?

Also, should I lap the DPM up above the floor and trim back down, this will still be above original damp course of brickwork.

I will also install an airbrick in the new front wall to allow ventilation in floor.

Regards

Jack

 

If your joists are supported by the concrete floor you don't really need wall plates etc. Ventilation of the timber will be tricky though, with no airspace beneath them there won't be any air circulation.
Might be better going for a full floating floor-DPM, 150mm Celotex (or 50mm sand below the 100mm Celotex) then slate laths then chipboard.
Yes lap the DPM up the wall.

 

Thanks for the reply.

The room is going to be a kitchen and will have a substantial island with granite fitted so I was wary of what floor construction to put underneath. Obviously I don't want any movement whatsoever.

Its something to have a look at.

Thanks

 

I'm not a builder but if it needs to come up 200mm then there's plenty of space for insulation and screed, you could do 100 of each.
If you prefer to be able to access things hence the floorboards then that's fine.

 

Ah, granite island wouldn't be a good idea on a floating floor. I'm still not convinced about the joists in contact with the DPM sketch due to the lack of air circulation but I'm prepared to be corrected.....suspect your better way forward would be the insulation covered with screed (I'd prefer concrete with some mesh in it but you might be struggling for depth to get a nice flat finish on top of it).
Plan C (depends how good the garage floor is) would be to hook that out to give you the depth either for suspended timber (I'd put some blocks in the middle where your island is going to be) or insulation/concrete/screed. If the floor is new then seems a shame to go to the trouble and expense.....

 

thanks for the replies.

I originally was going for a concrete floor as it seems the simplest and most solid. I didn't want a hollow sounding floor.
I'm planning in running some utilities through it.

To add another issue, my garage floor, as solid as it is, does run out on a slope slightly and I really need to keep this to maintain head height on the doors.
With timber I could play with it, not sure my screening skills are good enough to smoothly float it at a slight angle

My building inspector actually recommended the timber directly on DPM.

I'm racking my brains on how to maintain air flow through the joists.

 

If your BCO recommended the timber plan, ask him about ventilation (he's likely to come out with something like 'well, the original garage floor has a DPM under it, we know that cos we did the inspections when it was built. You're putting another one on top so odds of any water getting at the timber are low to nowt- not a problem, don't bother with an air brick'. He's the sort of expert I would happily defer to)
A sloping kitchen floor- that could be a nightmare, depending on how 'slight' the slope is. If ALL your units, cupboards etc are runnning across the slope then you might get away with it, if you're having a run of worktop in the direction of the slope you either need sloping worktop (rubbish) or significant height difference between 1 end and the other (if the slope is 1 in 100 and you have a 3 metre run of worktop, units etc there's 30mm to hide somewhere,might not sound a lot but unless your design is good it'll scream at you every time you look at it. Plus if you have a freestanding table it'll be on the slope. You can get used to sloping floors but some people (me for instance) find them massively annoying

 

Fill in-between the joists with insulation. No voids, no ventilation required. You could use a cheaper insulation fully, or partially to make up the difference to the Celotex

You need to level them up though.

 

I'm thinking, as the joists won't be suspended then the wall plate wont be bearing any weight. Its simply there to frame the joists and fix them to.
Therefore, I could reduce the size of the wall plate to maybe 5". Then there would be a 2" square gap at either end of the joists at floor level as the 7" joist will be resting on the floor.

Either that or I could do as woody suggested and fill the void avoiding need for ventilation.

An air brick will need to breach the DPM as it laps up the wall anyway if that was required!!

 

Thanks for the response.

I'm just coming back to this post as I'm about to start constructing the floor.
On your suggestion, would I need to fill the void below the celotex with a similar insulation or could I fill with rockwool/loft roll?

Cheers