Time to change my plan?

[andywatson]

Hi all, and thanks in advance for your time.

I am planning a garage conversion, and need to raise the floor by 150mm. The floor is concrete at the moment, and is in good condition and flat.

My original plan was to simply lay 5*2 joists on the floor, on top of a DPM and then put chipboard/ply on top (insulation boards between joists). I was advised against this as it will restrict air flow underneath the floor.

I then moved to the idea of a suspended floor. I was going to use 4*2 joists with a chipboard/ply covering. To cover the 4.5m span I was going to support the joists. This would give me 25mm under the joists to run any cables & pipes I might need. Again I was going to insulate beween joists.

On another post in the building forum I saw this:

http://www.planningportal.gov.uk/uploads/br/BR_PDFs_ADC_2004.pdf

Which on p27 shows a diagram for a suspended floor which must be at least 150mm abovethe subfloor. This suggests to me you cannot use a suspended floor if you are only raising the level by 150mm.

I think, realistically, I don't want to get into laying the new floor as a new concrete slab- so what alternatives do I have? Someone has suggested building it up with insulation boards but they recommended checking on here.

 

[freddiemercurystwin]

Using insulation would be an easy way to do what you require, assuming the concrete floor isn't too rough you could put a dpm down (if its too rough the dpm may get damaged (a thin blinding layer of sand would protect it in that case)) then say 90mm Kingspan insulation then a screed of about 60mm. Don't forget to have a thin insulation upstand at the edges and run the dpm up the edges too. Run your proposal past building control before you buy any materials.

 

[andywatson]

Thanks for that.........even though it was your link that blew my plan out of the water!!

Rather than screeding the top of the insulation could I either lay chip/ply as a floating floor, or actually go back to something close to my original plan and run a few joists around and nail ply/chipboard into it?

 

[freddiemercurystwin]

If you'd prefer, yes.

 

[andywatson]

Sorry Freddie, but just to clarify........

If I wanted to I could lay a DPM on the concrete floor, then lay out some 5*2 joists on the DPM. The joists can be fixed to the walls, and between them I can build up 5" of Kingspan. I can then nail some ply or chipboard flooring across the top.

Job done.

 

[freddiemercurystwin]

Well you could still have a timber deck (WBP ply, avoid chipboard like the plague), you'd just need to fit some treated timber wedges into the screed to fix it to. Going back to your timber joists is a slightly grey area with regards to building control, ideally it'd be suspended with the 6" void underneath which you obviously can't achieve. If you're determined to avoid the screed method though, I'd fit some noggins between the joists to stop them turning/moving about and to avoid fixing them into the perimeter walls and penetrating the dpm thats turned up the wall (if you see what I mean). You probably wouldn't need 5" of insulation if you went the joist route though. As I said this is a bit of a grey area and you should check with building control that they'd be okay with this method. If at all possible the dpm would lap up the wall higher than the dpc level.

 

[andywatson]

Thank for that- I'm pretty clear about what I am doing now.