timber base for shed over existing uneven concrete

[jell1234]

Hello

I'm planning a new shed/summerhouse for my garden. It's going to be approximately 5 x 3 metres. I've been researching the different options for a base, however the patch of ground I will be using consists of old uneven concrete around an existing, much smaller, shed, as well as paving slabs inside the existing shed as the shed floor (at roughly the same level as the surrounding concrete.

I would like to build a timber floor for my shed in order to have somewhere to put proper insulation underfloor, however I can't find any advice of supporting a timber floor on existing concrete.

Also, I want to avoid pouring a new layer of concrete on top as I know it has to be a certain thickness to not be liable to cracking, and I think this would add a lot of expense to the project.

What would you recommend for this? Thanks in advance for your help

 

[Deluks]

Build up low pillars of blockwork, bricks or 50mm thick paving slabs. Adjust the heights to suit by packing out the mortar beds with old roof slates, tiles etc. Sit your floor structure on this.

 

[jell1234]

Hi Deluks

Thanks for the advice - that sounds pretty sensible. Given the width of the shed (5m) I was thinking then to have 3 supporting "walls", which would effectively be a 1 brick high wall, bedded out to level as you suggest, then using 2x4s at 40mm centres for the main floor joists.

Because of the existing concrete / paving slab combination, this would mean the 2 outer "walls" would be on the old concrete, and the middle one would be built on top of the existing paving slabs. Do you see any problem with this or would this be sufficient? I don't think the existing base is going anywhere, it's probably 40 or 50 years old and it doesn't show any signs of having moved.

Then, another question - on top of these walls, should I put a timber wall plate between brick and main timber floor, or is it sufficient to build the frame directly on the bricks? I'd like to avoid the extra height if possible, but not sure if this would make it hard to get the floor completely level?