Shed base by a first timer - how thick a layer of dry mix?

[r_c]

I have a boring office job, so please excuse me if this seems obvious- I've never done anything like this before.

I am creating a shed base and am following the instructions of a B&Q video on YouTube. After laying the sub base and compacting it, they suggest to use a dry mix of 9 parts sharp sand to 1 part cement. They then lay concrete slabs on top of this dry mix.

But they do not say how thick this layer of dry mix should be. Is there a thickness I should aim for?

I notice they do not compact the dry mix at all - so maybe not so thick??

 

[pidgeon1]

All I did was to put down railway sleepers to kept sheds off ground and give air circulation, that was 35 years ago, shed floors still in great order. 6x4 and a 12x10 shed.

Times change with new ideas how to erect a shed,

 

[Mottie]

All I did, wrongly or so it seems, was just use a couple of inches of sand, no cement, with patio slabs on top but it’s been fine for nearly 10 years now so I think I’ve got away with it. 6x8 shed.

 

[r_c]

@pidgeon1 Interesting you say that, because my father in law was talking about something similar. Instead of sitting the shed on the concrete slabs, lay it on top of a few long bits of wood (e.g. those long blue roof battens). Over time these will rot due to laying flat on concrete, etc, and then every 10 years or so lifting the shed and replacing one by one. The argument goes that whatever wood that sits on the concrete slabs will rot, so why not use wood that can easily be replaced, instead of letting the shed wood rot.

But I don't know enough about these things to know if that's a good idea or not.

 

[pidgeon1]

@pidgeon1 Interesting you say that, because my father in law was talking about something similar. Instead of sitting the shed on the concrete slabs, lay it on top of a few long bits of wood (e.g. those long blue roof battens). Over time these will rot due to laying flat on concrete, etc, and then every 10 years or so lifting the shed and replacing one by one. The argument goes that whatever wood that sits on the concrete slabs will rot, so why not use wood that can easily be replaced, instead of letting the shed wood rot.

But I don't know enough about these things to know if that's a good idea or not.

Railway Sleepers ( mine were Second hand) are best, my Wife tells me it was 1977 when I put these down for the sheds base and they are still there. think they will outlast me.

 

[mikeey84]

You can get plastic joists (used for composite decking). These would work, and not rot

 

[r_c]

@mikeey84 Yes they would do a better job and not rot. But if this is a good idea (e.g. putting something under the shed to lift if off the ground), why isn't it standard practise? Or is it, and I just haven't a clue??

 

[mikeey84]

I've just installed a shed, and have used 3 3x2s to keep it off an old concrete base, so if it's not the right way, then I've done it wrong too!

 

[conny]

@mikeey84 Yes they would do a better job and not rot. But if this is a good idea (e.g. putting something under the shed to lift if off the ground), why isn't it standard practise? Or is it, and I just haven't a clue??

Most sheds are raised off the ground by the floor joists that help form the shed floor.
However, because these are usually quite small dimensionally, it is recommended to put the floor on further raised supports to protect the floor. The 'supporting' joists run at 90 degrees to the floor joists and can be any size you want though popular sizes tend to be 2" x 2" or 3" x 3" with a strip of damp proof membrane on top of each one to help stop rising damp contacting the floor joists.
Going back to your original question of how thick/deep the sand layer should be. I think a good depth is 3" to 4" supported by some form of external surround to hold it in place. These supports could be heavy timbers or a small brick wall all around. It is good practice to make your base slightly smaller than your shed base to allow rainwater to run down and drop off the bottom edge to prevent water pooling under the shed.