Concrete slab and walls for brick shed

12 Sep 2011
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United Kingdom

I am reading up on how to build a DIY heated, brick shed - which will have a gym and some rabbits, and I think I have the basics down, but I have some questions on the details.

My understanding is as follows:

1) The slab is basically a hole, with hardcore as the first layer, sand as the second layer (to even out the edges on the hardcore for the DPM), then the DPM, then 6 inches of protruding concrete on top (poured in by using a timber frame around it)

2) The foundations are a trench around the slab filled with concrete, with a layer of brick to keep the DPM around the protruding slab.

3) The wall is basically, brick on the outside, then some sort of insulation material, then blocks, then plaster (I wonder if this stage could be simplified, as I dont really need plaster in a shed).

My questions are as follows:

1) I have read that there needs to be room around the concrete slab, but I would like the shed to fit the width of the garden as close as possible to maximize space. What needs to be the depth of my slab and foundations and how wide can the foundation trench around it to have the least unused space between the border and the shed? The shed will be 8.5m x 3.5m.

2) Do I continue to build the outer brick wall on top of the bricks I used to keep the DPM stuck to the slab?

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You seem to be suggesting that the slab comes before the trench??

It's usually the other way round, ie you build up from a concrete foundation and when you get the brickwork/blockwork to ground level, you then pour the slab.

Do you feel confident building this structure??
I have read it both ways,

This guy suggests the slab comes before the trench:

While the concrete is going hard, without disturbing the posts (of which there should be at least 300mm in the ground) dig a small trench all the way around the base, right next to the frame, about 5 inches wide and 2 inches deep. Fill this with concrete also. It is the foundations for the brick wall which surrounds the base.

When I was reading a guide about extensions (habitable), they did the trench first

I am pretty confident because I do a lot of carpentry and engineering (robotics). As long as I know how to do it, I should be able to. Its a shed so its a good starting project for learning building. I have also done a lot of cement work before (mixing and helping other builders, when I was younger)

This guy suggests the slab comes before the trench:

That is ar$e-about-face and the most stupid way of doing it.

You dig the foundation trench first, pour the concrete foundation, build the masonry off that up to around 6" above ground level, and then use the masonry as the formwork for pouring the slab (remembering to put the dpm sheet in first, and overlap that on to the wall, with a dpc as well).
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That sounds more sensible than what that guy is doing. If you do it the way you say you don't even need timber frames for pouring the slab.

Whats the minimum width of the foundation trench? It can't be just the width of the masonry and outer brick right?

Do I need a mesh frame in the concrete slab?
Width of the foundation concrete depends upon loading, and ground conditions. For a structure of that height, and with no abnormal ground conditions, a concrete strip around 450 wide, centred under the wall, should be fine.

A slab that size might have a tendency to crack slightly when curing, so some light-guage mesh (A98) would help prevent cracking. Alternatively, you can get concrete with fibres added to prevent/reduce cracking.

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