Building Sunken Garden Shed/Summer House On Concrete Base?

4 May 2016
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United Kingdom
I'm planning on taking my current shed down and building a summer house type shed in the garden.

Here is a plan of my garden:


The 4.5m and 4.8m are garden width measurements (garden tapers narrower towards the back). The black pen "example" is the most likely size of the shed (3.8m x 3.9m).

1). a). The "shed" will be built from 100mm thick aerated blocks. Would a concrete foundation of 100mm thickness and footings of 450mm depth suffice for this type of build?

1). b). Can I mix old pieces of concrete and stones I have laying about in the garden to the concrete mix to save on costs? Should I thrown in some metal grids and do I need any expansion joints? I have mixed old concrete before into the foundations of a retaining wall without any problems.

1). c). Do I need any type of damp proof membrane under the foundation? For example: Dig hole, compact, lay DPM, add concrete over DPM... or should the concrete slap be in direct contact with the earth and the DPM added after (over concrete, under interior floor)?

1). d). Am I expected to lay the footings and base slab in one go to ensure they are all 1 piece of concrete? Or do I lay the footings first, wait to dry, then the 100mm thick base slab inside after?

2). I had an idea of sinking the shed into the ground (to allow greater height inside). Is it a good idea to start the build, from say, 30cm below the earth's surface or would I run into too many problems with damp etc...?

The aerated blocks will have a 45mm stud wall attached from the inside with insulation panels - this will also be the case for the floor (Celotex). Walls will be plaster-boarded and plastered. Exterior walls will be rendered.

Thanks in advance.
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starting below ground slightly would be ok as long as the water table isn't too high, and you have good enough drainage. I can just imagine heavy rain making a wet garden with a 1 inch puddle. that needs to stay off your shed.
if you have the floor level below ground, your shed can float away (upthrust) but I doubt you'll have to worry.
2). I had an idea of sinking the shed into the ground (to allow greater height inside). Is it a good idea to start the build, from say, 30cm below the earth's surface or would I run into too many problems with damp etc
I personally think it's a daft idea particularly in view of the efforts you will need to take in order to stave off the inevitable damp problems. Think wet winters. Its not just damp either - you will have temperature differences and mould etc.
If your going to do this then lay the building blocks on their side (so the bottom sub scruture has a thicker wall) and use tanking slurry on the walls inside and out then mastik painted on all the joins especially where the slab meets the blocks. When its been slurry'd and mastik'd then make sure the DPM is wrapped over the side and top of the tanking wall and inside the building. I would also have a fairly substantial porch on the front to stop rain trickling under the door. Also add some kind of anti heave board (ive used doubled up correx sheet in the past, its basically plastic cardboard and will be fine for a shed). Also look into adding a french drain all the way around.
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Back of the garden where it will be built:


It's not exactly a flood-prone area, especially in summer when the tree leaves shelter it.

A friend at work mentioned the French drain idea... kinda. He said dig a trench around the foundations and fill it with small stones... then have another small block wall built on the other side of the stones on a small, separate-to-the-shed foundation, so:

shed foundation || stones || block wall

Personally I think that's overkill with the extra block wall around.

Would it be just as bad if I started the build 10cm under ground versus the 30cm I mentioned earlier? Still prone to damp?

Any idea on the DPM on the outer side of the foundations, or should it be internal only?

I think if you dig down and fill anything with gravel it will improve drainage so lessen the damp. I quite like the idea of having a shed with a trapdoor and a vault for the more expensive tools hidden under a rug. Like a mini car pit I guess hidden under a mini carpet.

I wouldnt bother with the outer wall on a french drain the mud will be enough or add a correx sheet or something similar either side of the gravel trench if you want to minimise the gravel getting contaminated with mud over time then weeds growing out the gravel.

10cms down... sounds like the void youd fill with hardcore then put a slab on top. If you can have a dpm wrapped up and over the slab then it will reduce the chances of moisture getting into the building.

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