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Groundwork & Foundations

Garden office build foundations

Anonymous user 16/03/2024 - 2.50 PM

I am doing a garden office build. On demolishing the old shed that is there today I have discovered it is on a 4 inch thick concrete slab. This is about 2 metres squared, but my planned office is about 4 metres squared, so the slab is way too small. It’s going to be a right job to break and remove, so I’m wanting to understand my options. The slab seems to be in good shape though I don’t think it has any damp proofing - the chipboard shed floor is destroyed and soaking wet. Could I: 1) Install dense concrete blocks around the slab. 2) ? Put a damp proof membrane ontop of the concrete 3) Put my 2 x 4’s ontop of this - 2 metres of the wood would be on the slab, the other 2m or so resting on the concrete piers. 4 ) ? DPM under the 2 x 4’s. Build up from there In essence my finished room would have a mix of concrete slab and block piers for foundations. What am I missing? Can I do this? I’m London on clay if that makes a difference? Other thought is not to cover slab in damp proofing to let it breath / water to come up and only put DPM under the 2 x 4’s. I’d make sure my PIR insulation is off the ground so nothing in contact with the slab. This would allow air flow under the room, and should be ok?

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3 Answers

DMT Home Improvements

No reviews yet

Norwich
The latter would be fine. As long as there is substantial airflow and the timber is treated along with damp course underneath. Just be sure to lay a course of engineering bricks on top of the concrete slab where the floor joists are going to sit to give it a 75mm clearance from the ground to allow the air to pass underneath the flooring.
Answered8 April 2023
4

Anonymous user

I would drill rebars into the side of the old concrete with kem fix when installing the new section to make it into one
Answered10 April 2023
0

Top Trowel

Rating: 1 out of 5
St Helens
Break out the old concrete slab and install a new concrete slab with foundations. (Typical bricklayers answer)
Answered23 April 2023
0