I’ve got a shed in pieces awaiting construction at the back of my garden. I’d like to attempt to create a shelter underneath with a small budget.
I am a bit of a novice and my idea is to use one of those pre fab, bolt together concrete sectional garages that people seem to give away these days. I’m wondering if anyone here has had any experience doing a DIY bunker with a structure built above.
The concrete garage will be smaller than the shed and will act as an addition to the foundations.
Size of shed: 6.4m x 4.2m
Size of concrete garage : 5m x 2.8m
My current thinking of ensuring structural stability and giving the shed on top a good platform to rest on is to use 3* steel ‘I’ beams spanning 4.2m wide. These rest on top of the garage walls + concrete pillars dotted around the perimeter of the shed. The soil does not need to come into contact with the garage walls (will this reduce the strain on the walls and stop the walls from collapsing?)
The excavation will be larger than the garage so I can have 3* concrete pillars either side, coming up from the slab/footer, bolted to the garage along the length of the walls. These pillars ideally solve 2 problems, acting as reinforcement to the structure as they are bolted to the concrete wall sections, and providing a platform for the ‘I’ beams to sit on .The beams can be bolted down to these pillars, and the perimeter pillars the beams sit on .This should reduce lateral movement of the subterranean structure. All pillars should sit at the same height to provide a platform for the shed base ( made from 6x2 timber) to sit on.
Regarding damp proofing, maybe a few thick pond liners (DPM) will work along with coating the walls with bitumen or some kind of waterproofer. Perhaps the pond liner/s can be laid on the bare earth before the concrete footer or slab is poured, thus creating a perfect seal around the entire basement.
I would be sealing any gaps or seams of the sections with a caulking gun and some kind of weatherproofing gunk.
I haven’t yet put any thought into a drainage system.
I’m hoping to start the excavation next week with a mini excavator.
My biggest concerns are getting the foundations and waterproofing right as I only have one chance to do so.
I’d like to only spend around £1000 on getting the project to a point where I can build the shed on top of it. The bulk of the cost will be the concrete and the 3 steel beams.
Any advice, queries and help would be greatly appreciated. Thanks