Double skin block building on concrete pad?

[snsmurf]

Hey all, I’m in the process of a big garden renovation that I’m doing myself, including a load of bricklaying.
One of the things I’m going to be building is a garden office / drum room. For maximum sound isolation I’m planning on building it as a double leaf block work using the dense 7n blocks.

Now my biggest decision at the moment is how to do the foundations for it. Due to space I was planning on a concrete slab with rebar (and the standard type 1 mot underneath) just slightly wider and longer than the office size.

The ground under that is a lot of sandstone so it should have a stable base under the slab. But after some research I’m unsure if that will be safe given the weight of a double skin building near the edge of the slab, I’ve read that it could potentially lead to cracking of the slab and that I should really do trench footings for it.

Sadly though given how wide the trenches would need to be it would mean I have to reposition it and shrink the size of the room down.
So what I’m wondering is, would the slab actually work in this situation or would I be risking in cracking and damaging the building, anyone else had experience with something similar?

 

[stevie888]

Slab will work fine provided designed for what it has to do. We'd usually just thicken the edges of the slab to beef them up.

 

[stuart45]

If you look at raft foundation design you'll see what stevie888 means.

 

[noseall]

Hey all, I’m in the process of a big garden renovation that I’m doing myself, including a load of bricklaying.
One of the things I’m going to be building is a garden office / drum room. For maximum sound isolation I’m planning on building it as a double leaf block work using the dense 7n blocks.

Now my biggest decision at the moment is how to do the foundations for it. Due to space I was planning on a concrete slab with rebar (and the standard type 1 mot underneath) just slightly wider and longer than the office size.

The ground under that is a lot of sandstone so it should have a stable base under the slab. But after some research I’m unsure if that will be safe given the weight of a double skin building near the edge of the slab, I’ve read that it could potentially lead to cracking of the slab and that I should really do trench footings for it.

Sadly though given how wide the trenches would need to be it would mean I have to reposition it and shrink the size of the room down.
So what I’m wondering is, would the slab actually work in this situation or would I be risking in cracking and damaging the building, anyone else had experience with something similar?

If you have a slab size on a bigger footprint than the building at ground level or close to, you run the risk of rainwater entering at floor level, unless you design this out.
It’s important when designing a raft foundation that it is set at an adequate depth to accommodate the above issues as well as other over site detailing such as insulation and screed etc.

 

[Ivor Windybottom]

Get digging, find something that's not black or brown and build on that.

If you need to go a long way down then use a trench footing. If you have shallow soil on the rock you've mentioned then there's probably no need. No point hacking into rock to fill it with concrete.

I'd treat the walls separately to the floor. It makes more sense, and it also works better when it comes to the DPM - basically the floor is dry below DPC while the wall gets wet. You need a DPM somewhere under the floor.

Don't take short-cuts, you can't replace the foundations later. Treat it exactly like building a house.