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 stop there. 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, with a DPC at floor surface level. 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.

 

[snsmurf]

Ahh, this is all interesting stuff. I'd not come across raft foundation before.

I'm guessing you would need some kind of shuttering around the middle section while pouring and compacting hardcore then to stop that falling into the deeper edges? Then simply remove the shuttering when it comes time to pour the concrete?

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 stop there. No point hacking into rock to fill it with concrete.

Once I get passed the sand and membrane that was down for the paving bricks laid by the previous homeowners, the rock is basically right there. Maybe 10-20mm down. So i don't have to go down far at all to get to the rock.

I did want to go down the trench route, but as mentioned I'm in a tight space as i have foundations for a garden wall right near where this office is going. If I do trench footings I'd have to go down deeper usually i thought closer to where the foundation for the wall is so i'd need to move away from that to avoid undermining the integrity of that.

By doing a pad, i obviously don't have to dig down as far so can inch a little closer to the wall leaving more space for the office itself. That was my original thinking anyway. I am a total novice though so could be way off the mark.

 

[Ivor Windybottom]

If you're on solid rock then just scrape, sweep and jetwash it and start building. No point removing solid rock and replacing it with concrete. Set out a string line at your intended floor height and do what's needed to get up to there.

Then bring your floor up to the same level, you'll need a DPM somewhere beneath your floor - not directly on the rock, which would pierce it.

Then build your walls on a DPC.

Then add a roof.

Done!

 

[snsmurf]

Then build your walls on a DPC.

Then add a roof.

Done!

hahaha, if only it was as simply as that. I'm a web developer by trade, manual labour is still new to me!