Slab for outbuilding

[J4mes]

Looking for some advice... We had planning approved for an outbuilding with dimensions of 4.8m x 3m with a pitched roof (overall height around 3.6M).

Approved plans:

As the building in under 30sq meters it doesn't require building regulations so apart from the planning drawings with dimensions we don't have much to go on.

I have been quoted for a reinforced slab with the metal rebar stuff and fibers 300mm deep which the groundworkers seems to think is more than enough for a single story outbuilding. I've also been quoted for double skin blockwork walls using concrete blocks (the heavy ones on the outside and thermalite blocks on the internal skin).


The roof / windows / electrics etc i'm more comfortable with sorting but the foundation and walls are pretty fundamental and i don't have a lot of experience of these things, does the above sound reasonable or am i missing something. It seems simple enough but I know that building rarely is straightforward!

Any advice would be welcome, thanks in advance

[Big Jimbo]

It depends if you have trees, and what your ground is like. Sounds over the top to me . I did a workshop, about 10m x 8m. dug out 6" of mot, then 6" of concrete. 12 years later, it was still fine. No cracks. Nothing.

[markc]

Our workshop has a concrete slab varying between 150-200mm with one layer of mesh and we regularly have 20t point loads

[J4mes]

52 minutes ago, Big Jimbo said:

It depends if you have trees, and what your ground is like. Sounds over the top to me . I did a workshop, about 10m x 8m. dug out 6" of mot, then 6" of concrete. 12 years later, it was still fine. No cracks. Nothing.

Thanks, there is a youngish Pine tree within a few meters of the build. It was only planted about 5 years ago, it’s going to take many many years to reach any height. I assume the mesh reinforcement in the slab would help long term with any potential cracking? 

Edited September 14, 2023 by J4mes

[Temp]

My 5x6m outbuilding is on approx 150mm hardcore and 150mm concrete, no rebar. Timber frame with clay tiled roof. 

[joe90]

2 hours ago, J4mes said:

I've also been quoted for double skin blockwork walls using concrete blocks (the heavy ones on the outside and thermalite blocks on the internal skin).

So are you building with blocks?. I too put a garage on a 150mm slab with steel mesh but it was timber, not masonry.

[J4mes]

21 minutes ago, joe90 said:

So are you building with blocks?. I too put a garage on a 150mm slab with steel mesh but it was timber, not masonry.

Yes building with blocks and a tiled roof. So assume 300mm with reinforcement would be better. I’m no expert though 

[Conor]

6 minutes ago, J4mes said:

Yes building with blocks and a tiled roof. So assume 300mm with reinforcement would be better. I’m no expert though 

Way too uch. 150mm max. Single layer of mesh. What are you going to be using it for? It might be work putting insulation in. You'll not be able to do it easily in the future. 200mm of EPS under the slab won't cost much.

[PeterW]

Dig it out to 450mm all the way across, 150mm MOT1 levelled and compacted then put 200mm jablite or similar poly insulation in the centre allowing for a 300mm ring around the outside. Fibres in the concrete, and build up the external skin in 140mm 7N blocks, insulate on the inside using insulated plasterboard and job done .. 

[J4mes]

23 minutes ago, Conor said:

Way too uch. 150mm max. Single layer of mesh. What are you going to be using it for? It might be work putting insulation in. You'll not be able to do it easily in the future. 200mm of EPS under the slab won't cost much.

Thank you. It’s going to be a Gym so it will be occasional use. The groundworkers have quoted to lay the slab and then once block work is done come back and add celotex insulation and screed floor to final level, would that not work? To be honest I’m trying to keep track of it all and work out levels etc as there are a lot of separate trades so don’t want to get it wrong.