Concrete slab rules of thumb

[Barney86]

Hello,

 

Before I set off using my "that looks about right" engineering techniques, I remembered a few of you sounded clever Here's one for the builders:

 

I've just taken delivery of a second hand concrete prefab garage (minus the roof). Exceptionally heavy & there are no instructions for me to not read.

 

It'll need to go on a concrete base, but that's where my knowledge on the subject ends. I've read some internet, which suggests l'd get away with something like 250mm deep concrete under the walls, and 100mm in the middle.

 

Does that depth sound about right? Will I need re-inforcing mesh?

 

It's a 6m by 6m shed, I'm going to need a lot of concrete

[Beretta28g]

I did on my shed 2' deep 8'' wide trenches round perimiter and also through the middle, the rest was just 8'' deep prob very over engineered but i didnt have to pay for the concrete and yes i did use re inforcing

[RED BEARD]

an 8" slab with mesh should be fine,as long as the ground below is good.

[delburt0]

Personally contrary to other views I would personally like 8" maybe a extra inch or 2 around the edges ..

[lincs1963]

Personally contrary to other views I would personally like 8" maybe a extra inch or 2 around the edges ..

8"? For a shed? Driveways are only normally 4" thick! The strength comes from whatever you use as a substrate, a reasonable thickness of hardcore well rollered or wackered in with 4" of concrete on top, with or without mesh, willl be plenty.

You could increase the thickness slightly at the edges as its a prefab garage but if it was a wooden shed I wouldn't bother.

HTH, Neil

[Kes]

The most important element as said above is the foundation upon which the concrete is laid - if its rock then very little foundation is needed.

If its ***** then increase accordingly. Assuming its resistant like clay, some hardcore - 6-9"),using 2" down stone) well compacted should allow a 6" foundation (by 8") if its sloppy stuff which gives easily, double both the above and widen the stone trench to 12 ".

Mesh must be top and bottom as one (Bottom layer, 2" from base) is for resistance (concrete has no styrength in tension). The top is to control shrinkage cracking again 2" from the surface - all this allows for site methods but dont put mesh in the trencjh bottom or on the surface.

Good luck.

Always minimise the most expensive components of a build !

[goodo123]

Just put some cardboard down

[Barney86]

Thanks lads, looks like I wasn't far out.

 

I've been cursing that the ground is terrible for my vegetables, but looks like it could save me a couple of quid here! It's quite heavy & has set hard as a rock in this sun.