Slab size for new shed

G'day all
First time poster, so apologies if this question has been asked too many times. I'm looking to lay a slab for a 7.5 x 3.75m shed, and I'm unsure how much bigger the slab should be. Would 100mm be enough, or do I need to go 200 or even 300 ?

Cheers Phil

Hey there Phil - I just went through the same thing, although I had my slab laid by a professional concreter.

From my limited experience, I'd say the dimensions of the slab really depend on how you are going to construct the shed. For example, if you intend to set the side and rear walls on top of the slab, then the slab will need to be wider than the oustide dimensions of the shed. If not, the slab needs to be the same dimension as the shed, as the sheets will fit neatly over the concrete.

I prefer the latter method as then there is not such an issue with water getting in under the sheets as there would be if the sheets sit on top of the concrete.

To answer your question more specifically though, if you are setting sheets on top of the concrete, I wouldn't imagine there would be any main structual difference between 50mm to 100mm to 300mm - although the bigger the difference, the more water may pool on the edges and rust sheets quicker.

Don't forget, you may have a set back of side posts from the edge of the concrete to allow for side and rear battens which wall sheets are screwed to. So the outside edge of the posts will be in from the edge of the concrete by say 60mm to allow for the protrusion of the wall batten.

Hope this helps. Importanly, I hope the rationale is sound!

cheers
DNL

Hi Phil
I think DNL has covered most of it except to that if you're buying the shed in (not building it from scratch) then discuss it with your supplier. The main point I would recommend is don't have the top surface of the concrete at ground level - err on being higher rather than lower. I have seen a number set at ground level which when you get heavy rain the surface flooding flows into the shed. Check your property for slope and run-off from the neighbours as well. It's easier to put in a ramp/step to get into the shed than to have to bail it out :)

I think the time on my tuppence has just run out.

Ken

There is of course another way, pour the slab inside the shed after it is erected. This is the method I used my current shed and the last one.

Makes it a little harder to erect the shed, but it pays of in having a good seal between slab and shed, no nasties and water can get in.

This current shed used brackets and plates that were concreted into 600mm deep holes, then the frames were bolted on top, 100x50RHS, nice method of construction.

Last shed was C-channel with frames concreted in holes, bit harder to do.

G'day DNL

Quote:

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Originally Posted by DNL

Hey there Phil - I just went through the same thing, although I had my slab laid by a professional concreter.

I'm going to have a crack at it. I've only laid two slabs before, one for a pool pump filter, and a short pathway, so they were obviously MUCH smaller. But hey, same principal right, just bigger.....Wish me luck !

From my limited experience, I'd say the dimensions of the slab really depend on how you are going to construct the shed. For example, if you intend to set the side and rear walls on top of the slab, then the slab will need to be wider than the oustide dimensions of the shed. If not, the slab needs to be the same dimension as the shed, as the sheets will fit neatly over the concrete.

OK, that makes sense. I just assumed the shed would sit on top, and I'd seal up the bottom from the inside.

I prefer the latter method as then there is not such an issue with water getting in under the sheets as there would be if the sheets sit on top of the concrete.

Very interesting. I will do some more homework on which would be my best option.

To answer your question more specifically though, if you are setting sheets on top of the concrete, I wouldn't imagine there would be any main structual difference between 50mm to 100mm to 300mm - although the bigger the difference, the more water may pool on the edges and rust sheets quicker.


Don't forget, you may have a set back of side posts from the edge of the concrete to allow for side and rear battens which wall sheets are screwed to. So the outside edge of the posts will be in from the edge of the concrete by say 60mm to allow for the protrusion of the wall batten.

Hope this helps. Importanly, I hope the rationale is sound!

Thanks for the detailed reply, it was extremely helpful.

Cheers Phil


cheers
DNL

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If you are pouring the slab before hand you will need to be relatively exact in the squareness of it, which is no big deal really.

I also suggest boxing a 25mm recess around the edge of the slab to stop water seeping under the walling IF your iron doesnt continue down past the level of the slab.

My last two sheds I've built myself have had the slabs poured after the shed was erected. As has been said, this provides a seal around the bottom of the shed without much extra work.

However...
...concrete is corrosive and over a long period of time can rust away the bottom of the sheets. In my current shed, I simply painted the bottoms of the inside of the sheets in Bitumen paint, providing a protective layer for the sheets.