Easiest way to extend shed base width

[Lex90]

I have an area where I want to put a shed that currently has a nice level concrete slab 3ft 7’’ wide, next to grass. I want to put a shed there that has a footprint width of 4ft. I also have some spare sharp sand and some gravel (though the stone size is quite large) if that might be useful. What are my easiest options for extending the width so the shed will be fully supported and no moisture issues? It’s a KETER plastic shed. Thanks for any help or ideas

 

[Deluks]

Dig out next to and slightly under the existing slab. Use neat gravel at the bottom of the hole. Tamp it down well with a sledgehammer or an old fence post.
Use remaining gravel and sharp mixed in with cement to make concrete and fill the hole.
If you don't have enough material buy a few small bags of ballast to make up the difference.

 

[martygturner]

You can normally find paving slabs free to a good home in local ads, as its under a shed just dig out gravel, sand blanking and pop the pavers down. All paving slabs are just concrete in a easy to move lump.

 

[Lex90]

Thanks, that’s really helpful. This is where I’m at so far - now down to the chalk so a bit harder going, but I’ve got under the existing slab so do I need to go down much further? I can’t see what base they used but now would be the gravel going underneath the old slab then build from there? Thanks again

 

[Lex90]

Photos for context

 

[Deluks]

If you are going with the self mixed concrete route, you want the wet stuff poked in under the existing slab, this will help to lock it together.
Ideally you want to go down 150 - 200mm below the grass.

 

[Mottie]

and no moisture issues? It’s a KETER plastic shed.

When I was looking to get a shed on my allotment plot, I was advised to get a wooden one. I was told that metal and plastic sheds suffer greatly with condensation.

 

[Lex90]

When I was looking to get a shed on my allotment plot, I was advised to get a wooden one. I was told that metal and plastic sheds suffer greatly with condensation.

Ee, shed already purchased and half built unfortunately! Thanks though

 

[Mottie]

Ee, shed already purchased and half built unfortunately! Thanks though

If it hasn’t got any fitted, fit a couple of air vent grilles.

 

[robinbanks]

this will help to lock it together.

Nah, not concrete that's been buried in mud for years. It won't matter though

I was told that metal and plastic sheds suffer greatly with condensation.

Only because the material they're made of is non porous. Wooden sheds still have the same amount of condensation, you just don't see it because it's soaking into the wood, helping it to rot away if it never gets a chance to dry. Ensure a ventilation strategy for the plastic/metal sheds combats build up of internal moisture

 

[Deluks]

Nah, not concrete that's been buried in mud for years. It won't matter though

I'm currently doing the same job, the underside of the slab is rough and uneven, it provides a better key than just butting up.