Starting the foundation

[boschy]

Hello all,

Im just about to venture into finally building a workshop in my little garden 2.25 x 4.55M. Now I have helped build a workshop in the past but it was on a concrete base with access and a much bigger budget than I have.

I feel a the hard part is starting!.. I was looking at going the rod system, so burying some threaded rods and cover with concrete then build the wood structure. I lifted a paving slab where the shed will soon live and discovered its concrete underneath. The whole garden is paving slabs so I expected some packed down sand but not concrete. I'm Not entirely how to proceed now, should I chisel away and just see how deep it is? or rethink this as a good thing as build on top on this with timber bearers. It has to take the weight of things like a motorbike, lathe, etc so have some concern about it.

 

[martygturner]

dig out the side or pop a drill through the concrete and work out is depth and therefore its loading...

When you say rod were you planning to make some piers and use a ring beam or drill in the rod type foundation screws ?

 

[boschy]

thanks for the reply @martygturner. Yeah was thinking of poking a hole and seeing how deep it was, good place to start. Id imagine these slabs have been here for decades. They are stable and level... I was going to dig out 60cm holes bury some threaded rods with a plate and fill with concrete, but I think its over engineered if it already has concrete and is flat.. its got a smaller cheap crap shed on it at the moment that was here when we moved in thats just been laid over the top of the slabs and concreted in.

 

[Mr Chibs]

Are you sure it's concrete under there, not just a sharp sand/cement layer for bedding the paving on?

I'd lift up a slab and dig out some of the 'concrete' to see what's what and how thick.
Concrete will have larger pebbles in it, bedding mix will be finer.

Can you not dig the soil away at the edge of the slabs to see how deep it is?

 

[johnny2007]

Are you sure it's concrete under there, not just a sharp sand/cement layer for bedding the paving on?

I'd lift up a slab and dig out some of the 'concrete' to see what's what and how thick.
Concrete will have larger pebbles in it, bedding mix will be finer.

Can you not dig the soil away at the edge of the slabs to see how deep it is?

+1
That looks like sand and cement.
Stab it with an old screwdriver and if you can chip it, it's not concrete.

 

[boschy]

Digging from the soil makes sense not sure why I didn't think of that.. I didn't want to start messing with it incase I could just build straight on top. Yes its does seem finer than concrete which makes sense. I thought it would be much easer to chip if it wasn't. Do you think I could just lay piers straight onto and build from there or is that risky, if so why?

Thanks for all the help.

 

[boschy]

It’s deeper than I thought!

 

[Deluks]

Dig down til you hit firmer ground, lay concrete blocks on their side, build up to above the existing slabs, then build your framework, obviously get the slabs level first.
If you use proper concrete blocks you can just stack them dry. Or use hollow blocks and backfill them with weak concrete mix.

 

[boschy]

Looks about 15cm of compact sand. Initially I was going to dig 18 holes 60cm deep for the rods. I think its going be really hard getting through so just want to make sure I should go this route! whats the best way of braking this compacted top layer? big hammer?

thanks for the help all.

 

[Mr Chibs]

If you want to dig it up get a Concrete breaker (Titan breaker) will shift it in no time, look for a second hand one on Facebook, will be cheaper than renting one.

The base is not sand if it's hard to break, it's bedding with sand & cement.

If the slabbed area is large enough, why not just build off the slab?

You can put wooden bearers down, some use concrete fence posts... or concrete blocks, then put your timber down and carry on.

 

[boschy]

You think just going off the slabs would be okay. I mean it makes sense. It would certainly save a lot of time, disposal etc.
this is the “garden” at the moment. I’m replacing that shed with a workshop that will run across the whole of the back wall, same depth. So it’ll be replacing the back fence and some of the side fences.

https://www.diy.com/departments/ver...15FhP1qL4EVQKd1dS_BoCyecQAvD_BwE&gclsrc=aw.ds did find this for some hole digging

 

[cdbe]

If you have access, I'd shutter up and pour a metre of concrete on top of those slabs - they don't seem to have moved or settled much and a slab is great for load spreading, and will raise your floor to avoid damp issues.