Workshop base

[domdee]

Afternoon ladies and gents.

I’m going to be building a workshop in the next month or so as my 30th birthday present and will create a build thread in due course on the ‘your project’ part of the site.

I have a question regarding the base. How thick do I need to go???

The workshop is going to be 12ft x 18ft built like so:

Concrete base 13ft x 19ft.
Concrete blocks 3 course high (maybe 6 high if I sit them on sides for added rigidity).
Timber structure then built on top of block.
Tin roof.
Ply lined.
8ft from floor to gutters. 14ft from floor to ridge.

The timber frame will be a 4” posts construction covered externally in Yorkshire boarding. And osb internally lined.
Roof will be pitched made up of mainly 4”x2” a-frames tied together. Again lined with osb.
Corrugated tin roof.

I don’t think it’s going to be that heavy compared to brick and it won’t have heavy/regular vehicular use. It’s just a place for the gardening equipment and tools and a work bench etc, (maybe a classic car restoration or two), so I thought I’d proceed like this:

Take the top soil off down to firm ground (I’m in a heavy clay area and the land is a bout a 1:10 incline)

Compact recycled crushed brick to a depth of 6” (calculated at 7ton online)
Soft sand binding layer (1ton)
1000GA DPM.
6” concrete slab. 5 ton of ballast to 1 ton of general purpose cement (40 25kg bags). Mixed with old diy mixer and trying get it done all in 1 go. I am also going to be adding 1kg of fibres to every ton of ballast to give a little added strength and flexibility.

My old man thinks I could get away with 4” of concrete. What do you guys recommend? Am I missing something? Should I go deeper? I don’t want to dig a strip foundation around the edge to keep cost down and I don’t fancy shelling out for any kind of rebar or mesh either.

Any input greatly appreciated.

Thanks in advance, Dom

 

[23vc]

Are you trying to do a raft then? If not surely you can’t get away with just building the masonary off the slab. I’d still do trench foundations dug down to firm soil, but I’ve never built one quite like this so happy to stand corrected

 

[domdee]

I suppose it is like a raft yes. Never thought of it like that.

How deep/wide would you go with strip foundations?

 

[charliegolf]

I built my 6mx5m garage on a 4" reinforced slab with a 9" deep, 12" wide 'foundation' incorporated in its edge. Rendered concrete blocks with piers, trussed roof with concrete tiles and a 16' sectional door. Twenty years on, still there.

 

[SFK]

I would use Course sand, not soft sand as soft sand can be washed away.

All you thicknesses seem a bit thick or rather a bit too heavy duty (is suitable for heavy vans rather than a shed.
But but with you being on clay that might be for the best.

Ensure slight slope on concrete to make sure water does not collect under shed.

Or concrete same size as shed, and with slight slope.

This is a great website:
http://www.pavingexpert.com/concrete.htm

Sfk

 

[Notch7]

you could do a bit of a heel around the edge like a raft -itlll prob be strong enough for a workshop base

make concrete base same size as blockwork -or you will have probs with damp

if nearer than 2 metres to boundary you will need planning permission in England.

you dont need 4" posts, just use 4 x 2

externally clad studwork with breatheable membrane
then screw on 50 x 25 battens flat
then cladding
this is a far more weathertight construction as you have a cavity -weatherboarding will last longer and no chance of leaks if done properly

 

[cdbe]

Don't know what your clay is like, but mine is like rock - I put 100mm of concrete straight on top of it, a bit thicker on the edges, and put my prefab concrete garages on top with no problems. My dad's was done exactly the same 59 years ago and is absolutely fine. For that quantity of concrete it's probably worth pricing up barrow mix.

 

[domdee]

I built my 6mx5m garage on a 4" reinforced slab with a 9" deep, 12" wide 'foundation' incorporated in its edge. Rendered concrete blocks with piers, trussed roof with concrete tiles and a 16' sectional door. Twenty years on, still there.

what kind of reinforcing did you use? I’d imagine yours would be a much heavier old lump with block, render and tiles than my timber tin construction.

 

[domdee]

I would use Course sand, not soft sand as soft sand can be washed away.

All you thicknesses seem a bit thick or rather a bit too heavy duty (is suitable for heavy vans rather than a shed.
But but with you being on clay that might be for the best.

Ensure slight slope on concrete to make sure water does not collect under shed.

Or concrete same size as shed, and with slight slope.

This is a great website:
http://www.pavingexpert.com/concrete.htm

Sfk

Is course sand sharp sand? Like I said my dad thought it was a bit thick. But would rather be safe than sorry due to clay.
What do you mean by slope on concrete? I’d tamp it level wouldn’t I? Or do you mean slope around the edge down to soil?

 

[domdee]

you could do a bit of a heel around the edge like a raft -itlll prob be strong enough for a workshop base

make concrete base same size as blockwork -or you will have probs with damp

if nearer than 2 metres to boundary you will need planning permission in England.

you dont need 4" posts, just use 4 x 2

externally clad studwork with breatheable membrane
then screw on 50 x 25 battens flat
then cladding
this is a far more weathertight construction as you have a cavity -weatherboarding will last longer and no chance of leaks if done properly

is a heel an angled edge around the base? Like say 45deg chamfer down to the soil?
Yes I was going to use a membrane, just forgot to list it. Going down the theory of
Less 4x4 Post spaced further apart in a barn like config than 4x2 every 16” or what ever.
Do you mean pvc when you say weatherboard?

Also boundarys arent an issue.

 

[domdee]

Don't know what your clay is like, but mine is like rock - I put 100mm of concrete straight on top of it, a bit thicker on the edges, and put my prefab concrete garages on top with no problems. My dad's was done exactly the same 59 years ago and is absolutely fine. For that quantity of concrete it's probably worth pricing up barrow mix.

Unfortunately we have soft boulder clay here in East Yorkshire. Hence the compacted crushed brick to try and firm the ground up. Maybe I could get away with 100mm if your prefab has lasted that long problem free as I’ve tried lifting them panels and they are pretty heavy! Priced up ready mix and barrow mix. Seems quite expensive compared to buying ballast and cement in bulk from a local quarry. Although it will take much much much longer mixing it all I know! A full 9 hour day no doubt. Good job my dads a dab hand with a shovel.

 

[charliegolf]

what kind of reinforcing did you use? I’d imagine yours would be a much heavier old lump with block, render and tiles than my timber tin construction.

As I recall, 8x4 mesh sheets from the local steel stockholder.

 

[domdee]

As I recall, 8x4 mesh sheets from the local steel stockholder.

i have priced that up, seems expensive compared to the fibres you can use, which apparently offer better propperties. wheather thats the case or not i dont know?

 

[SFK]

Domdee, when I said course sand I meant to type in "Sharp Sand", which is what you should use over "Soft sand binding layer (1ton)".
https://www.wickes.co.uk/search?text=sharp+sand

And the reason I suggested putting a 'slight' angle on the whole concrete slab is because if you make it perfectly flat then you might have some water collecting under shed.

 

[domdee]

Right thanks for clarifying.
The concrete base is going to be my floor internally. So will have the dwarf walls around the sides with a raised single brick threshold where the door will be. I would like to think water will not be collecting as it shouldn’t be able to get in if I get some good mortar laid around the bottom of the block/brick work.