Shed Foundations

[Mark-UK]

Hello all

I'm completely new to building almost anything so I hope someone can help me out.

I am building a 20' x 14' shed in my back garden, truth be told it might be smaller but that's the size I would ideally like.

Not sure if it’s relevant, but my garden has a high clay content

The actual timber shed construction seems to be relatively straight forward as most seem to be constructed the same way. I am though, having some trouble deciding on a foundation. This is what I’ve considered:

(a) Placing the timber joists directly on the ground, but that’s just inviting damp into the shed so it's not really an option.

(b) Digging to a depth of around 4″ and filling with gravel and tamping it all down - the wooden joists lay on top

(c) Placing the wooden joists on top of dense breeze blocks – this seems to be the better choice as it will cost no more than £90 to £100 as I'll need quite a few

(d) Digging to a depth of 4″ in filling with concrete, but ready mixed sends the price up to just over £300

Of them all, even though it's expensive, I'd prefer option (d) as it seems the most straight forward.

Any advice would be appreciated.

Thanks all

 

[theprinceofdarkness]

It depends on the state of the ground, D is better as it stops weeds growing under the shed. 4" seems excessive for a wooden shed but the ground must be really well compacted. Its a big shed so I think you will have to spend at least a whole day compacting and rolling the surface until it is smooth and shiney. You also want keep an eye on the level as you don't want your lump of concrete to turn into a swimming pool, its best if it at least a few inches above the general level.
I built my shed in 1982. It had a retaining wall around it which varied from 6" to 9" above the lawn to get the door/patio level right. This box was then filled with the earth(clay) dug out from the stepped wall foundations, the centre section had about 2" of concrete over it. My shed joist rested on this with a layer of felt (DPC). In the end the shed had a 1000 kg milling machine in it and a 500 kg lathe as well as a lot of other engineering gear. The base and shed were perfect 30 years later when I sold the house. The big weakness with sheds is water running down the wall and splashing back under the wall/floor joint, perhaps a short plastic skirt?
There is an awful amount of info on home brew sheds on the web.
Frank

 

[Mark-UK]

It depends on the state of the ground, D is better as it stops weeds growing under the shed. 4" seems excessive for a wooden shed but the ground must be really well compacted. Its a big shed so I think you will have to spend at least a whole day compacting and rolling the surface until it is smooth and shiney. You also want keep an eye on the level as you don't want your lump of concrete to turn into a swimming pool, its best if it at least a few inches above the general level.
I built my shed in 1982. It had a retaining wall around it which varied from 6" to 9" above the lawn to get the door/patio level right. This box was then filled with the earth(clay) dug out from the stepped wall foundations, the centre section had about 2" of concrete over it. My shed joist rested on this with a layer of felt (DPC). In the end the shed had a 1000 kg milling machine in it and a 500 kg lathe as well as a lot of other engineering gear. The base and shed were perfect 30 years later when I sold the house. The big weakness with sheds is water running down the wall and splashing back under the wall/floor joint, perhaps a short plastic skirt?
There is an awful amount of info on home brew sheds on the web.
Frank

Thanks for the advice Frank

I've been told my ground gets boggy due to the clay underneath, though that's predominantly in the front part of the garden, it's not nearly so bad toward the back, where the shed will be.

What is 'D'? Sorry, complete novice here

You're right about the 4", not sure why I said that. I'll be digging down 6", covering over with 3" of gravel and then 3" of ready mix cement.

I've read that making the concrete base about 2" to 3" larger than the shed will help prevent the rain splash back. Hopefully that, and a plastic skirt will solve that problem.

 

[theprinceofdarkness]

Sorry it should have d re. splash back, the DPC on houses are spec'd at 6" above ground level to over come this. Your base seems to be a better affair then mine! Remember that its consolidating the ground that does it, loose gravel is all smooth and slippery, you want quarry scalpings which is the small rubbish they can't sell.
Frank

 

[Mark-UK]

Thanks for clarifying Frank

The more I think about it, the more I'm tempted to go for a ready mix cement and just pour the mix out over the quarry scalpings (thanks for the tip).

£300 should do it :/

 

[Mark-UK]

Would you recommend these 70mm bolts for fixing two 4" x 2" joists together? Obviously they will be going straigth through the two 4" depths

Bolts

Thank you

 

[theprinceofdarkness]

"Bright" zinc, rusts also high tensile bolts are for bridges and cranes, these are a bit OTT for a shed. Unless you wish to lift the shed and its contents on one of them
Frank

 

[JohnD]

Would you recommend these 70mm bolts for fixing two 4" x 2" joists together?

No, because they won't reach.

 

[Mark-UK]

Ah, thanks for the advice

Any suggestions on what I can use to screw one 4" of a 4"x2" to a 4"x4"?

@JohnD I'd say that is what I get for posting so early in the morning, but the reality is, I would have made that mistake any time of the day

 

[Mark-UK]

I found these timber screws 150mm (6 inches or near enough)

Here

Will they work? Difficult to gauge how thick they are

What does the 7.0 in 7.0 x 150mm mean?

 

[JohnD]

M8 x 100mm hex bolt if you are going through the two short sides. On softwood framing I would use galvanised penny washers under head and nut. Not coach bolts as they are liable to turn.

BZP will prevent them going rusty in the warehouse, but will not last long if the shed is at all damp. If you hope to undo them one day, use copaslip or grease on the threads. If you use a small stiff brush after pushing the bolts through the holes, it will only take a moment.

I am on the coast and now use stainless for outdoor work.

If you need to go through the long sides, try 8mm threaded studding, it is handy and you can cut it to length required. I found it works out cheaper than extra-long bolts.

 

[Mark-UK]

M8 x 100mm hex bolt if you are going through the two short sides. On softwood framing I would use galvanised penny washers under head and nut. Not coach bolts as they are liable to turn.

BZP will prevent them going rusty in the warehouse, but will not last long if the shed is at all damp. If you hope to undo them one day, use copaslip or grease on the threads. If you use a small stiff brush after pushing the bolts through the holes, it will only take a moment.

I am on the coast and now use stainless for outdoor work.

If you need to go through the long sides, try 8mm threaded studding, it is handy and you can cut it to length required. I found it works out cheaper than extra-long bolts.

Sorry John, are you saying to use M8 x 100mm hex bolts?

 

[JohnD]

yes

 

[Mark-UK]

That's weird, I quoted you but the quote is different to the original post

Anyway, thanks for the advice

 

[Mark-UK]

Given a 19' x 16' shed, will 4" concrete atop DPM and 1" of sand, all on compacted earth be enough of a solid base, or do I also need to lay down a few inches of quarry scalping? It's for woodworking, and at some point in the future I'll be hoping to house a lathe and a table saw in there.

I've been told I don't need the scalping but I'm not convinced.