Shed Bases.........Directly onto soil?

[Alie Barnes]

I am making a shed base this weekend for a 12 x 8 shed, i was thinking of excavating 15" of soil and pouring concrete directly into the frame onto the soil. Do i need to lay hardcore realistically if its just for a wood working shed with no significant weight involved?

Thanks

 

[MikeG.]

I don't understand pouring "into the frame".

You don't need hardcore if you have got to firm ground within a reasonable depth. It's just that hardcore is a lot cheaper than concrete, so if you have to remove 15 inches of soil but only need 6 inches of concrete (structurally) then hardcore fills in the gap saving you hundreds of pounds.

 

[geoffshep]

You only need about 3" for something like a shed but check the firmness of the soil beneath. If the base is bigger than the shed, you really need to raise the shed on wooden bearers to stop water splashback onto shed walls. Putting it on wood also helps to keep the shed itself out of the wet - it is better for the bearers to rot and be replaced than the shed floor structure.

I've also put sheds straight onto wooden bearers on the ground. I dug a small trench to accommodate each bearer and used mortar/concrete to level each piece along its length and to each of the others. The wood bearers need to run perpendicular to the ribs on the underside of the base - so you need to know how they are orientated.

There is plenty of info online - or the trusty old Collins DIY book - this looks quite simply laid out: http://www.fountaintimber.co.uk/advice- ... or-a-shed/

 

[Seiken]

I put my 20' X 10' shed on paving slabs laid on sand, I picked up a load of slabs cheap on an auction site ( these are decent pavement slabs not the B&Q biscuits), its been there 10 years now and not moved and no signs of decay. I figured there would be little risk of water sitting under the base this way as any that could get under would drain between the slabs.

 

[MikeG.]

Yeah, but don't ever have timber in contact with the ground. Never, ever. Not if it is to do with a building. Put your concrete base down, then build a low plinth.......and there are many ways of doing that without involving timber, or even bricks if you aren't sure about your bricklaying skills.

 

[Alie Barnes]

Thanks Guys, its a prefab shed, but we have ordered it without a base because we will have woodwork machines (lathes) in there which we intend to bolt to the floor, can i lay timber runners around the edges of the shed, which will increase the height by a marginal amount but leave the shed not touching the concrete, i was also going to place a thin rubber liner between timber runner and shed timber for added protection, with this in mind is 3" enough? i was thinking of using 5" of concrete because it was going to have machinery bolted down to it.

 

[MikeG.]

No, it's the same problem. You'll still have timber in contact with the ground level concrete. The only solution is to have a small masonry upstand. This doesn't have to be brick. You could, for instance, lay concrete lintels or fence posts down on the concrete, and stand the building on that. There are even recycled rubber posts available which would do the same job. Sit your shed on that and have the bottom board of the cladding hanging down below the junction and you'll be OK.

 

[julianf]

I don't understand pouring "into the frame".

You don't need hardcore if you have got to firm ground within a reasonable depth. It's just that hardcore is a lot cheaper than concrete, so if you have to remove 15 inches of soil but only need 6 inches of concrete (structurally) then hardcore fills in the gap saving you hundreds of pounds.

Subbase isnt just "cheap concrete" - it has a function beyond that to allow for small amounts of movement.

A sheet of glass bolted entirely without give to a sheet of wood would crack if the wood moved at all.
Same sheet of glass with a slightly flexible packing would survive longer.

Same reason why soil pipes are run through pea shingle. Pea shingle isnt used for subbase in this country, but its common in other countries.

 

[julianf]

Depending on the size and construction of the shed, what about putting it on pillars?

I mean, plenty of sheds are just on bricks - somthing along those lines, but maybe a bit more fancy.

Indeed, you could dig holes, fill with concrete, and build you pillars off them. No subbase needed then, as its not the same situation. Depth of hole will control movement, but the wooden structure will tolerate more than a 100% rigid structure would.

If going down this path, put DPC between the tops of the pillars, and your wooden structure. Make the air gap large enough for a cat to get in and remove the rodents...

 

[Alie Barnes]

appreciate the views and points raised, i have opted to go down the sub base and concrete route, because of the need to have machinery bolted to it so makes sense to make it as strong as i can.

I was thinking of going 2" of hardcore and 4" of concrete, this would give me the 15cm for the standard size gravel board (give or take) is there any reason i should be having a thicker concrete base or is that enough?

Thanks

 

[julianf]

Noone can ever answer this question on the internet with any great amount of certainty, as its so dependant on soil conditions.

The soil in our garden is such that, to move it, you need a pickaxe very quickly, as its, basically, bonded rock.
Where my folks live, in essex, its clay, which expands and contracts with moisture.

My old man is a (retired) buildings surveyor, but he is useless for any information of that type for the soil that i have here - he could spec foundation depth required for the area where he worked, but its just not the same all over.

Regs are a bit of a blanket coverage, but youre not dealing with them for a shed... (and if you were, you would be hiring in skips for the amount of spoil you would need to remove).

All that said, 4" of concrete, over 2" of decent subbase does not sound bad, provding you have solid soil that's not going to move about too much with moisture content. For what it will cost you, put in some mesh also, but make sure its centralised, as it should be 50mm from ether face. You will probably need another inch in there though, if you want to blind the subbase, and put in a DPM at that point.

 

[julianf]

ps.

Concrete itself is not crazy money. If youre not fussed about levels, putting another inch in is not going to cost you much more. On your area, my sums make it less than 0.25cube more.

I find getting shot of the spoil more trouble than laying more concrete!

 

[MikeG.]

150mm is plenty for most domestic outbuildings. 100mm can be enough. Unless you have some some unusual stresses on the base, 150 is fine.

Frankly, I'm not concerned about your concrete design. It'll be fine. What bothers me is the thought of sitting your shed straight down on top of it. That's just a good way of wasting the money you spent on the building.

 

[MikeG.]

.......All that said, 4" of concrete.........For what it will cost you, put in some mesh also, but make sure its centralised, as it should be 50mm from ether face......

Huh? Reinforcement on the line of neutrality helps in neither tension nor compression. All it can do is reduce micro-cracking from shrinkage, and there are better ways of achieving that.

 

[Alie Barnes]

150mm is plenty for most domestic outbuildings. 100mm can be enough. Unless you have some some unusual stresses on the base, 150 is fine.

Frankly, I'm not concerned about your concrete design. It'll be fine. What bothers me is the thought of sitting your shed straight down on top of it. That's just a good way of wasting the money you spent on the building.

I have taken on board peoples views here and will not be putting the shed directly on to the concrete, it'll be raised slightly.

 

[MikeG.]

But on what? Last time you said on timber, and that is the issue.

 

[Alie Barnes]

But on what? Last time you said on timber, and that is the issue.

so will be looking at bricks or conctrete lintels, not sure what else to use, i was looking for suitable suggestions.

 

[MikeG.]

Bricks look best, but require a little skill. Concrete lintels or fence posts just need cutting to length and bedding on mortar, but don't look quite so great. Both will need straps building in underneath, for screwing to the shed walls to hold everything down when the wind blows.

 

[transatlantic]

I have just installed my shed on a concreate base, with a plastic shed base kit between the concreate and the bearers. The plastic pieces then get filled with gravel. It's not cheap, but I am very happy with the result. The plastic pieces are very robust and prevent damp rising up to the bearers. The gravel is to allow for drainage.

 

[Geoff_S]

This is a coincidence as I have just posted this link on another thread.

http://www.supremeconcrete.co.uk/genera ... d-dekpost/

No, I don't work for them.