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Laying down a shed base

Posted: Sun Oct 25, 2015 12:01 am
by Barry
Hello all those good at calculating amounts of material!

I am shortly going to buy a 6' x 4' shed.

This will be supported on a base consisting of paving slabs of a just slightly larger size.

First, you have to mix one part cement to eight parts of sand to achieve a 4cm dry mix onto which the slabs can be placed.

Can anybody give me even the vaguest idea of how much sand and how much cement in kilos I am going to need?

Also, why does this have to be a dry mix? I am assuming that moisture will be drawn from the ground and the dry mix will solidify, binding the slabs together. Is that right?

Re: Laying down a shed base

Posted: Sun Oct 25, 2015 5:21 am
by Pa Snip
Morning Barry

3 years ago, Sept 2012, I made a dry mix base for my new 7 x 5 shed. I probably made it a lot thicker than needed (about 4 inches) but I did not use slabs. Just the dry mix sharp sand and cement.
Doing it this way I was instructed to make the dry mix and then use a watering can with rose to lightly wet the mixture.

Your assumption about drawing the moisture if you lay slabs is correct. Don't forget to keep some dry mix back to fill between the slabs.

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I tried using a couple of the sand and cement volume calculators available online to answer your question and can't say as I believe the suggested figures. , they seemed light to me.
They suggest 1 x 25 kg bag cement to 1 or 2 bags sharp sand.

A 'ready mixed wet concrete' site suggested you would need a total of 0.1167cu. yd.

Re: Laying down a shed base

Posted: Sun Oct 25, 2015 8:36 am
by Geoff
Your area is 2.23 sq metres, call it 2.5 to be oversize, 4 cms thick means you require 0.1 cubic metres. Internet says a cubic metre of concrete weighs about 2,400 kgs but just sand and cement will be less dense so you could try 1 bag of cement and 8 bags of sand equals 225 kgs and see how you get on.

If you keep searching you find all sorts of answers http://www.wickes.co.uk/Blue-Circle-Sla ... g/p/153707 says 20 kgs bag does 0.6 sq metres 25mm thick, I think that comes to 6 bags.

If you have access to a mixer making 6" of concrete and not buying slabs might be the cheapest option, but then I use concrete for everything, made 25 mixes last week.

Re: Laying down a shed base

Posted: Sun Oct 25, 2015 10:38 am
by peter
The dry mix serves two purposes.

First it is very easy to get level and lay slabs on, just like sand.

Second, unlike sand, once it has been moistened, it sets and will not get washed away from under the slabs. :wink:

Re: Laying down a shed base

Posted: Wed Oct 28, 2015 11:33 pm
by Barry
Hi Everybody,

Thanks for all the feedback.

The reason for using slabs is because I have loads I can use for free and it saves money to recycle these.

I just measured them and found that they are 45cm by 45cm, which is bad news, because I need 3 x 5 slabs and not the 3 x 4 that I thought to support a 6' x 4' shed!

Geoff, based on the Wickes product you found, bearing in mind I need to go 4cm thick, would that approximate to about 8 bags?

The total area in the new calculation is one of 3.0375 square metres.

What do you think.

PS. Sorry it has taken so long to reply, but I am having to carve out land on my new allotment at around one square metre per hour, given the astonishing amount of roots I am having to dig out, plus the soil is heavy clay, so just turning this stuff over is taxing, to say the least! I am now having to cover land to stop it getting wetter and therefore unworkable. Great fun though and I have a real sence of progress. I just now need to get a shed up to catch the free water falling from the sky, since we have no ground water!

Re: Laying down a shed base

Posted: Thu Oct 29, 2015 10:07 am
by Geoff
Yes I reckon 8.1. They say you can use less than 4 cm so you could keep a bit back for pointing.

Re: Laying down a shed base

Posted: Thu Oct 29, 2015 11:55 am
by Shallot Man
If your shed has it's own floor. Would humble suggest you place three bits of quartering down, to allow drought under floor of shed. This will help to stop the base rotting.

Re: Laying down a shed base

Posted: Thu Oct 29, 2015 12:03 pm
by Barry
What do you mean by "three bits of quartering"?

Re: Laying down a shed base

Posted: Thu Oct 29, 2015 4:41 pm
by peter
Put treated timbers, like fence posts, on the slabs right angles to the measly skinny little joists under the floorboards (assuming a bought cheap shed) and sit the shed on top. Some long screws throgj floorboards and joists into the fence posts to hold it all down. I usually add angle iron or galvanised strapping concreted into the ground and screwed or coachbolted to one of the larger timbers under the floor, wont blow away or be tipped over.

Re: Laying down a shed base

Posted: Thu Oct 29, 2015 4:44 pm
by peter
Then air can circulste and reduce chances of rot.

Re: Laying down a shed base

Posted: Thu Oct 29, 2015 8:01 pm
by Barry
Peter,

I have absolutely no idea what you last sentence means :D

Thinking back, I do remember seeing shed bases that have the arrangement you refer to, whereby the post timber is nailed across the base at right angles.

This is the shed I propose to buy:

http://www.acesheds.co.uk/products/Stan ... /140615011

It looks to me as though air flow has been taken into account, since the base of the shed is clearly raised on timbers of some sort.

What do you think?

Re: Laying down a shed base

Posted: Thu Oct 29, 2015 11:51 pm
by peter
Ah, the lovely OSB, aka flaked wood glued back together, very susceptible to falling apart like weetabix once wet, but fine if kept dry.

My allotment neighbour s cross the main path dug a shallow grave 6" deep to sit his shed in, I hate to think what the floor is like now.

All my diy built structures sit on either brick pillars and horizontal fence posts or on concrete with fence posts as joist/runners. Get that air through under the floor, so my workshop shed has rock wool on chicken wire under the floorboards, recycled decking. :wink:

Re: Laying down a shed base

Posted: Fri Oct 30, 2015 5:50 am
by Pa Snip
Hi Barry

Using that link showed that a 6 x 4 shed of that type is £338.00 so your shed should be of reasonable quality although I am surprised it has a osb floor at that price.
At least it doesn't look as if the front and sides are made from feathered overlap which soon distorts

It does show cross bearers underneath so that point is covered.

Once you are ready the first thing to do is make sure you give it a couple of good coatings of preservative. But not at their prices !!!

Do treat yourself to a good quality lock, and fix all lock fittings to the shed using coach bolts not screws

The most important bit comes once you have dealt with the installation and security and that is ......................................






enjoy your shed

Re: Laying down a shed base

Posted: Fri Oct 30, 2015 8:40 am
by Shallot Man
Barry. I think Peters reply covers it.

Re: Laying down a shed base

Posted: Fri Oct 30, 2015 10:42 am
by JohnN
I put 4"x4" tannelised posts under my summer house 30 years ago and they're still sound and supportive. I laid 8" wide strips of roofing felt under and up the sides of each post, which stops them getting wet from water that can run under the shed in heavy rain.