Building advice please

Im building a shed and want to put a concrete base down, its going to be aprox 102 ins x 44 ins and 4 ins deep, Ive worked out I need approx 11 cu feet of concrete, is that right ? and how many bags of cement and sand would I need to complete the job ? any advice will be gratefully recieved
Thanks
Stew

[embee]

In new money that's about 0.3 cubic metres. A big builders bag is about 0.6m3 so I guess you plan to use small bags from Wickes/B&Q etc.

For general purpose concrete you want gravel+sand+cement (Ordinary Portland or "OP" cement) in a ratio of 4:2:1 by volume (i.e. buckets full). You don't end up with "7" volumes though, the sand fills the gaps between the stones and the cement fills the gaps between the sand, so you get only slightly more than the volume of the stones. For concrete use "sharp" sand, not "building" sand, which is a lot finer.

If you're only doing the concrete, then you can get bags of "ballast" which is 20mm stones+sand in the right proportions, and makes life easier. Roughly 5 bags of ballast to one bag of OP cement will work fine for a shed base. Mix the dry ingredients before adding any water, add it a bit at a time while mixing until it just becomes "wet" (i.e. it's beginning to get a shine and will smooth out when trowelled, but not runny). You should be using water equivalent to about half the weight of the cement as a guide.

I believe the small bags are about 25kg, or 40 to the tonne, which means you'll need around 15-20 at a stab, and 3-4 bags of cement. Rough guide anyway. Remember concrete takes best part of 4wks to reach full strength, so treat it carefully for at least a week while it's still "green".

[Luckypants]

I have a half of a one tonne bag (big builders bag) of ballast (sand and stones as above) you can have if you have access to a trailer stewie. It is just sat on my drive being un-tidy at the moment. Sounds about the right amount for what you need. You can always buy a load of smaller bags of ballast from Wickes (or wherever) in case it is not quite enough. Most builders merchants will take any un-used bags of ballast back without charge.

have a look here at the bottom of the page...

http://www.diydoctor.org.uk/projects...g_concrete.htm

worked a treat for me..

oh yeah and dont do it the week it ****es it down as it makes a mess and takes ages to go off..

[timwilky]

4" deep?. round here that would shift/crack so quickly. The clay we live on moves with the weather. People have spend 10 of thousands on nice new pressed concrete drives to see them crack in 5 years.

Only answer seems to be about 12" with lots of rebar

Have you considered getting your concrete ready mixed? The minimum they are likely to do is 0.5 cubic meter but you only need to increase the thickness to 7 inch to use the lot. It's a lot easier than mixing it yourself!

Originally Posted by Mogs

Have you considered getting your concrete ready mixed? The minimum they are likely to do is 0.5 cubic meter but you only need to increase the thickness to 7 inch to use the lot. It's a lot easier than mixing it yourself!

depends where you are but its more likley to be £180 for a cube and a cube will be the minimum,

it only took me about 4 hours to mix .75 of a cube with an electric mixer so give yoru self a saturday and youll be good to go..

Originally Posted by Luckypants

I have a half of a one tonne bag (big builders bag) of ballast (sand and stones as above) you can have if you have access to a trailer stewie. It is just sat on my drive being un-tidy at the moment. Sounds about the right amount for what you need. You can always buy a load of smaller bags of ballast from Wickes (or wherever) in case it is not quite enough. Most builders merchants will take any un-used bags of ballast back without charge.

Cheers Mike, appreciate that mate, I dont have access to a trailer unfrotunately but if I do I will let you know, thanks

Thanks guys, appreciate the advice given
Cheers
Stew

[johnnyrod]

I'd use slabs and a dry ballast/cement mix, one bag of ballast per 18x18" slab, it's only a little shed