Shed foundations: How much ballast and cement

Abu Baniaz

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I am in the process of building my shed and would be grateful for assistance with finalising things and also calculating the ballast and cement I require.

The intended size of shed is 4m x 2m, using single layer of blocks. Height will be 2m. The external dimension of foundation trench are 4.2m and 2.2m to give a width of 30cm. The trench is 70cm deep. I intend to put about 4 inches of hardcore at the bottom. I have lots of it to get rid of from garden. The concrete will not go all way up to the top.

Q1.
Is the 10cm allowance at top correct?

Q2.
Based on above, my calculations of the concrete required are:
[(2.2 x 0.3 x 2 sides) + (3.6 x 0.3 x 2 sides) = [1.32 + 2.16] x 0.5 (concrete thickness allowing for first brick layer and hardcore) = 1.08 m3

Are these correct?

Parking is an issue, so getting pre-mixed concrete is going to be difficult. I will have to mix my own. I have looked online for the calculators and cant make sense of them.

Q3.
How many jumbo bags of ballast do I need?
Q4.
How many bags of cement do I need?

Thanks in advance.

Shed.png
 

silverdale

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Get some old flags 3x2 or 4x2 the big heavy fookers, someone's always giving them away on the free ads.
Lay them on 3-2-1 mix, sharp sand builders sand cement , lob shed on jobs done.
Little tip my Dad use to do. Get a roll of chicken mesh and roll it out when laying the sand cement.
It just ties everything in, it's a bit belt and braces I know but I've always done it that way when laying weight bearing flags like a shed or drive.
 

FocuS

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hey, the first thing i would ask is what is the ground you have dug down to is clay? hard clay, is it soft? then you can decide the foundation concrete thickness, as for putting hardcore in the bottom of the trench are you going to be able to compact it? , are you saying you want 300mm deep 300mm width by 9600mm long concrete foundation, when i was doing this sort of thing i would say each cubic meter of concrete roughly weighs 2.4 ton so for you your foundation at what i have put down you would need 0.9 of a cubic meter, as for bulk bags they don't weigh a ton mostly, you would have to find out at your local builders on theres, so if you aim at lets say the bulk bags have 900kg that would be 1.8ton for 2 as a rule i would use 6-7 bags of cement per bulk bag dependent on strength, so if you 12 bags cement 300kg that would give 2.1 ton in total so you wouldnt be far off your target for 0.9 of a meter give or take.

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Abu Baniaz

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The trench is 700mm deep. I have reached clay in parts, but it is not solid hard. Other sections have had rubble chucked there. I think it is next to an old cess pit. Tell the truth, not 100% sure what the difference is between soil and clay. It is light brown and pliable. So think it is clay.

The calculators say for a 1:5 mix, I need 16 bags of 25kg cement and 2.398 tonnes (1.4 cubic meters) of 20mm ballast. Is that two jumbo bags?
 

FocuS

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sry i miss calculated 1.044 cubic meters, take the weight of the ballast so you need 2 ton 20mm ballast, but like the bulk bags aint always a ton, as you say just order 2 bulk bags and 16 cement, gives you 2.4 tons IF the bags are indeed a ton.
 

silverdale

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If you get the heavy flags and bed them on 3-4inch of mix theyll be right. The shed will spread the weight so unless your laying in a swamp it wont go anywere.
I've a 12 x 8 shed laid on 3x2 flags heavy jobs on a 3" bed of mix and it's still solid 10 years on.
If you wanna go OTT mix in a towel of postcrete with your mix for each flag. There'll be enough moisture in the sand but move quick :)
 

FocuS

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hey Abu, you could always put a bit of A142 mesh cut to width 200mm to give it some strength if the ground aint too good.
 
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