Concrete - does this seem correct?

matt · 20 Jan 2012

Hi All,

Long time no posting - so much to do, so little time! Anyhow, I'm hoping for a quick sense-check on the following before I go spend more than I need to, and end up with more than I need!

I have to build an extension to an existing shed base tomorrow. The size (of the new bit) is 3ft x 10ft.

I've used an online calculator to estimate the volume of cement, sand, and gravel I need; however, I'm slightly sceptical about the numbers and would be grateful for an experienced view on whether the volumes looked broadly correct.

This is what it calculated:
Cement 134kgs (6 x 25kg bags)
Sharp Sand 252kgs (10 x 25kg bags)
Gravel 504kgs (20 x 25kg 20 bags)

Also, am I correct in saying I can substitute the gravel and sand for 30 bags of ballast?

Thanks in advance.

Matt

jasonB · 21 Jan 2012

Without the thickness of the slab we can only guess at what you need.

Also are you actually going to buy sand and separate gravel, far easier to buy "all in balast" and you will need 1.6 tonne per cubic meter and say 250kg of cement to go with that. Just pro rata for teh volume you actually need

J

henton49er · 21 Jan 2012

Matt,

as you have mixed metric and imperial measures, I will do the same:-

assuming a 4" thick slab, I think you need 150kg sand, 300kg gravel and 70kg cement

assuming a 6" thick slab, I think you need 225kg sand, 450kg gravel and 105 kg cement.

Each of these is without any allowance for overspill, wastage etc so it would be prudent to increase by 5% if you are using formwork and have a sound and flat screeded foundation on which to lay the concrete and maybe up to 15% or more if not. So I guess that your figures are not too wide of the mark.

Mike

ian_in_the_midlands · 21 Jan 2012

Does a shed need such a substantial base?
Maybe an extension to an existing base is a different problem, but with my shed I dug 8 individial holes, concreted in 9" concrete blocks, spanned them with tanalised fence posts (bolted down), and put the shed on that. It hasn't moved.

Digit · 21 Jan 2012

I'm with Ian on this, concrete is expensive, heavy to mix, transport and lay, and bloody cold to the feet.
I used concrete spots poured into holes, concrete blocks on top then 125mm square beams on top followed by 18mm OSB floor panels.
I made no great attempt at getting the blocks all to the same height, I found the highest then packed on top of 'tothers to bring the floor to level.
That was nearly five years ago.

Roy.

matt · 21 Jan 2012

Thanks guys, much appreciate your input (esp. as I forgot to include the thickness - duh!). I based the original est on 6" to allow for it being deeper at the edges (which will be visible) and approx 4" in the centre. Thanks again!