BIG shed base

[williamglossop]

Long story short, I need to pour a concrete base for a 24 ft x 12 ft shed.

I plan on levelling the ground and removing the top soil. I will then construct formwork and place 50 mm compacted type 1 in the bottom. I will then fill with 100 mm concrete.

I have done a a few bases before, but nothing of this size - as such, I was womdering if I could pick the brains of those that have....

Firstly - any tips on levelling a 24 ft piece of timber? May have to consider Road Forms, but im not sure how you form 90 degree angles with them.....

Secondly, would a slab of this size need some kind of control joint? If so, whats the easiest way to achieve it?

Thirdly - would it be advisable to build in a slight gradient?

Fourth - would a shed base of this size require some 142 mesh?

Any other tips, greatly appreciated

Bill

 

[John D v2.0]

1 make sure the water doesn't run off the shed onto the base
2 any kind of dpc? How are you going to keep the shed dry?
3 50mm hard core won't do much, you'll need more like 75-100mm

 

[williamglossop]

The shed will pretty much run right up to the edges which will keep the water from the shed roof off it
Ill stick a 1200 dpm under the conc
Easy enough to up the type 1 to 75 mm

Just dont wsnt to end up over engineering the base - was wondering if 75mm on steel reinforced conc would be sufficient....

 

[chappers]

I agree up the hardcore , level your formwork by screwing through support stakes on the outside. With regards to a control joint your ratio is spot on but the total area is borderline, the std formula for 100mm concrete is largest ratio 2:1 no area larger than 20sqm. a single cut cutting it in half across the long side will do, cut through to half depth. if it is particularly hot cut it in wet, if not the next morning should be fine. silicone it in afterwards.
Otherwise A142 mesh supported at mid depth should stop it cracking

 

[williamglossop]

Cheers guys

I am still wondering what to use as a straight edge for tamping it down - im concerned that finding a 14ft long piece of timber thats straight enough will be nigh on impossible....

Was also wondering whether I should make the formwork up into a box first, then site it and level it, or whether it'd be better to place one side, level it up then fit the next side before moving on to the next piece etc....

 

[John D v2.0]

When I did our drive I set some old bits of conduit as screeding guides at whatever level I needed, did the layer and then removed them and filled in straight after. I reckon the same would work, you'd have to pull them out quick and fill the hole if you're curing cement which is an additional complication.

Regarding the edge and water, you'll have to detail the edge carefully to allow ventilation under the shed but not allow water to run onto the slab. Otherwise the concrete will be constantly wet and the air humid there..

 

[tomfe]

A length of 4x2 will be fine, just have a look through the stack and find a straight length.

It's not rocket science, start one side and level as you go along, screw the form work from the stake inwards so you can take the screws out.

I presume you are using the concrete as your floor, otherwise you may as well use concrete blocks and suspend your floor.

 

[williamglossop]

The shed manufacturers have stated a 24 ft x 12ft concrete base - that's all. They've not stated any particular detailing at the edges. When you say detail the edge carefully - what exactly do you mean? I'm not sure if the concrete base is going to be the floor or not..... I'm basically helping a mate out.

 

[John D v2.0]

If it's a designed product they should have taken it into account already. But what you want to minimise is the amount of water getting on top of the slab when it rains. A common mistake is making the slab a few inches bigger then the shed, then the water can run on top and rot the shed.

 

[williamglossop]

Thanks again John. Whats the best timber to use for the formwork? Was thinking of 25mm thick 150mm deep gravel boards.....unless there is a better way...

 

[John D v2.0]

if you have them lying around then use them! otherwise cheap plywood is traditional I think? you'd be best off asking others tbh.

 

[r896neo]

You dont want plywood as it will give you loads of joints at its max 8ft lengths or 10ft for shuttering ply. use whatever you have or can get cheaply in longish lengths. If you use 4x2 then it'll be stiffer and need less staking.

You will find pull screeding 14ft of wet concrete almost impossible physically even with 2 people so you will need to rake it pretty close to finished level and then use a longer 15/ 16ft length to get a sawing action going. Simply join 2 4x2s and sister a length 5 feet or so either side of the joint and screw it well.

Use a dpm under, not for moisture protection but to stop your concrete drying too quick. Have you considered how you will trowel the slab? If you have lots of room around it a ladder could be suspended over it but a bull float might be worth hiring if you want it smooth. If its only for under a shed then your sawing will get it flat enough and get some cream up so you can walk on it when its firm enough and give it a bit of a go with a steel trowel to tighten the surface a bit and take off any high marks.

 

[chappers]

yep that's what I would do I would dig your ground out, compact your hardcore and then stake your shuttering over that.
That's what we do when imprinting concrete to divide the pours. We stake and screw the ends and centres of each length of timber shuttering (4x2) and then hammer in 20mm rebar pins as intermediates to strengthen the whole thing up, make sure all your stakes and pins are below the shuttering edge as it makes like so much easier when screeding it off

 

[williamglossop]

Thanks for all the help n tops. Sheds in - the installers stated the base was a pleasure to work with.....