Shed base - Will this suffice?

[kiddo]

Hi guys.

Still trying to workout what to do for the base on my shed-build. The shed will be built from 4x2timber sitting on 3 courses of engineering bricks that I will lay around the perimeter of the slab once cast.

I was gonna compact 75-100mm of MOT1 and then cast a 200mm slab from one of the mix-on-site companies.

I was also gonna use 1 layer of A142 rebar.

1) Will that be enough, or should I make it thicker around the edges where the bricks are?

2) How far in from the edges of the cast-slab should the rebar be?

3) Its at the bottom of a long slopping garden, and the grass gets boggy in winter. Do you think there is a danger of the slab (5.2m x 3.5m) cracking becuase I have not used enough MOT etc ?

Thanks.

[dig dug dan]

200mm shed base? are you planning on parking a chieftain tank in it??

I have never built a shed base thicker than 100mm yet, and have yet to have one fail.

I would perhaps raft the edge so it is 150mm where the bricks are, and 100mm the rest.

That size of slab will crack through expansion, and not because you have not used enough type one. However, you must make sure the type one is well compacted.

rebar is perhaps over the top, but its not too expensive so chuck it in.

[Tony McC]

Dan's right - a 200mm thick slab is a serious industrial specification. 100mm with 1 sheet of mesh would be more than ample for a shed base.

You could form a keel at the edges if you wanted, but for just three courses, I wouldn't bother. I's set the brickwork back from the edge of the slab by at least 100mm, though.

The steel *must* have at least 50mm cover, top and bottom and at the edges.

Read the notes on the main website about concrete slabs and the bit about span not exceeding 30x the thickness. Your 5.2m span should be split in two with a control joint, otherwise it *will* crack, regardless of whether you have a boggy garden or not.

[kiddo]

Hi all, thanks for the words of advice.

I have decided to reduce the base thickness to 100mm, and approx 300mm from the edges, I will thicken to 200mm.


1) Tony suggests laying the bricks inside the slab by 100mm, which is for strength I guess? However, won't water collect on the edge of the slab? Wont this pool and make the shed base damp inside ?

2) I wanted to get cracking on this, but have heard you shouldn't lay concrete if there is a chance of frost, especially if the concrete is structual. Should I wait until the weather gets warmer, or just go ahead ? If its okay to go now, should I instruct the mix-on-site Lorry guy to add an additive etc ?


Thanks alot you guys. You really know your stuff!

[Tony McC]

1 - not if you use an arris trowel around the edges. This has the effect of smoothing the concrete surface to a level that is often a millimetre or so lower than the rest of the base. You can also create a slight towards the open edge, if you really want.

However, a trick known as hydrostatic gradient will cause water to flow off the edge of the cast slab rather than into the shed, so none of the above is strictly necessary.


2 - yes: wait for a warmer spell (like now)

[GB_Groundworks]

12 degrees today go for it

[kiddo]

Yo, been progressing with this. Have been digging and suverying - and have ran into problem. Basically the garden slopes so much, that I will need to dig out 500mm from one end, or have 500mm of concrete rising up at other end..... so I have decided to build two seperate smaller sheds/workshops and cast the slabs on seperate levels..


Can I dig out the earth on a slope and lay a 100mm of MOT1 on a slope, but cast the slabs level ?

Er - the picture will probably explain better than what I just said?! lol.


http://img.myph.us/0cJ.jpg

[GB_Groundworks]

No do them level, but stepped

Or use the cut and fill technique to create 2 level areas

[kiddo]

Hi, thanks for the reply.

These two slabs are still going to rise from 100mm to 250mm due to the slope of the land.

a) I was going to use 5mm rebar. Do I angle the rebar sheet so that its central to the slab across the slope (i.e., 50mm to 125mm) or do I lay level in relation to either the top or bottom (i.e. 50mm from the top at both ends)

b) Trying to cut down on concrete that I have to pay for. I have large chunks of concrete from a previous slab. Can I chuck them in, so long as they are not too close to the edges of slab ?

c) I haven't got any concrete chairs from the rebar to sit on yet. Can I get away with laying the sheets on bricks or small pieces of smashed up concrete paving slabs etc?


Thanks very much.

[kiddo]

oh, got one more question.

d) i estimate that I need 4cu m. There are some readymix places that do 4cu m lorries, but the barrow run is very long. I have help but its still gonna take close to 2 hours to barrow it all (my extimate). Should I get a mix-on-site instead of a readymix, so the concrete doesn't go off in the lorry ?

[lutonlagerlout]

the concrete wont go off in the lorry but the £1.50 a minute waiting time might put the pressure on you
if its a long push you will need 4 lads and 3 barrows
1 tamping and 3 pushing
LLL

[kiddo]

Yo. So, now making the formwork for my impending pour and have a couple of questions.

1) How do I go about sticking metal strapping into the web slab so I can tie the shed walls down? Wickes have these rolls of metal strapping - Im not sure if they are the right things to use though, cos they are very bendy etc? Do I cut lengths I need and then bend a right angle into it before sticking it in the wet concrete? Or are the special things made for this kind of thing ?

2) I want to lay either 2-3 courses of bricks, or 1 course of Blocks to raise the shed up. Obviously the Blocks will be quicker to lay than the Bricks, and cheaper I would guess? Will Blocks be just as stong as the bricks or will bricks be stronger due to staggering of joints. I wouldn't want the whole thing to rock sideways etc?

3) Previously, Tony recommeded that the bricks are set back on the slab 100mm all around. Would I be asking for trouble if I laid the bricks on the edges of the slab ? It will be 100mm thick, rising to 250mm and have Rebar set into 50mm from all edges?


Thanks for any help you can offer!!

[GB_Groundworks]

Make your slab bigger and set them back 100mm

Get some wall plate straps and string line out your shed either before or after so it doesn't get in the way when tamping on the shutter then once tamped just push the small L into the Crete. Alternatively just drill and plug the slab once set

Use concrete commons not block