Footings and DPC (long)

[David WE Roberts]

Onwards and upwards with the shed/store/workshop.

Getting closer to laying some concrete, but slowly due to the wet weather.

The current plan is to build a single block wall approximately 2m high to all four sides and roof it with steel sheet. I was planning to have 4"/100mm hard core and 4"/100mm of concrete for the floor, and have an extra 4"/100mm for the footings to support the single block walls (that is, 100mm hard core and 200mm concrete for the footings). Comments please - is this adequate (or over the top)?

I am planning to lay the floor and footings as a single raft (one of the options in one of the books I have read). Dubious ascii art of a cross section:

___________________________ |-----------------------------------| L-J L-J

However most of the examples show a footing then a block wall below ground level, with the floor poured as a seperate raft after the footings and wall have been done. This allows you to lead the DPC under the concrete then up the inside of the block wall to fit into the DPC one block further up.

The thing that is puzzling me is how to incorporate a DPC in the concrete raft if you lay it as one unit - how to bring it up to where the blocks are going to be laid so it can be incorporated into the DPC in the wall. The easy (?) way would be to lay it under everything but this would bring it up outside the block wall. I can see that it could then be incorporated into the DPC from the outside, and the plastic protected from sunlight by render but this seems a strange way of doing things. Bringing it up at just the right point to be inside the block wall but right alongside the blocks seems a tad complicated.

Also, do you lay 2"/50mm concrete, then the plastic sheet DPC, then another

2"/50mm of concrete, or do you put the plastic sheet directly on the blinding sand and lay all the concrete over it?

As far as I can tell when building habitable accomodation you tend to do footings+block wall course, concrete floor, DPC over floor lead into DPC in blocks, screed over DPC. However this seems far too complicated for a shed/store/workshop which will be unheated most of the time and is likely to have a bare concrete floor which is likely to reduce the incidence of damp problems.

What is also puzzling me is the effect that a plastic sheet has on the general structural integrity of the floor. I can see that 4"/100mm of concrete is pretty robust. However 2"/50mm of concrete floating on a plastic sheet over another 2"/50mm of concrete seesm much less robust and more likely to crack.

Chasing my own tail at the moment.

Comments and suggestions more than welcome.

Cheers

Dave R

[js.b1]

Comments?

Steel mesh in the floor? Makes a big difference to to the strength, cracking resistance. No idea if glass fibre flakes would assist on a whole floor (costly?)

Single block is not very waterproof? I assume you are external cladding over say black bitumastic paint.

Remember to insulate the floor too? Cold calf muscles make for a miserable experience.

Electrical services? Provide an underground duct to bring cable in. Be generous re bend radius of SWA, say 240mm radius.

[Phil L]

you don't need hardcore unless the ground is not suitable - IE topsoil - if you've dug the roots and grass out, and/or the ground has previously been covered with concrete / paving / another building, then you just need 100mm of concrete - you can make it slightly thicker at the edges for the walls, say 150mm

200mm is overkill as 2 storey houses only get 200mm under their walls and they are double thickness and holding up a roof and second floor

It doesn't need to - if you pour the whole thing as one, then the DPC is underneath the footings as well as the floor, wheras with strip footings, the DPC is the first and only defence against damp, the concrete and brickwork above remains wet all the time, which is why when a floor is then poured, the DPC under that is incorporated into the brickwork - otherwise the edges of the floor slab would be in contact with wet brickwork

Yes, that's how it's done.

It doesn't need to go anywhere near the wall, no damp can rise up through your raft because of the DPC below it, so a single DPC under your block wall is sufficient.

on the sand

no, the floor dpc is underneath the concrete - screed is optional, I never do it because I take time to get the floor right, but if it's a bad batch of concrete or a very hot day, then it's proably wise to leave the conc 50mm low and lay a screed at a later date

a 100mm layer is also likely to crack unless you use steel mesh, but is it that critical that your shed floor doesn't get a hairline crack or two in the next 20 years or so?

[Phil L]

i've uploaded a small jpeg here:

[David WE Roberts]

Thanks Phil - exactly what I needed to know :-)

[David WE Roberts]

Magic :-) Much appreciated.

[David WE Roberts]

The slab is quite large - roughly 8m * 3.5m - and is over mixed ground. The site was slightly uneven so some areas have had a little taken out (still some topsoil) and others have had a lot taken out. There are still roots from dead trees and some live ones going under the site. The ground was a mixture of lawn, flowerbeds, and a previous shed base for a

7' * 5' shed. I have also had to dig out two tree stumps (one of them large). All in all I think I will go for a layer of hardcore so that I can be sure of a reasonably consistent base. I tend to over-engineer but this is going to be initially a very large store but eventually a garden store/workshop at one end and a garden room at the other. Double doors at either end of the front, and windows in between. Solid fuel stove in the middle of the front for occasional heating during the winter.

With footings it looks like about 3.5cu m of concrete in all. That would be another 3.5cu m of hardcore (I am expecting to use a stone mixture such as carrstone, not crushed bricks etc.) The hardcore should be compacted - but how much? Should I be tamping it or should I be looking at one of those vibrating plates to really bash it down?

Cheers

Dave R

[Phil L]

you will need hardcore by the sound of it.

MOT type 1, or crusher run as it is more commonly known

They don't get it down very far, but it's better than any manual method, expect to pay £25 for a day's hire