On Sat, 5 Jun 2010 09:53:37 +0100, "The Medway Handyman"
wrote:


Customer has had a shed base laid (not by me, I turned the job down) which
has turned into a bit of a disaster. Its off level by about an 1" in both
directions and the surface looks like a roller coaster for mice.

They want a metal shed erected on top, but there is no way thats going to
work until the base is sorted.

Initial thoughts are to erect level formwork & top the base with a little
more concrete.

Is it going to bond OK to the exciting base, especially where it
'feathers'.? Would PVA help?


I wouldn't use concrete that feathers because thin concrete will
always crack, no matter what you put in it.

If the out of level is only an inch, I would erect the shed level
using steel packers where the base is low, then fill the gap between
the base angle and the concrete with expanding grout underpinning.

"Expanding" is a misnomer - the admixture is just enough to prevent it
shrinking.

For the steel packers, go to any fabrication shop and ask for some
offcuts in an assortment of thicknesses. Used in combination, these
will give you packers in a range of different heights. For a small
fee they will flame cut them to the size you want. For a slightly
larger fee they will grind the edges flat.

Hardwood folding wedges would be an alternative.

For the expanding (non-shrink) grout, you can either buy it ready
mixed in pourable or trowellable form, or use an admixture from a
builder's merchant and add it to a 1:3 cement:sharp sand mix. Don't
add too much admixture or it will really expand and bend the shed
base!

http://www.bcsproducts.co.uk/frames/grouts.html

To make the grout frost resistant, SBR would be ideal if you have
some, but some waterproof PVA is a cheaper alternative. After all,
it's only a shed.