On 30/04/2014 10:54, mogga wrote:
On Sun, 27 Apr 2014 14:28:03 +0100, John Rumm
wrote:

So far so good....

and top off with sand.
Don't use any cement, there's no point, it's just a bodge method.

Could I suggest you go read some of the British standards on paving
techniques? To describe using a bound bedding layer (i.e. one containing
cement or lime) as a "bodge" is inaccurate and misleading.

Generally, "sand only" screeds tend to be used for smaller element
paving such as blocks and sets, and screeds with a binder used for
larger slabs - however this is not a hard and fast rule.

However the critical requirement of any sand only screed is that the
edge is adequately retained - typically with a mortared in place bedded
on course of blocks or edging stones, but sometimes just a concrete
haunch. Without the edge restraint, the sand will migrate out, and will
also get migrated by insects. and worms etc.

So greenhouse bases put solely on a slab sand base - especially in a
very wet area - will be fairly wobbly after about how long?

They may never get wobbly - although the levels will usually shift.
Often there will be no trace of the sand if you lift a slab later.

Frequently wet areas are worse - but the effect will vary with soil
conditions.

If the slabs are above ground level (rather than on it "in" it) then the
edge ones usually tip outward as the sand escapes from the edge.

What about postcreting posts in for the corners for my greenhouse
base, someone's suggested pressure treated boards around the bottom to
make like a raised bed to sit it on.

What kind of soil do you have?

For slabs used as a shed or greenhouse base, adding enough cement to the
sand to let it set a little once laid is usually all that's needed to
keep it in place. (used as a dry mix - no need to actually turn it into
mortar prior to laying)

--
Cheers,

John.

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