Fwom:Autolycus )
"N. Thornton" wrote in message
om...


I have another building of around 120 square
metres which is structurally sound but has a number of leaks in its
big-6 asbestos sheets as well as quite severe condensation problems
on
the underside of the roof. I don't relish removing the existing
sheets,
and I certainly don't relish paying for their disposal.

Does anyone have experience of "oversheeting" with steel, onduline,
or
other materials? I believe it can be done directly onto the sheets,
having first removed all the nails, or onto battens.

Re your 2nd building, I presume cement sheet, which is what asbestos
sheet is, could be cement lined to renovate it. Mixing fibres in for
reinforcemenr would make it behave musch like the asbestos there
already. Presumably one could make a good superstrong mix and trowel
it thinly onto the asbestos already there? Certainly be a big cost
saving.

Assuming that by "cement lined" you don't mean trying to render the
_underside_of corrugated sheet, three problems spring to mind:

Preparation of a moss and lichen-encrusted, slightly friable area of
asbestos-cement sheet to accept a trowelled-on cementitious mix would
be
non-trivial:

Power wash it to get the bulk off, the PVA content in the mix should
provide enough adhesion to deal with any remaining bits, and resolve
the friability problem. Bear in mind the new stuff would only need to
stick in patches for it to be fully successful.


filling the troughs completely, and having say an inch of
render above the peaks would require around 6 cubic metres of
concrete -
over 12 tonnes more load on the structure, and a hell of a lot of
mixing
and lifting;

The original sheeting is 5mm thick: why ever would you want to put a
couple of inches onto it? Thats a completely untenable idea, your
structure will only take so much load.

A more sensible 5mm gets rid of that one.


then, of course, if (when) any cracks do develop through
thermal or structural movement, water could well run down the very
corrugations that are leaking now.

You'd be replacing like with like, asbestos cement sheet with d-i-y
asbestos cement replacement bedded onto asbestos cement sheet, so
performance should be similar.


I don't know whether the extra thickness would provide enough
insulation
to solve the condensation problem.

unlikely. That would need a bit of insulation if its a problem.


You could either treat the whole roof like this or just spot patch.

Not easy to spot patch _and_ profile it so water running from further
up
the roof doesn't pond in a corrugation

As long as the patches are trowelled so the leading edge is sloping
slightly they should work ok, the slope at the patches will be very
reduced of course, but as long as it doesnt slope the wrong way it
should still function ok. Even horizontal will function.


- and there's still the
preparation problem.

not really

Thanks for the ideas - but I don't think we're there yet.

Maybe 'we' arent.


NT