andrew wrote:
You could simply increase the ventilation in the roof space above the
plasterboard just as much as is needed to stop the condensation.

there is good ventilation as the gable ends have the equivalent of 4 air
bricks in each end and the eaves and ridge have the area under the
corrugations. we have a lean to adjacent which is open fronted with no
ceiling and the condensation is visible on the inside of the roof.

Andrew, I am surprised by some of the advice you have been given by
other posters above. I studied the whole business of insulation two
years ago when we were refurbishing this house by importing an excellent
book from the States. What I learnt is that condensation will occur on
the underside of roof-tiles, tin-roof, or the felt or canvas directly
beneath it if the air in that space is not the same temperature as the
air outside. Hence, the advice was that one must make absolutely
certain that your attic has good and free ventilation. Now, obviously,
beneath the ceiling you want to keep warm, so beneath the ceiling you
want none of that cold that is directly above your ceiling. Hence a
highly effective thermal barrier must be created a ceiling level.

From my understanding all you will achieve in the circumstances you have
described by adding canvas adjacent to your tin-roof is that the
condensation on the underside of the tin will be caught by the canvas
and dribble downwards to the eaves. However, you are likely to get much
the same problem on the underside of your canvas! In fact, I distinctly
remember that being one of the issues of one of the chapters: how do you
ensure you do not get condensation on the underside of the canvas over
which your roof-tiles are laid? Answer: increase your ventilation.

You say you have four air-bricks in each gable, but are these actually
allowing free and immediate movement of air? We were so concerned
about this matter when this place was being refurbished that we forced
our builders into putting one bloody great big vent, 12x12" at one gable
end of our roof. It's as good as 1-foot hole but for mesh which stops
birds etc. getting in. It's damned cold up there in our attic, as cold
as it is out in the garden, but we don't get condensation on the
underside of our canvas.

The odd thing though is your saying that you have a totally open lean-to
with a tin roof and that there is condensation on the underside of it!
Do you live in a very moisture-laden area? Are you in a dingle in the
hills of Wales, Cumbria, or Scotland? Do you have a stream and/or ponds
in your garden? Are there constant mists? If you have a significant
condensation problem inside a totally open lean-to, I don't think
there's much you can do, except lay some large plastic sheeting over the
upper-surface of your plaster-board ceiling. Surely there isn't enough
condensation to drop onto such and form massive puddles that would need
to drain away? You might find that the moisture that gathered on the
sheeting will also be constantly evaporating.

I suggest trying this easy and inexpensive route first. Nip into Wilko
and buy a plastic-dust or two first. They're massive and they only cost
£1. Lay it over your plasterboard, wait a month, and observe.

But if you want to keep the area beneath the plasterboard ceiling
"warm", in comparison to the space between the plasterboard and the tin
roof above, then consider adding a strong thermal barrier against the
ceiling. Â (And this may possibly stop daylight warmth escaping from the
tool area into the roof-space, so that you then won't have to increase
the ventilation up there.)

In anticipation of wishing to warm the area when maintenance is done I did
put 100mm fibreglass insulation between the joists above the plasterboard
when it was fitted in March and it is this that is getting wet from the
drips.

Fine, so start by laying a large dust-sheet over the insulation. If
you need to use more than one sheet, edge it upwards where they meet so
that you create two potential "paddling pools". And then wait a month
and observe.

Good luck.

Eddy.