Transparent or opaque sheeting fastened to the underside of the rafters?
The water should drop onto it, and then run down to the edges, where you
can deal with it easily by some form of gutter arrangement.
Of course, this won't deal with the cause of the condensation, just the
My garage roof is made of box profile steel and in the centre there's one sheet of see-through plastic to make a skylight. The metal sheet has a lining of OSB board but of course the window bit is not lined with OSB and that produces lots of condensation just like you describe.
I don't think changing the roof slope would be practical - you'd still get as much condensation - but it might all run down to one end.
Could you line the ceiling with some sort of boarding? That would stop it.
I have a similar problem in similar Scottish conditions, and do have
to just live with it. In my case there really is nothing I can do as
it's clear plastic and is the only daylight source for the lean-too
section of the garage and contains my lathe, and is in near enough
daily use. Certainly the angle of my roof is pretty low to the extend
that in the recent winters I have to go and remove the snow from it.
But the condensation is not significant - if it was the lathe would
suffer and it is quite OK under an old groundsheet, so I do wonder
quite why you are getting so much moisture - I guess there may be no
damp proofing on the floor. If in fact your shed roof does not
require to give light then lining it would solve the problem.
sheet of see-through plastic to make a skylight. The metal sheet has a lining of
OSB board but of course the window bit is not lined with OSB and that produces
lots of condensation just like you describe.
If you don't mind what it looks like you could try lining it with
horticultural grade bubble wrap available in large sheets. That should
provide sufficient insulation and decrease contact with cold outer skin.
Insulation and/or stopping saturated air reaching the cold surface and
possibly with active ventilation when the relative humidity is low.
Maybe an application at last for a solar powered fan ;-)
A subject close to my own heart as well ! Scotland too ..
I have a largely sandstone outbuilding with a clear plastic corrugated
roof, that suffers lots of condensation.
About 18 months ago, I (in my wisdom at that time), I lined the roof
with 6mm ply, the idea being I created a "cushion" so that the water
couldn't condense on the cold plastic, instead I had "warm" plywood.
It's not really helped ! The ply is rotting and going black. Well, it
was anyway .. Left a half door open for months and months and the roof
seems to be clearing a bit, but am not sure if it's the open door, or
the large bucket of road salt that has helped !!
Anyway, have now closed the door and removed the bucket of road salt,
and instead stationed 3 diy dehumidifiers around the shed, which are
basically road salt in a muslin square or tea towel hanging over a
bucket, the salt draws in the water, then drips it into the bucket.
So, over the next few months I'll see if the underside of the roof stays
drier or not. I can still see water condensing on the cold walls.
I have a theory that one wall in particular is sat in water and the
water is drawn into the shed that way. The whole outbuilding, which is
pretty big, is on an incline and I've convinced myself that the wall at
the bottom of the incline is the one causing trouble. The parts of the
building higher up the slight incline are definitely drier.
Am currently lining one wall with shelving which as part of the prep,
I've put polystyrene in and will board over with 6mm ply. Of course,
this means more water will condense onto the other bare walls !
Anyway ... line the roof, don't bother ! Of course though, there's not
as many drips of water into the shed as the poor old ply lining gets
most of it.
I think the diy dehumidifier is worth a shot. I also intend to scatter
around the shed some tins (with holes) with wood charcoal in them to act
as dehumidifiers as well.
Albeit not a steep enough roof to allow the water to run down I suggest that
if you cannot make it steeper to dry thoroughly, spray with silicone spray
to provide a less water adhesive surface and allow the water to run more
easily to the edge to a collection gutter.
Apart from that install double wall polycarbonate sheeting such as eBay item
320705397522 underneath to give better insulation and therefore less
I don't store rottable things in there. Bicycle, mower, hedge trimmer, electric
saw, that kinda thing. But large amounts of water falling on them might cause
I'm currently planning (form advice in here) to spray the roof with silicone
spray to make the water slide to the bottom, and if that fails, add some
polythene sheeting below it to catch the drips.
You don't say whether it is clear or solid opaque plastic, so this may
or may not not be suitable. I had the same problem with a corrugated
steel roof on an outbuilding. I found that pushing 2" thick expanded
polystyrene sheet (something I had lying around - thinner may work as
well) up between the rafters and making sure it was well sealed around
the edges cured the problem.
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