Condensation on underside of corrugated plastic roofing sheet

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9 Sep 2023
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We have a pergola type of thing covering an area of decking outside our summerhouse. The pergola is covered in opaque corrugated plastic roofing sheet and the ends have reclaimed twin wall polycarbonate sheeting (thanks to our neighbour removing a conservatory) from 2" above the decking to the roof. The front is open apart from an area where I built a workshop which is closed at one end, semi open at the other end and at the front. We are experiencing high levels of condensation on the underside of the sheeting. Previously, before we reroofed the pergola and built the workshop, the roof was covered in clear corrugated sheeting (which had gone cloudy over the years) and the ends had bamboo screening instead of the polycarbonate. We never had a condensation problem with that set up.

So the changes we made are the type of sheet used - opaque rather than clear, and the closing off of the ends, although the front width is open to the elements. Is opaque sheet more susceptible to condensation or do you think it is the closing of the ends or a combination of both? Any advice will be gratefully received.

Many thanks
Just guessing - "closed off" means less ventilation, which could very likely be the cause?
Photo might help.
"Twinwall" sheeting is going to be warmer and less condensation prone than single thickness. Have you changed from one to the other?
Thanks for your reply. The previous roof was the same stuff - mini profile corrugated sheet, but clear and not translucent. The translucent is slightly rough on the underside. The cost of twin wall was ridiculous in comparison (largely the support bars needed). I have attached some photos that hopefully show the layout. The pergola roof slopes down to the gutter and the workshop area roof slopes in the opposite direction to the gutter. Hopefully you can get some idea of the layout from this.


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Guessing again - the roughness of the new stuff slows down condensation run off? Shallow pitches not helping
Even though its open to the front, the enclosed 3 sides are preventing air flowing freely across the inside.
Try vents on the sides to get a crossflow of air.
Is there any heater in the space?
It's not that rough. It's the coating that tints it blue. It's a re roof of a roof that lasted 16 years using the same profile but translucent, on the same frame, and we never had problems with that roof. All I can imagine is the lack of direct cross ventilation with the polycarbonate replacing bamboo screening
Even though its open to the front, the enclosed 3 sides are preventing air flowing freely across the inside.
Try vents on the sides to get a crossflow of air.
Is there any heater in the space?
Thanks for your reply. That was the conclusion I was drawn to, the lack of cross ventilation. There is no heating - we ain't that rich 🤣. I have removed the panels from the left side and will open the doors on the right side to see if that makes a difference before cutting holes in them for louvre vents.
I built a large 'lean-to' the full length of my bungalow. The joinery was very simple and I used Industrial grade GRP sheets, meant to allow up to 92% light. Over the years the GRP has become dirty and probably needs a bit of a clean. However, the only annoying thing about the 'lean-to' was during freezing/ very cold cold weather I would get condensation/meltwater drip the following morning along the exposed under-battens. Even though the 'lean-to' had exposed open sides, the drip still occurs. I should say that earlier this year I installed a partition divide to keep my turning workshop separate from our seating outside area.


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Quite similar to ours in design. You seem to have plenty of cross ventilation which I was hoping may be the issue with ours. I dropped the polycarbonate panels from one side and opened the doors and removed one panel on the other side yesterday to test the theory. By the time I had done that and came to dry the underside of the sheeting it had frosted over so I couldn't dry it off properly to see if it had worked 🙁. That's todays job now. Strange this is it never did it before with the bamboo screening ends and clear sheet
By way of update. I removed the panels on the right hand side and dropped the ones on the left side to leave a gap of about 9" at the top. The roof was completely dry on the underside this morning - there was rain on one end of the decking which had blown in, but the condensation seems to have been cured by increasing the ventilation. It was very windy last night so it may not have been a true representation of the effect though. I have replaced a few of the panels but left some openings to see how little I can get away with - trying to find the balance between opening it up enough to be effective but not opening it too much to allow the elements in. Thanks all for the input.
Well somebody opened their mouth too soon. Checked this morning and we have condensation again :(. Last night was less windy than the one before so less through flow. In the words of Dick Dastardly, 'drat! drat! drat! and a double drat'.
Well somebody opened their mouth too soon. Checked this morning and we have condensation again :(. Last night was less windy than the one before so less through flow. In the words of Dick Dastardly, 'drat! drat! drat! and a double drat'.
Hi! Is the condensation on the underside e.g potentially dripping on your head, or on top e.g sits on top of your plastic roof?

You may need to consider the type of material you are using. Maybe it's not as breathable as your previous material.

Maybe also consider if you have added or removed any other things to that particular area of your garden recently?

I am following your journey with this as I am thinking of putting one up ☺️
If that is a seating area, getting a lot of wind blowing through is not ideal, I think.
The issue is that warmer air, which can contain more humidity, encounters a colder surface, the underside of the corrugated plastic. If you choose a material which insulates a little better, but still lets light through, that will solve it. Just to give the warmer air underneath the roof a little bit of time to escape before releasing the humidity contained.
The polycarbonate channel sheet material is really nice, quite a bit more expensive though, and you need to close off the ends and make it airtight for it to work.

You can get to up to almost an inch in thickness.

Two layers of corrugated sheet will work also, but as above, it needs to be an airtight construction.
Hi there - just read the above with interest. I am turning an old swimming pool into a shed. The room is about 45' x 20'.

Before I put on an opaque roof I thought I'd

1) try repairing the existing roof (twin wall polycarbonate sheeting, see photos) where it's letting water in and then add some vents at either end of the room to help with condensation/damp/humidity.

2) I had thought that putting on a 40mm composite roof might just solve the damp problem instantly but I think in any case I'd need vents (is this correct?).

3) I was literally going to whack 3 of those window vents in (you know, with the pull strings and windmill opening) but would welcome any advice to the contrary.

4) Also should they be high or low on the windows? Corners or centre of the wall?

5) Am I right in thinking ventilation is better than insulation especially as they are just breeze block walls (and no point doing the roof if I am not doing the walls?).

I just want it to be a DRY space for storage, workshop, games etc. Many thanks in advance.

Before you go to do anything, have you drained the pool and let things sit for a few weeks to see if there is a condensation problems?
yes - the water you see is just from this year - prior to that it was empty for about 4 years
wondering if actually a ridge vent is the way to go across the roof pitch