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Advice needeed on leaking roof


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Established Member
23 Mar 2005
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Mid Derbyshire
Help please guys :-

I've just put up a new workshop which has a corrugated PVC roof - mainly to let in good light. The roof panels overlap side to side by 150mm and so do the ends. Everything looked hunky dory but this morning I woke up to a big puddle on the chipboard floor. Not good. Floor has not yet been sealed on top side but doesn't look to have blown with getting wet.
Looking at the roof I'm getting a problem with capillary action dragging water between the panels across the lap joint which then drips down when it reaches the end of the underneath panel. Leaving a gap between the panels would prevent this but with it bring other problems.
I seem to have two options:-

1. Loosen the panels and run a bead of clear silicone between them and re-tighten giving a physical block to the water tracking through.
2. Overlay with adhesive flashing tape on the end of the overlapping panel to seal it from the outside.

My preference is 2 and this could look ok as well as being effective as long as I'm careful to get the tape runs straight.

There isn't much fall on the roof overall but well enough to allow water to run off into the guttering and the roof is well supported by substantial timbers to cope with wind and snow.

I haven't had this problem with other roofs I've put up over the years and obviously need to crack this one before I go any further with the build. Luckily there were no tools beneath the leak except my shop vac, which hasn't yet blown up so that seems ok.

Any suggestions gratefully recieved.

Your humble apprentice hh.

Knot Competent

Established Member
25 Apr 2004
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You're a busy guy, Mike.

If you run mastic down the edge, it might well seal and thereby solve the leak problem, but water would sit in the overlap and go green, which wouldn't look good and would annoy you for years.

Having said that, I'm sorry but I don't know the answer to this. Hope someone else can solve it!

Regards, John


You say there isn't much of a fall on the roof, and that points to your problem. Sealing the joints in some way is the only way to stop it, but my solution would be to loosen the panels off and run the clear mastic on the end of the top panel instead of running tape. As for algae growth - you don't need standing water for that -- the shaded area at the back of our house is 6' deep, which is just one panel, and they gradually green up. Takes about 2-3 years and the only way to get rid is to powerwash it - can't stand on PVC panels!

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