Shed roof leaking by capillary action?

[Lobster]

During the recent bad weather, I braved the elements and went down the garden to my shed, which is just a glorified storage cupboard really.

Having spent a fair old time during last summer replacing the roofing felt, I was a bit miffed to find a puddle on the floor inside. However it's not at all clear where this has come from - after the snow had all gone I checked the felt and it looks fine.

However, I'm wondering... the roof is pitched, and there's a 3" overlap in the felt running the length of the roof, and the puddle was more or less below this. The overlap is not nailed down, but glued with bitumen-type stuff from a tin - admittedly not particularly well, but certainly enough to stop the felt from getting wind under it, say.

At the time the snow was thawing - which was when I saw the leak - I realised after the event that melting snow was being retained over the overlap area by leaves, twigs etc which had landed before the snow. So I'm wondering - is it conceivable that the soggy mess of snow and leaves during the thaw could have allowed water to travel uphill, up the overlap and through my roof?

If not I'm a bit stumped!

David

[robgraham]

Is it possible that there was frozen condensation on the inside of the roof that thawed when it got warmer ?

Rob

[alans-computer.local]

(snip) It could be condensation on the ceiling because the ceiling was kept below freezing by all the snow on top even when the air temperature went above freezing.

[Hugh Jampton]

I think you're right - that's the reason. Went into our loft just before Xmas to get decorations and stuff. Was very worried when I found lots of things wet - drops of water covering things. Could not find any leaks in the roof. Took things back up there a couple of days ago and all was dry again (thank heavens).

Also found similar in a shed - also no sign of leaks in the roof.

[george [dicegeorge]]

I get leaks in my valleys with snow cos the snow and ice lower down stop the melt water taking the usual path, it finds its way between the lead sheets of the valleys, up a bit, then drips in the house. [g]

[Lobster]

Although condensation was the only other possibility I could come up with, I rather discounted it as there seemed to be quite a lot of water present in one puddle (half a cupfull?) whereas everywhere else was bone dry - can't see why it would be so localised if condensation was the cause?

David

[Donwill]

Perhaps the localised condensation inside was caused by low temperature imposed by the localised snow retention over the overlap area as described. Don

[dennis]

Three inches of overlap isn't going to stop the water going up between the strips if there is three inches of ice and snow covering it. The ice and snow act like a dam and let the melting water underneath rise up. The glue might, but only if its perfectly sealed.

I have seen the same thing happen on my shallow angled plastic roof and that has about 12 of overlap but no glue. Its not an issue on a carport though so it doesn't need fixing for the odd day it happens. I have seen the condensation dripping too, but that happens along the length, not just at the joints.

[Tim Lamb]

In message , "dennis@home" writes

Condensation will run down under a sloping roof and then drip at purlins or some other discontinuity.

regards

[Lobster]

ISWYM - yes that answer does make sense. Certainly the seal won't be watertight in that context... but don't think I'll be ripping it all off to start again any time soon!

David

[robgraham]

Yep, I've seen that in a Scots' slated barn roof during a blizzard - well slated but no membrane. Sarking would have got slightly damp but was well ventilated so would have dried out quite happily. Rob.

[Skipweasel]

You also get snow and ice forming little dams that allow water to build up and overflow a barrier which in normal circumstances would suffice to exclude water.

[John MacLeod]

Yes. Worth remembering that most Scottish roofs are both close- boarded with sarking and roofed with Ballachulish slate which is much rougher than Welsh slate and provides natural but controlled ventilation for the sarking.

[Mr Pounder]

Strange things happen in very cold weather. My garage roof leaked; it no longer leaks. My driveway gates would not close, they now close.