Onduline roofing sheets leaking

[AlwaysBestToAsk]

Hi all,

I put Onduline bitumen roofing sheets on my garage this year (about 8 months ago) and it's constantly leaking. I've overlapped the sheets fine but there are massive patches of wet on the underside of the sheets i.e there's not a few leaks where it's dripping out of a 1cm wide gap or hole, it's almost like a condensation kind of dripping in the size of the wet patch where the waters dripping from?
I can see water sat on top of the patches but it's not what i thought would cause this in that the water sat on top is really just a wet film rather that anything substantial but my problem is that i thought Onduline bitumen sheets were waterproof ?
I couldn't find any instructions at the time to say they need painting or treating before use so am i supposed to paint or treat these first and i've just missed an obvious thing here or SHOULD they keep the water out as they come off the shelf ?
I've adjusted the angle of the sheets a few times as well to make sure water doesn't sit and it flows off ok.
Any advice would be great.
Thanks

 

[datarebal]

may well be condensation, its that time of year. where did you place the fixings?

 

[AlwaysBestToAsk]

So I've got numerous 2 x 4's across the span of the roof of the Garage and then used the Onduline black capped screws into the wooden beams every 2 or 3 feet. There's plenty of ventilation in and out of the garage.
Just to confirm though that these ARE waterproof aren't they ? I can find a lot on Bitumen being waterproof (as in the paint) but not these sheets ?

 

[AlwaysBestToAsk]

One thing with you saying condensation is that my Daughter moved back in with us for a while and she's been sat in there with a blow heater on for a number of weeks but has now gone back home to her own house about 5 days ago and since then the heater has not been used ? Could this be a result of that but it's only just started showing itself ? It WAS fine before she moved in but i've not seen it leaking/dripping until today.
It seemed to be better later in the day when the sun was up ?

 

[Old Salt]

I have Onduline sheets on my garage roof supported by marine ply without any issues.

 

[AlwaysBestToAsk]

I've not put ply under mine, mine is supported every foot or two with cross member joists but i can't see that causing the issue as it's not appearing to be sagging anywhere

 

[AlwaysBestToAsk]

I've been out early this morning and noticed that the inside of the roof was covered in frost (as it's -3c here) and gone back a bit later and it's all dripping so i think i went in yesterday after it had thawed and so yes, looks like it was condensation in the form of the frost melting when the sun came up. This would also account for the massive "patches" of wet as opposed to a single or few small sources in gaps etc.
Thanks all.

 

[Eccles]

Did my shed a while back and man in Wickes said make sure you do a double overlap because strong winds can drive rain under it unless you do.

 

[Harry Bloomfield]

I've been out early this morning and noticed that the inside of the roof was covered in frost (as it's -3c here) and gone back a bit later and it's all dripping so i think i went in yesterday after it had thawed and so yes, looks like it was condensation in the form of the frost melting when the sun came up. This would also account for the massive "patches" of wet as opposed to a single or few small sources in gaps etc.
Thanks all.

The only way to prevent it, is to add some form of insulation on the underside.

 

[datarebal]

I would use a different roof covering .

supported by marine ply without any issues.

Have you seen between the ply and roof covering

 

[Old Salt]

I would use a different roof covering .

Have you seen between the ply and roof covering

Ignorance is bliss

 

[datarebal]

I guess