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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthortony
    • CommentTimeFeb 9th 2019
    I have been asked about this problem


    Roof is on a covered outside store, There seems to be be a problem with the underside of the top sheet getting wet/damp above the join, pitch is only 7.5 degrees area is very well ventilated.

    Any help or recommendations please.
    • CommentAuthorCliff Pope
    • CommentTimeFeb 10th 2019
    It depends on the length of overlap. The shallower the angle the greater the overlap needed. At 7.5 degrees I'd say from experience you'd need at least 2 feet, ideally 3. If you get a gale blowing straight up the roof water can easily be forced up the slope underneath the upper sheet.

    Another factor is how scrupulously you keep the roof clear of moss. If you allow moss to grow in the troughs at the lower edge of the top sheet then water can build up behind it.

    Both problems are greatly exacerbated with a shallow pitch roof.
    • CommentAuthortony
    • CommentTimeFeb 10th 2019
    No moss and 650mm of overlap, I think the sheets have a condensation mesh on the back. Could this be capillary sucking up moisture?

    • CommentAuthorphiledge
    • CommentTimeFeb 10th 2019
    Id guess is condensation running down the underside and dripping off when it hits the top of the lower panel. Our sheds regularly drip off the purlins where the sheeting rests on them
    • CommentAuthortony
    • CommentTimeFeb 10th 2019
    Thinking about it. Could it be that the underside of the roof gets damp with condensation everywhere and this percolates down the slope. Where the sheets overlap it gets stuck and can’t get out very easily and to a small extent prevents the area above from properly drying out as it can cappilaryate back up. So the area above the overlap stays wetter for longer and is picking up moulds and looking progressively darker the nearer ir gets to the join.
    • CommentTimeFeb 11th 2019
    Condensation mesh AFAIK does next to nothing to reduce total condensation but by being a bit absorbent in a capillary way, 'soaks it out' sideways, even uphill, so it doesn't accumulate enough to drip.

    That wd mean that if as tony says the lap forms a 'hanging dam' that prevents condensation from running on down, that's a concentration of water that stays wetter longer; the condensation mesh then 'soaks it uphill' so wetness extends upward from the 'dam'.

    Just what tony says, in fact.
    • CommentAuthordickster
    • CommentTimeFeb 11th 2019
    Maybe try a counter-intuitive spacer (5mm?) wedged in overlap at the underside join, this might allow air to dry out the damp area that shows and move the problem under the overlap (out of sight, out of mind), but it might allow water in!

    There's no mention of water actually dripping into shed, so is this just a cosmetic problem? If so, the rest of the shed might want a facelift as well!
    • CommentAuthorgyrogear
    • CommentTimeFeb 11th 2019
    How about suspending a tarpaulin etc. from the bottom of the joists, and blowing a bit of air through the open plenum, using a cheap fan ?

    (not being a drip or trying to damp anybody's enthusiasm, just trying to shed some light on the problem...)

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