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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorRex
    • CommentTimeNov 30th 2021 edited
    What ho one and all.

    Below is a photo of the front of my house. The roof has a 1 meter overhang one all four corners, but only one has a 'dripping' problem.

    The construction is 300mm deep PosiJoists, ventilated at the top and bottom; they are supported on the purlins. In the living area of the house, the PosiJoists are filled with Warmcell insulation, but the overhang areas obviously, have no insulation, just empty space. There is also sarking boards under the tile battens. None of the tiles are broken.

    I noticed yesterday, water dripping from the lowest corner (arrowed.) When there is a frost, I frequently have icicles hanging from the purlin (arrowed.)

    The other three similarly constructed corner overhangs, no problems.

    I can only assume it is condensation but why only one area and not the other three? If condensation, how can I reduce it? More ventilation and if that is the solution, how to implement it?

    Thanks and toodle pip
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    • CommentAuthortony
    • CommentTimeNov 30th 2021
    Which way is North, is there a boiler flue round the corner or exhaust from a fan?

    Still thinking , tricky
    • CommentTimeNov 30th 2021
    Which way does the wind blow?
    Definitely not gutter overflowing with leaves from those trees?

    Is it a metal roof by any chance? We have a pressed steel tiled shed roof, which gets drippy on the underside when a cold night is followed by a damp morning.

    Is the void in the uninsulated bit of roof overhang sealed or ventilated? How?

    What rooms are upstairs in that corner but not the others- bathroom?

    The matching overhang on the left is over the garage, would you know if it was dripping?
    • CommentAuthorRex
    • CommentTimeDec 1st 2021
    Thanks for all the thoughts.

    No, it is not a metal roof, ceramic tiles (NuLok) on counter battens so fully ventilated below the tile. Then there is breather paper and sarking board. the verge is a metal edge, but it only goes over the tiles bay around 150mm.

    The facia and soffit are both uPVC (hate it but at least it never needs painting!)

    The room with that roof is a bedroom but we us it as a TV watching room. Leaves, not a problem as I try to keep them from overhanging and certainly they do not cover the purlin area. This is the south facing side of the house and certainly does 'benefit' from the winter winds!

    Having answered these point, I can now throw another thought into the mix. The white walls are rendered onto thermolite block so there is a 2" cavity. But the timber clad area had to be spaced out to keep the front flush with the outer face of the render. Result is that behind the cladding, there is around a 6" cavity, all ventilated at the base with breather bricks. The soffit also has ventilation along both the lower and upper edges, but the wall cavity, does merge into the roof, uninsulated space.

    The ground floor rooms are a guest bedroom and en-suite; all rooms have UFH. Although the 180mm of insulation keeps the warmth in, perhaps the size of the bedroom cavity is causing too much 'warm' air to rise into the roof space and cause condensation?

    Would (could?) fitting some of these, top and bottom help?


    Thanks and toodle pip
    • CommentAuthorphiledge
    • CommentTimeDec 1st 2021
    Posted By: Rexthe verge is a metal edge, but it only goes over the tiles bay around 150mm.

    On a cold damp night the underside of the corrugated metal sheets on our outbuildings gets soaked with condensation and all other materials nearby are dry. Be worth having a close look at your metal verges first thing in the morning to see if thats where the water is coming from
    • CommentAuthorRex
    • CommentTimeDec 1st 2021
    Don't think it is the metal edging as that is nothing more than a cloaked verge. It sits over the edge of the tiles on the top and covers the facia. No part of it is within the roof void.
    • CommentAuthorphiledge
    • CommentTimeDec 1st 2021
    It doesnt have to be within the roof void. The condensation Im suggesting is from atmospheric moisture condensing on cold steel and running down the facia.
    Not sure of the solution but what a fantastic looking house
    • CommentAuthorRex
    • CommentTimeDec 2nd 2021
    "It doesn't have to be within the roof void. The condensation I'm suggesting is from atmospheric moisture condensing on cold steel and running down the facia. "

    The dripping I noticed a few days ago, possibly but I don't really think so. Certainly not when I get icicles from the purlin!

    "Not sure of the solution but what a fantastic looking house "

    Thanks. It still looks good after ten years, mainly because the roof overhang prevents water dripping down the walls and as a result, algae growing; unlike some neighbours with no overhang!

    This was taken while the garden was relatively 'fresh' and low; now it is quite well established and the house does not look like it is in a field. Fortunately, woodland behind.
    Something I've experienced on the painted timber cladding on my own house is condensation developing on the external surface of the timber cladding, due to a cold night turning into a warm, damp morning - 10oC temp fluct not unusual.

    This can appear more on the east/south east side as the sun warms the air locally. The same is evident on the 3G windows, as the outer surface is cold, and so condenses the warmed morning air, again prevalent only on the E/SE elevations.

    Just a thought, but could you be experiencing the same effect, with the sloped soffits encouraging the external surface condensation to run down (painted surface), till it hits the discontinuity of the purlin outrigger, and so shedding as a drip? I think you said it only happened on the southern side, and I'm guessing the trees to the rear may also block any early sun to the "non drip" sides, resulting in one corner only dripping?

    Diagnosis via internet is always very difficult, but I don't really buy the internal air escaping, travelling along to that one soffit, and condensing. It's a ventilated roof design (IIRC), so I'd expect it to have dissipated before it got across to the soffit.
    • CommentAuthorRex
    • CommentTimeDec 3rd 2021

    Thanks for your thoughts. Was speaking with a neighbour who is a retired architect and his thoughts are as yours. Particularly, in his opinion, exacerbated by the soffit being uPVC.

    As for the icicles that appear under the purlin, not only on the outer edge but along its length, he thinks that condensation is probably running off the front of the facia, down the 'drip' edge (an algae trail is evident) and probably running back where the soffit meets the purlin because may be, the levels are not 100% level.

    Essentially, he does not believe that there is internal condensation as there is sufficient ventilation. I will investigate the same design on the front left side of the house, where, if a similar thing is happening, it will run off the slight slope of the roof to the gutter and would not be seen at all.
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