Some 6 years ago, with me helping a time served builder, we attached a brick extension as a garage with separate accommodation for a bedroom and shower room. Overall, the build is 30 x 18 feet, with 50mm cavity insulation enclosed by 100mm grey insulation blocks. The garage has 75mm PIR covered by 125mm concrete. This is separated from the 10x18 foot accommodation area by a single grey insulation block wall up top the roof for fire retardation and with a fire door. The floor in the accommodation area is suspended 18mm T&G. None of the ceiling voids have yet been plaster-boarded
To retain the view from an upstairs bedroom, we had the 75mm trusses manufactured to 20 degrees strengthened with horizontal and diagonal pieces + 25mm ply wood sarking boards.
Because of the shallow gradient, the sarking boards were covered in 2 layers of breathable membrane with a 225mm overlap. The 30x60 slates were fixed to a 110mm cover. There's also a glass fibre based gully running down some 20 feet alongside the house. All of this was inspected at each stage and OKed by building regs, including the under eve venting.
Until this winter we'd not noticed any problems. But, several weeks ago, whilst taking down some garden furniture stored on the ceiling cross beams, I spotted a number of wet black stains bordered by a white chalk like edging in the angles where the rafters and board supporting cross beams meet, and a sort of black/grey marbling on a number of the boards. Both these situations were most prevalent at the corners not protected by the higher house wall. In fact, the pitch closest to the house from where warm air might be most abundant is completely clear of marking. Some of the long, heavy 100x50mm diagonal strengthening runners also have a black, soot like covering at intervals, perhaps resulting from where items had been stored on the ceiling beams
My first thoughts were that the driving wind and heavy rainfall this winter, combined with capillary action, had enabled water to be driven up under the membrane and perhaps tugged it sufficiently to cause damage at the nail points under the retaining vertical batons.
However, I'm now stating to learn that this could be the result of 'Roof Sweat....that is condensation!
I've carefully scrapped away one of the larger black areas and find that as this is removed, the normal wood colour is showing. The boarding is virtually dry now following the recent winds.
Assuming this is roof sweat, and given the rafter depth is only 75mm, the advice so far is to create a warm roof by first fixing a vapour barrier to the rafter underside and then fix 75mm PIR.
If this is the answer, what quality of vapour barrier should be used and how should it be fixed, bearing in mind that a supporting spar interrupts the sweep of the underside every 600mm?
Also how best to fix the PIR. Is 75mm OK, and does it need another vapour barrier on top?
Finally, if I fix the PIR to the rafter, will there be any issues if I add additional PIR at the ceiling level at a later date?
Grateful for all and any advice.
To retain the view from an upstairs bedroom, we had the 75mm trusses manufactured to 20 degrees strengthened with horizontal and diagonal pieces + 25mm ply wood sarking boards.
Because of the shallow gradient, the sarking boards were covered in 2 layers of breathable membrane with a 225mm overlap. The 30x60 slates were fixed to a 110mm cover. There's also a glass fibre based gully running down some 20 feet alongside the house. All of this was inspected at each stage and OKed by building regs, including the under eve venting.
Until this winter we'd not noticed any problems. But, several weeks ago, whilst taking down some garden furniture stored on the ceiling cross beams, I spotted a number of wet black stains bordered by a white chalk like edging in the angles where the rafters and board supporting cross beams meet, and a sort of black/grey marbling on a number of the boards. Both these situations were most prevalent at the corners not protected by the higher house wall. In fact, the pitch closest to the house from where warm air might be most abundant is completely clear of marking. Some of the long, heavy 100x50mm diagonal strengthening runners also have a black, soot like covering at intervals, perhaps resulting from where items had been stored on the ceiling beams
My first thoughts were that the driving wind and heavy rainfall this winter, combined with capillary action, had enabled water to be driven up under the membrane and perhaps tugged it sufficiently to cause damage at the nail points under the retaining vertical batons.
However, I'm now stating to learn that this could be the result of 'Roof Sweat....that is condensation!
I've carefully scrapped away one of the larger black areas and find that as this is removed, the normal wood colour is showing. The boarding is virtually dry now following the recent winds.
Assuming this is roof sweat, and given the rafter depth is only 75mm, the advice so far is to create a warm roof by first fixing a vapour barrier to the rafter underside and then fix 75mm PIR.
If this is the answer, what quality of vapour barrier should be used and how should it be fixed, bearing in mind that a supporting spar interrupts the sweep of the underside every 600mm?
Also how best to fix the PIR. Is 75mm OK, and does it need another vapour barrier on top?
Finally, if I fix the PIR to the rafter, will there be any issues if I add additional PIR at the ceiling level at a later date?
Grateful for all and any advice.
