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New Build Passive House Roof Leaking or Sweating?
This house was probably built to what's known as Passive house standards, i.e. extremely high standards of airtightness and insulation. We don't know if the problem to the roof was totally caused by leaking, or if it was the way the membranes over the roof were fixed, that stopped the Passive house from 'breathing'? Although I agree with the idea of Passive house standards, extreme care has to be taken when building a house with so much insulation. Management of the internal environment has to be strictly managed, the transfer of energy to the outside is severely reduced because of the insulation, this also severely stops the transfer of moisture that has to be managed iternally to stop interstitial condensation from happening. When we stripped this flat roof, we found different membranes fitted in the incorrect place, thus holding back any moisture that got into the roof.
The other thing to take into consideration is that this roof is formed from cross-laminated timbers and is fully structural. This is a good strong platform from which to form a roof's sub base. However, if the roof leaks and the timber is compromised by rot, then the cost to replace it will be enormous. If we had to replace the cross-laminated roof section on this roof, we would have had to completely dismantle the stairwell and the surrounding areas.
Building in redundancy into a building should be a big factor in the design. When we re-roofed this roof we made sure that the falls to the roof were extremely good; any small settlement in the building would not change the falls to the roof, and the roof would still drain. We enlarged the scupper (rainwater outlet) so that it would drain more freely and be less likely to get blocked. We added through-ventilation all around the roof so that if the roof did get wet or there was moisture that needed to escape from the roof, it could freely get out of the structure. We covered the roof in a roofing membrane that was laid in one piece so as to make maintenance and roof inspections easier in the future. The decking that was placed over the top of the flat roof was laid on plastic supports and the decking itself is plastic so that it will be maintenance-free for its lifespan.
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Steven Dickinson
London flat roofing
07802300099
info@londonflatroofing.co.uk
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