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Rotten Roof Sheathing in Winter

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  1. #1
    Apprentice (new member)
    Join Date
    Dec 2020

    Exclamation Rotten Roof Sheathing in Winter

    We purchased a house this fall (it is roughly 30 years old). A few days ago, we went to check the attic and found either wet OSB sheathings or what seems to be rotten sheathing as well.

    1) Some OSB sheathings appear to be black and we suspect they may actually be very wet/soaked sheathings (we found one section of 30 x 30 cm black, and another of 60 cm x 30 cm). When we look from outside, the OSB sheathing is slightly deformed/wavy. When we look at it from the attic, we see that the OSB sheathing is currently black (wet/soaked with water). The sheathings seems to be holding its integrity, but seems waterlogged and we suspect it has went through (and still is) wet and dry cycles causing it to deform. Also, some parts started delaminating (wooden chips falling). Since winter is beginning, we will received more water/rain/snow, so it is unlikely that this will dry. We are in Canada, Quebec.
    2) What do seem rotten is a portion where two OSB sheathings are supposed to join. We suspect they have lifted and water seems to have infiltrated through that lifted portion. Overall, we have 1.5 m x 5 cm of OSB sheathing, between two wet rafters, that is completely black and flaking (we see bit of black flakes falling from it - you can see these black bits on the isolation when looking at the picture)
    Overall, we suspect that water condensed and accumulated due to bad ventilation (we found isolation blocking the soffits on the problematic side of the house). For now, we have pushed down the isolation and created about 15 cm of air flow between the wet OSB sheathings and the isolation (which itself was partially wet as well).
    When the house was inspected in the fall prior our purchase, it was reported that there was less than 6 in of isolation and so we initially had the intention of waiting until next spring to add isolation and wanted to take advantage of that time to renovate/fix any weaknesses or problematic areas in the roof. The first inspection was done in warm and dry weather, and so we do not know if the problematic sheatings (OSB) were overlooked by the inspector or if the problem started during the fall after our purchase (we had a period of heavy rain).

    Knowing what we know now, can we still afford to wait until the spring to work on the roof? With the holidays and confinement, we will likely not receive any profressional help until mid/end january. Any constructive thoughs appreciated.

  2. #2
    Seasoned DIY droog's Avatar
    Join Date
    Sep 2005


    That type of roof construction is rarely used here in Australia so you may not get much input from members on this forum.

    You may get better input from forums primarily located in your part of the world.

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