2

So we just started replacing an old roof as the forecast seemed promising. All of a sudden, it changed to full on rain every day...

We have managed to remove half of the old roof (2 out of 4 sides) and put OSB3 sheathing covered with underlayment before the rain hit (no shingles). I'm not sure if it's standard or I might be using a wrong term, so just in case let me clarify - we're in Europe and the underlayment membrane we got we were told was waterproof and supposedly allows vapors from underneath to pass through - they call it "breathing membrane" or "vapor permeable membrane" I guess are the closest translations.

Anyway, we put it on using a hammer tacker, and as you probably can figure, it's far from perfect. It is wrinkled here and there and water seems to accumulate. There are about a dozen wet spots on the roof floor. As it was raining today, looking at it from the ground I could see a bunch of water leaking from between the membrane and the OSB end (the membrane sticks out a bit, so it's easy to tell).

Nevertheless the majority of water is being blocked by the membrane, if I'd have to guess I'd say less than 10% of the total OSB surface is being exposed to water directly.

Here is a very vague and disproportionate drawing of what I tried to explain:

enter image description here

  • black lines represent OSBs
  • blue lines represent the membrane which is laid out in 3 rows which overlap at the ends
  • green dots represent staples, which are all over the place
  • red represents where water is supposedly entering and reaching the OSBs, then continues on to drain to the end of the roof on top of the OSB and below the membrane.

I am tempted to put shingles on as fast as possible to prevent any more water from going in but I'm worried if the water that has already entered will be able to evaporate, or are the OSBs going to rot?

There seems to be more than a week straight of just rain every day, so I don't see the OSBs drying off anytime soon, it'll only get worse.

What's my best bet?

php_nub_qq
  • 123
  • 5

1 Answers1

2

If you were in the US, I would say "install the roofing," but I'm unfamiliar with European standards for OSB. I see "water resistant binders" associated with OSB3, so that's probably the same stuff that we use for roof paneling in the US (listed as "Exposure 1" grade). I saw an APA conformance document indicating that US suppliers ship to Europe, and I perceive that as corroboration.

Your subroof assembly can dry to the interior, so an installed roof will be a net good. You might place a fan in the attic to accelerate drying (removing the moist air as the water evaporates from the underside of the OSB). With only one panel face for the water to diffuse out of, I expect that it will take about twice as long to dry (twice as long versus a simplified model of the alternative case with no sun and no moisture barrier to maintain high humidity beside the top surface).

The barrier is punctured all over the place, so obviously it's imperfect, but if the shingles handle 99.5% of rain and snow, then 80% effectiveness from the barrier still takes that 99.5% up to 99.9%. Your 90% sounds pretty good to me, but that's just the personal feelings of an idiot on the subject.

For OSB rot, the problem is from persistently high moisture content, not a one time deal during construction.

popham
  • 10,617
  • 1
  • 18
  • 33