I am currently living in Vancouver area, recently I got a leak in my roof but my roofer already fixed it yesterday.

After the day it was repaired I went up to our attic to check if if everything was OK and no more leaks but I noticed this white something (maybe mold) growing in some of part of my wood.

I read on the internet that you can kill molds with a bleach solution and wipe it with paper towel? I am not sure if this would be safe to do in my attic.

Is this dangerous that I need to call professional immediately?

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I’m 99% sure this is mildew/mold caused by the high humidity level in leaky attic. Hopefully the moisture levels will come down now that the leak is fixed.

It would be wise to spray the areas with a non chlorine mold killer like Concrobium

For the next few weeks check humidity levels in the attic. If they remain high you may need to ventilate the attic better.

  • I havent use Concrobium before, is it safe? Should I just spray it on wood directly and wife it with paper towel afterwards? – Hawk Dec 25 '20 at 23:39
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    Yes it is safe and no obnoxious chlorine odor. Wiping is not necessary – Kris Dec 25 '20 at 23:40
  • ok ill buy one tomorrow with this stuff thank you.. What should be the humidity level in the attic, around what percent? – Hawk Dec 25 '20 at 23:41
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    This site says 30-50% humidity level should be maintained – Kris Dec 26 '20 at 0:18
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    @Hawk Yes I did misread. Sorry – Kris Dec 27 '20 at 21:53

I'd put on a N95 mask. Take a bucket of water with some bleach and a dish sponge. Scrub the surface of the wood down with the bleach solution. Let it dry, once dry I'd use a white vinegar spray to spray the affected wood, let it dry. If it looks good then you can either leave it or paint it with a mold encapsulating primer/paint like extreme stain block.

I wouldn't worry too much about the mold in general as warmer lower density air moves up and is pulled into the attic. Air that is already in the attic is unlikely to come into your living space. Mold isn't good for you but varies based on exposure, species and sensitivity. Mold spores exist in fresh outside air and it grows naturally outside so even fresh air will have a spore count to it. The more toxic species of mold typically have a black look - even then something of this size you should be able to self remediate.

You should get a hygrometer and find the RH of that attic it should be below 60% RH. You can do a quick check of square footage of attic and count how many air vents you have - most attic are under vented. You want soffit intake venting and ridge or high output venting. You should ensure that the insulation that is in your attic has not clogged the soffit vents.

  • Actually I noticed some insulation are clogged to the side of my roof prevent some air flowing air inside the roof. Maybe 3-5 out of 16 holes are clogged.. Is it safe to use bleach on wood I read that its just add moisture to the wood – Hawk Dec 25 '20 at 23:57
  • I want to cleat it but the side of my roof is so stiff that only a long stick can reach and get to it. Do you have any tips on how can I clear it out? – Hawk Dec 25 '20 at 23:59
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    Totally agree with the mask. I breathed something in from a ceiling space and ended up hospitalised with a lung infection. Warm, damp and dark is a perfect breeding ground for badness. A better tool than a bucket might be a garden pressure sprayer with a wand, to help OP get further out with less risk. – Criggie Dec 26 '20 at 1:24
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    The interior window in my bedroom was the color of your wood. I hadn't moved the curtain in a year and dust had gathered, water had condensed and mold had eaten the dust/grown on top of the painted surface. I used bleach water with dish scrub sponge and it looks pretty good today. I had a mask. It is the spores of the mold that are inhalation hazards if you have a bleach spray bottle and you spray before you wipe spores shouldn't be put into the air. If you let the mold dry out and try to clean it dry that would the worst. – Fresh Codemonger Dec 26 '20 at 21:59
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    @Hawk good work - I meant that a N95 mask is your minimum for health. If I had one, a respirator would be more reassuring and comfortable. But KN95 vs N95 are pretty close. smartairfilters.com/en/blog/… suggests they're almost the same so I wouldn't stress over KN vs N. – Criggie Dec 27 '20 at 21:43

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