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Community and Q&A

Mold on roof sheathing

JonPope | Posted in GBA Pro Help on

I am currently building a large shop in rainy Oregon and I just noticed a little bit of mold on the underside of OSB roof sheathing. The problem is only occurring on the north side which doesn’t receive much sun in winter. Problem is worst near the eaves and subsides about halfway up the roof.

Humid air coming through eaves is condensing on cold sheathing. Each eave is vented and ridge is vented. 56% intake, 44% exhaust and following 1/300 rule. Framing and roof was completed in August, so this is not from wet building materials.

Any thoughts on a solution?

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Replies

  1. Malcolm_Taylor | | #1

    Jon,

    Probably night sky radiance. It's endemic to the PNW. Usually consists of some discolouration on the underside of the sheathing. In the absence of other moisture issues I haven't seen it cause more than cosmetic damage - but maybe it can.
    https://www.greenbuildingadvisor.com/article/moldy-roof-sheathing-in-the-pacific-northwest
    https://www.greenbuildingadvisor.com/article/night-sky-radiation

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