Mold growth on attic roof sheathing is a common issue in cool climates such as the pacific northwest.
Underside of roof sheathing is wet.
It is never a good idea to install a roof over wet plywood or any type of roof sheathing.
Mold growing on the sheathing in your attic is a serious problem that shouldn t be ignored.
Mold on attic sheathing.
When this phenomenon occurs the sheathing or framing may become slightly wet on the surface.
If the temperature drops below the freezing point often at night after the sun goes down this moisture will freeze.
Mold growth provided it is not just cosmetic mold which can simply be left alone can be cleaned from wood surfaces by blasting scrubbing or even simple surface wiping depending on the surface accessibility and.
There were also several issues with the intersection of the roofs where the 2 apartments meet.
The following picture is of a long standing moisture condition in an attic where much of the roof sheathing has turned black.
The sheathing at the peak of the roof was saturated with moisture.
Those often are the underside of the roof.
In the vast majority of cases the mold growth is caused by condensation.
This occurs when the temperature of the sheathing drops below the dew point creating a thin layer of moisture on the substrate.
Installation tips osb roof sheathing the ol sage builder trubord.
This can be caused by the action of fungal growth and molds and the affect they have on the tannins in the wood.
When moisture condenses on plywood roof sheathing repeatedly or continuously the wood will turn black.
On going back inside the attic the frost had melted and the underside of the sheathing was wet not so much that it was dripping but enough that it was damp.
The osb and in fact any wood panels must be allowed to dry completely depending on how wet the panels are this could be a few days before any roofing material is applied to avoid trapping moisture under the roof material that could cause edge swell and buckling.
Generally tearing off a roof and roof sheathing and perhaps roof framing to remove mold would not be sensible nor cost justified.
The ventilation is needed not only for moisture control but also to reduce by 20 40 high summertime attic and roof sheathing temperatures which are bad for roof trusses rafters and the roof sheathing as well as the shingles and underlying waterproofing membrane which can reach 160 180 degrees over an unventilated attic.