Another common cause is exposed soil in the crawl space.
Mold on roof joists.
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Once the wood dries out the fungus stops growing.
At the time of inspection.
If left alone mold in joists can eventually become such a problem that it can only be removed by professionals.
Project report inspection for mold on joists mold growth.
Generally tearing off a roof and roof sheathing and perhaps roof framing to remove mold would not be sensible nor cost justified.
The subfloor and joists often become saturated by rain before the roof is installed.
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 smoothness.
Inspection of crawlspace revealed moderate mold growth throughout significant areas of subfloor and floor.
In an area of high levels of mold growth or moldy dust and debris the irregular surfaces formed by wood trusses and also by older wood cross bracing between floor joists prevents thorough cleaning of surfaces and creates many dust collection points.
Over the subsequent weeks mold growth forms before the building materials can dry out.
Blue black or gray mold on floor joists may indicate sapstain fungi which are similar to surface mold but go deeper into the wood.