They will not weaken the floor joists but the presence of this fungus is an indication that the wood has been moist or wet for a long period.
Mold on roof joists.
Generally tearing off a roof and roof sheathing and perhaps roof framing to remove mold would not be sensible nor cost justified.
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.
The best course of action is always to remove any spores in the earliest stages of development so follow the simple steps below to effectively get rid of all of it.
At the time of inspection.
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.
Over the subsequent weeks mold growth forms before the building materials can dry out.
Another common cause is exposed soil in the crawl space.
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Blue black or gray mold on floor joists may indicate sapstain fungi which are similar to surface mold but go deeper into the wood.
Damaged insulation damaged throughout the crawlspace area and must be removed.
The subfloor and joists often become saturated by rain before the roof is installed.
If left alone mold in joists can eventually become such a problem that it can only be removed by professionals.