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The structural floor system on the 4th and top floor of a house in Rotterdam that was built in 1906 consists of 20cm x 7.5cm wooden joists which are spaced at 67cm center to center. There is nothing between the plaster ceiling and the wooden floorboards.

Floor joist view

After removing the floorboards (already done in the photo above Note, the insulation in the photo above was just added, it is part of the reason I am removing the floorboards) and inspecting the floor joist I found some white spots on them. I believe these spots is white rot. This white rot occurs at the same location on the two floor joists which are visible. It is directly above a 12cm x 7cm wooden beam girder which was installed in the early 2000s.

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In the early 2000s the house was renovated. A mason wall on the 3rd floor was partially removed to allow for a more open kitchen. The 12cm x 7cm wooden beam girder was installed to replace the mason wall’s role of supporting the floor joists. Note: Wooden beam girder covered by drywall in these images below.

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After brushing the wooden beam with a wire brush the white rot was able to be removed, it does not seem to have caused any structural damage.

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What do you think caused the white rot?

I’m hoping the white rot was caused by the 12cm x 7cm wooden girder beam having a bit too high of moisture content when it was installed. The moisture from the girder beam got into the dry floor joists and caused some white rot. This rot wasn’t able to grow any further since there was adequate airflow in the 20cm tall cavity between the floorboards and ceiling.

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  • Moisture from some source, at some time, anyway. Possibly from the masonry wall that the beam replaced. – Ecnerwal Sep 14 '20 at 13:46
  • @Ecnerwal do you think there is reason for concern? I'm planning on putting insulation between the floor joists. However, my fear is that if I do so it will make it very difficult for moisture to escape if it is somehow still present. – Eric Sep 14 '20 at 14:00
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    White rot dry rot or whatever fungus we decide to call it is usually caused by water or moisture, keeping the area dry is the best possible fix for inside structures (may not want to use nasty wood preservatives). Hydrogen peroxide is really good for killing organic fungus 3-7% solution on the area around where you brushed would be a good idea because the wire brush spread the spores further so keeping the area dry will keep any spores dormant and a good spray with a solution like hydrogen peroxide or another anti fungal may further reduce the chances of it returning. – Ed Beal Sep 15 '20 at 14:38
  • @EdBeal I was advised by a worker at a local hardware store to use this paint translation from dutch "on turpentine base is a lead-free product that is intended to protect wood that after finishing becomes difficult to reach or comes into contact with masonry and concrete". Would you advise to paint the beam with this after spraying the area with hydrogen peroxide? – Eric Sep 15 '20 at 14:54
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    If you paint it that should keep air and moisture off the wood. I buy concentrated hydrogen peroxide for mold and mildew remediation and use as a sanitizer (So I have it, but if you are painting that should be ok) if you want to treat the entire area it can’t hurt in my option but the paint will do the job. – Ed Beal Sep 15 '20 at 15:08
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As discussed in the comments the cause of the white dry rot had something to do with water and moisture.

Since the damage was only on the surface, there should be no reason for grave concern as long as the area can stay dry in the future. To prevent further potential rot this terpentine based lead-free paint was used to treat the three reachable surfaces of the beam. This should be enough to prevent future rot.

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