Anyone know what this stuff is? bad boards

Had boards similar looking to these on our deck two months back, ripped the bad boards out, but the neighboring boards acquired the same ailment since then. Ripped them out too (and anything with any sign of white stuff) and attempting to order copper-green wood treatment to paint underside -- would this be enough to stop this from happening? Does anyone know what this stuff is? Was told that it's mold and to just clean it off, but the affected boards broke very easily so I'm worried that it's some sort of nasty rot. Can mold weaken boards to this extent? Does it even matter what this is or is it just a question of getting rid of anything affected and reducing moisture in the area?

This is a fixer upper house and the deck has been very neglected for a number of years. The deck lacked any sort of stain/seal and the gaps between the boards were so small they would disappear completely when it got wet. We also had a VERY wet spring and summer. Only about two weeks ago we cut the gaps. Couple of days ago stained it and redirected the gutters away from the area. The entire deck is massive (probably 800 sq ft) and the remaining boards seem fine, so would really not want to rip the whole thing out. Would this in addition to copper-green treatment on the underside be enough?

  • Looks like normal old deck issues to me a strong cleaner even high pressure cleaning may help, you probably did not notice before ,,, a little fungus among us is not hard to clean. Did you change wood types ? To me that can help growth but what do I know just a guy on the internet...
    – Ed Beal
    Jul 22, 2020 at 2:51
  • interesting.. change wood types -- I have no idea. We got whatever pressure treated decking was available. There is none available now, so whatever boards we took out will not be replaced for a while. The boards with the white stuff were taken out, and the middle one was definitely weaker than a normal board -- split in two when I thew it off the deck. Sounds like you are of the opinion that cleaning, treating neighboring boards will likely prevent further issues? Jul 22, 2020 at 3:09
  • Cleaning and treating And sealing can stop the wood rot There are some products that harden damaged fiber but I would not use those on exposed decks mostly for the joist or beams under the decking.
    – Ed Beal
    Jul 22, 2020 at 14:17

2 Answers 2


I would check the wood itself before trying to clean. Cleaning has not been an option for us and I've been told to check/contact the wood manufacturer to see about warranty. It has to do with the treatment itself on the PT wood and stain and/or cleaner is not working. I think this is a valuable answer and others should know as well. My issue, that looks just like yours, is most of the substructure and wood hardeners as described did not work.


I have had the same issue since 2020. The decking begins to exhibit the white substance that grows on the underside and subsequently the board begins to rot in that spot. Even after removing it and using bleach the substance comes back and eventually the board completely rots in that area. I’ve been replacing the boards since 2020. I finally got disgusted with the whole situation when another six boards and three of my joists rotted and called the installer. When I told him when the deck was built (2012) he said that many of his customers who had decks installed around the same time have had the same problem. He said that the problem was that the manufacturer did a poor job of pressure treating the lumber and any of the decks they built using it have rot problems. He is currently trying to help many of his customers, but the manufacturer has gone bankrupt. he had me send pictures of the problem, but there isn’t a lot he can do, So,I’ll have to pay to replace all of the rotted wood myself at great expense.

  • Welcome to Home Improvement. While there does appear to be an answer in this, it reads mostly as a (well deserved) rant. Please take the tour and read through the help on how to write a good answer, then edit this to remove the more ranty bits and to make the answer bit more obvious. It might help to mention the name of the manufacturer, if you know it, to help others. You've got the basis of a good answer here, it just needs a bit of work.
    – FreeMan
    Jul 7 at 13:56

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