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We bought a townhouse a few years ago and they had just put this wooden patio in. It was just unfinished wood and I was lazy, then we had a baby, and never got around to sealing it.

We have finally gotten around to wanting to improve our backyard and I am planning to replace the rotting boards then stain and seal it.

It seems like the first six or seven boards closest to the house are the ones rotting the ones further away from the house seem dry.

I pulled a rotting board and it looks like there is a lot of mold on the underside of the boards. Is this expected? Is there anything we can do to prevent it or is it just part of having a wooden patio so close to the ground?

Not sure if you can see from the picture but under the deck seems to be a combination of pavers and gravel.

Picture 1

Picture 2

Picture 3

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    That looks like pressure-treated lumber. If it's just "a few years" old it shouldn't be rotting like that. It must've been a low-grade treatment. How old is the wood, exactly?
    – isherwood
    Commented Oct 13, 2023 at 14:29
  • We bought in 2020 and the wood looked new so about 3 years old. We do live in the mid-Atlantic area and it stays pretty humid during the summer months
    – Kyle
    Commented Oct 13, 2023 at 14:59
  • Still, that's an absurdly short timeframe for treated lumber to rot. I've had garden boxes buried in soil for twice that long with no discernable rot appearing.
    – isherwood
    Commented Oct 13, 2023 at 19:22
  • Does the area beneath the deck collect drainage water from anywhere? Diverting water away with grading, a french drain, and/or gutters on the roof could mitigate the problem.
    – popham
    Commented Oct 13, 2023 at 19:27
  • Is the framing pressure treated? The boards obviously are not. If they're the exact same lumber, then is there evidence of the same mold on the framing? Is the framing equally wet and/or moldy as the surface boards. If the boards are saturated and the framing is not, then I would conclude that undrained surface water is saturating the boards and causing the mold.
    – popham
    Commented Oct 13, 2023 at 21:05

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You have moisture wicking up through the ground, and you have moisture dropping from the sky. The only way to mitigate mold and rot in that scenario is airflow. If there isn't an open space below the framing things will never dry out. If you get frequent rain and it's on the north side of the house even that may not help, though. Synthetic decking (or a return to a masonry patio) may be the best strategy.

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  • It rains here a good bit (Virginia) and the underside looks pretty closed off. Wonder if it would help to put vents into the sides of the deck to allow air to get under it (until we feel like redoing it completely)
    – Kyle
    Commented Oct 13, 2023 at 15:02
  • Probably not. There are already "vents" between each deck board and through small gaps elsewhere. You need full airflow.
    – isherwood
    Commented Oct 13, 2023 at 15:36
  • Look how tight the board edges are, though. Are there "vents" between each deck board? Those boards may have been installed tight, or it could be a seasonal humidity thing.
    – popham
    Commented Oct 13, 2023 at 19:18
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    There are a million tiny gaps in the structure. A few small vents doesn't change the current situation enough to be worth trying. Also, you can't usually cut large holes in deck framing anyway.
    – isherwood
    Commented Oct 13, 2023 at 19:21
  • Looking at it again, without drainage between boards, water may be collecting on the surface and saturating them.
    – popham
    Commented Oct 13, 2023 at 19:36
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That much rot is pretty conclusive evidence that the deck boards aren't cedar or redwood and that they're not treated. Definitely seal them. There exist copper-based wood preservatives that you could use to kill the mold on the underside and keep it from coming back. They can be price competitive with deck sealers, so you might instead use such a product on the underside if you intend to use sealer underneath. You might not like the color of these wood preservatives, so on top you would probably not want to use them. Definitely take care not to get blots of stain on the exposed surface.

The tightness of your board edges looks like a problem. This configuration won't allow water to drain through from above, and it won't allow water vapor to vent from below. The International Residential Code requires vented crawlspaces to have 1 ft² of ventilation for every 150 ft² of floor space, so I would space the boards to roughly achieve this level of venting between the boards. Remember that the boards swell with seasonal humidity, so provide a bit of extra space if you do this work during a low humidity season.

Ground water from below could be a problem source. Grading the surrounding area can mitigate this some. A french drain is an alternative option. Gutters on the roof edge above the deck can also move water away from the deck. I suspect that the roof edge and poor surface drainage caused your really badly damaged boards, but I'm uncomfortable asserting it without more images.

While you have the deck boards removed, take a look at the framing below. If this framing is untreated, then the bearing points on top of those pavers is a problem. Especially if you decide against any ground water mitigation, consider spraying wood preservative around the wood in contact with the pavers.

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  • Thank you for all this. I will take a look at the treatment for the underside and will try to give the boards as much space as I can. Unfortunately they’re all pretty tight right now, but I also just power washed them not long ago so they could be a little swollen. Luckily the framing looks to be in better condition except for some of the 2x4s that go along my house. They look pretty bad but the rest is solid. This area that is rotted never gets direct sun so that can’t help.
    – Kyle
    Commented Oct 13, 2023 at 23:22
  • We’re probably going to try to get 3-5 more years out of it by sealing and replacing any rot then just redo it with synthetic or take it out completely. I’ve been suspicious of it since we moved in because having a wood deck at ground level in a humid area doesn’t make much sense to me.
    – Kyle
    Commented Oct 13, 2023 at 23:25

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