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I'm currently in the beginning stage of a bathroom remodel. All the flooring and subfloor has been removed, and I'm at the point where I am getting ready to replace some joists and sister one.

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The first question I have is about the joist that I'm sistering to (video 0:50 to 2:00). I don't want to replace that joist because it is under a load bearing wall, and it is already up against several other 2x8s. It doesn't have much damage or deterioration, but there has definitely been water on it, and the top of it may be slightly rotted in a few places. Since I'm planning on leaving it in and just sistering a new joist on to it, is there something I should do to treat the existing joist to keep any mold/mildew/rot from spreading?

Second (related) question: For all the joists I'm replacing, and the one I'm sistering, I plan to use pressure treated 2x8s, since this is going in a bathroom. I just want to make sure that I never have to worry about water damage again, and one way to do that is to use pressure treated lumber for the joists as well as the sheathing/subfloor. Is there any reason I shouldn't use pressure treated lumber?

Any other tips or ideas for how I should go about this are appreciated.

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  • Welcome to Home Improvement. If you'll take the tour, you'll see that we prefer 1 question per question, not a series. As it stands, your first 2 questions may be closely related enough to get answered together, but the 3rd isn't even about sistering joists, so should stand on its own as a separate question. Please edit your post and feel free to ask as multiple questions. Nobody will look down on your for doing so, and you may find that an answer to one means you don't even have to ask a second.
    – FreeMan
    Feb 18, 2021 at 11:57
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    As to PT lumber, basically nothing means you "never have to worry about water damage again". Wood is a natural product and water will damage it. Work on preventing leaks/spills and proper & quick cleanup & repair (instead of letting leaks go on for months/years as the previous owners seem to have), and worry less about waterproofing your structural lumber.
    – FreeMan
    Feb 18, 2021 at 11:59
  • @FreeMan Thanks! I just edited to remove the third question. I agree with what you said about wood and water. I am confident that all the leaks are fixed and have been for some time. I think the leaks existed for years before someone repaired them, but never repaired the water damage they had caused. This place was a rental before I bought it, and after I've lived here a while, I intend to rent it out. So I want to make it as tenant-proof as possible. But I also intend to keep everything under control and not let any leaks go untreated in the future. What can I do to prevent leaks?
    – chen
    Feb 18, 2021 at 17:50
  • Don't use PT joists. It's not allowed to do so except for specific cases, and if you have enough moisture to rot joists you have other problems as well.
    – isherwood
    Feb 18, 2021 at 19:11
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    I can't cite code, but the gist of it was to minimize exposure to chemicals used in treatment by restricting usage to just where it's truly needed. This isn't such a case. It's possible that more recent preservatives are less restricted in application.
    – isherwood
    Feb 18, 2021 at 19:59

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In terms of sistering you don't need to treat the joist there if it has dried just sister the joists and attach the new subfloor to the new joists.

Pressure treated wood for joists and subfloor is going about this wrong. If those get wet from above things have already gone awry. If you end up with another leak in the future you'll still need to pull up everything to determine the extent of the damage.

If you are planning tile above the subfloor invest in a waterproofing system like schluter ditra as well as a good waterproofing system in the tub/shower area. Other systems are available as well.

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    Thanks! I definitely intend to keep everything under check to make sure that any future leaks are taken care of as soon as they happen. But as long as I'm replacing joists and subfloor, I thought that pressure treated lumber could be one extra line of defense (just in case) for long term. Does that make sense?
    – chen
    Feb 18, 2021 at 17:54
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    Also, I do want to make everything as leak-proof as possible, but if the current plumbing is not leaking, what more can I do to help ensure that it won't leak in the future?
    – chen
    Feb 18, 2021 at 17:56
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    Plumbing will always leak eventually. If done correctly it should be at least 20 years (usually more like 40 or 50 in a lot of cases) barring external circumstances (extremely hard water, chemical leach, freezing and bursting). If your plumbing is already very old and you want to future-proof it, consider replacing (and see if the cost to do so makes sense). Feb 18, 2021 at 19:25
  • I'm not sure if you've ever priced out pressure treated plywood, but it's pretty insane. You'll also also need to use specific fasteners since the pressure treatment corrodes normal screws etc. For very limited benefit since it's not supposed to remain in constant contact with water regardless of the pressure treament. There are just more direct ways to ensure your system is water tight. For plumbing in particular I think bracing and strapping are often neglected, but foot plumbing should last half a decade unless you are in extreme conditions. By that time a remodel is often in order anyways
    – redlude97
    Feb 18, 2021 at 21:18

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