How do I waterproof the joists under my bathroom floor?

I recently decided to replace the floor tiles in my bathroom and found that there had been a leak under the shower. This has meant that the bathroom T&G chip boards have rotted away because of water damage and need to be replaced.

I have also noticed that the floor joists have a little bit of wet rot because of the leak. This has been addressed by using Ronseal Wet Rot wood hardener and filler on the joists.

I want to know if i should and how i should protect the joists against future leaks? The 2 options that i have seen after doing a little research are:

  • Cover top of joist with flashing tape such as Bostik flashband
  • Paint top of joist with black bitumen

Should i do any of these or would this introduce further problems? Do i have any other options?

  • 4
    @noybman your comment is an answer. Copy it into an answer and then delete the comment. Commented Oct 27, 2018 at 10:52

1 Answer 1


I've never seen or heard of this being done.

Water in a bathroon should never make it to the studs or supports in any significant supply such that it actually creates or promotes damage or rot.

The appropriate solution is to remove/stop the water source that caused the damage before it gets that far/in the first place. The flooring choices is most important in these cases. Protecting the joists wont hurt them, but is most likely wasteful from a time/cost=reward/value perspective. Spend the time and money on quality water supply, drain, and routing (removal/protective, like caulk, grout, etc) components.

  • Just so you know, posting this as an answer as requested, but note, that even if time and money was spent to protect the joists, water travels via gravity and will damage ANYTHING it encounters given enough time (just ask Niagara Falls), so the importance of my answer is that the owner/resident be aware of leaks and not allowing them to exist for prolonged periods of time. A little water wont ruin a beam for short period as long as it is corrected.
    – noybman
    Commented Oct 27, 2018 at 18:32
  • 1
    I fully agree there should not me water getting to the structure.+
    – Ed Beal
    Commented Oct 27, 2018 at 19:42

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