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There is a damp line being formed behind the toilet commode, as seen in the picture. Any suggestions on where the problem lies would be appreciated.

enter image description here

Edit:

I've added pictures of what it looks like with the lid off (the lid is usually left on) . I don't particularly see any signs of condensation on the inside. Also attached is a picture of the wall right behind the commode so you can see how far down the wall the damp line runs (annotated the level with a red arrow)

The toilet is on the 1st floor of a 2 story home. However, on the 2nd floor, there are no drain pipes right above this toilet that I can think of (there is a game room right above).

enter image description here enter image description here

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    That is odd. A water supply line is always under pressure, so a leak is constant. A drain line is not under pressure so a leak depends on use, but going up is very weird, pumps required. Are there drain(toilets/sinks/tubs) pipes for above this floor?
    – crip659
    Commented Dec 31, 2023 at 22:42
  • Can you ad a pic of (and behind) the tank with the lid off?
    – brhans
    Commented Dec 31, 2023 at 23:06
  • Is there a continuation of this dampness downwards behind the toilet as well ? If so, there is a tiny leak inside the cistern that is spraying water out.
    – Rohit Gupta
    Commented Dec 31, 2023 at 23:06
  • @RohitGupta Usually the cistern/tank not under pressure. If the lid was left off, then the fill tube could spray up. I am assuming the lid is always on and not only for the picture.
    – crip659
    Commented Jan 1 at 0:13
  • I've added pictures of what it looks like with the lid off (the lid is usually left on) . I don't particularly see any signs of condensation on the inside. Also attached is a picture of the wall right behind the commode so you can see how far down the wall the damp line runs (annotated the level with a red arrow) The toilet is on the 1st floor of a 2 story home. However, on the 2nd floor, there are no drain pipes right above this toilet that I can think of (there is a game room right above).
    – cd234
    Commented Jan 1 at 6:18

4 Answers 4

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The mark could be from condensation on the cold water pipe. As the pipe warms, when not in use, condensation stops. If that is the cause, there likely would be condensation on the toilet tank.

Some fixes for condensation:

  • Easy: Reduce humidity in the room. If there's a shower, there should be a vent fan.
  • Harder: Open the wall and insulate the supply pipe.
  • Harder and less efficient: Use a mixing valve to warm the cold water supply line. One reason to do so in very cold climates is to help prevent pipes from freezing.
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    Good answer. It would match with the problem better than most other ideas.
    – crip659
    Commented Jan 1 at 0:10
  • Would have expected this to have gone (or will eventually go) mouldy if it's continually damp and a black line to appear where the wall is damp.
    – D Duck
    Commented Jan 2 at 0:26
  • Use a metal detector (electric wire detector) to confirm or deny that the line marks the route of the water supply pipe behind the drywall. Or you could carefully scrape a small hole to investigate (not drill, so as not to puncture the assumed pipe!) [Second thought] metal detector won't work with plastic plumbing!
    – nigel222
    Commented Jan 3 at 12:10
  • @nigel222 At least where I live, PVC is only allowed for drainage, not supply.
    – OpenAI was the last straw
    Commented Jan 3 at 18:39
  • @A.R. It's not PVC. It's PEX or Polybutylene screwfix.com/c/heating-plumbing/pipe/…-plumbing--speedfitpipe
    – nigel222
    Commented Jan 4 at 9:47
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Temporarily tape a piece of paper so it hangs loosely between cistern and wall. A sheet of newspaper would be fine.

Intention is to figure out if the damp is coming from inside the wall, or from the room. Maybe there's a spray of water from a pipe when you flush - if so the paper should catch it and show this.

If the paper stays dry then the source is inside the wall, and you're up for some exploratory wall excavation and some plaster work afterward.

Do you have access to a bore-scope camera? If not, it might be worth buying a cheap chinese one, the sort that connects to a cellphone. This will allow you to drill a small access hole in the wall which is easier to patch than a head-sized hole.

My suspicion is that something's leaking or sweatting inside the wall.

On the other hand you have a Bidet which contain pressurising pumps and pipes. So a tiny crack in a pressurised line could result in a fine mist that sprays, and drips down the wall. Need to confirm or eliminate if the source is in the wall or not.

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    May be worth taping a piece of plastic sheet (e.g. Saran wrap) on the wall before the paper, just to eliminate any porential water transfer if the source is in the wall.
    – Huesmann
    Commented Jan 2 at 19:13
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    Don’t bore near the line: very likely that is a pipe!
    – Michael Lorton
    Commented Jan 3 at 18:04
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I would not think condensation would seep through a 3/4" thick drywall and then even if it did, dry up so quickly.

I would guess you have a very small leak that sprays a fine mist when you use the toilet. I would flush the toilet and run your hand all around and feel for when your hand gets sprayed. Shine a flashlight as well as it will pick up a mist spray quite well

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  • 3/4" drywall is very thick! Even fire rated drywall for use between a garage and living space is only 5/8"...
    – FreeMan
    Commented Jan 3 at 17:02
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Drill a 4 or 5" hole using a Hole saw ( being care full as you drill) . Either side of top water stain. put your hand in to feel for any water lines / drain lines etc. You may also be able to shine a light . Has to be a source of leak in the area Other option is to cut a square hole , possibly 5"x5" immediately where the water stain begin on top.

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  • How would one go about patching a 4-5" round hole drilled between studs? Cutting a rectangle that goes from stud center to stud center makes the repair job much easier.
    – FreeMan
    Commented Feb 13 at 13:01

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