Finding a Small Water Leak in a Wall

Best approach for Finding the Source of a Small Water Leak in a Wall

By Mark J. Donovan

Question: I have a small water leak inside a wall. There are water supply lines and a waste line inside the wall that lead to an upstairs bathroom. Near the baseboard there are paint bubbles and moisture so there must be a leak on the other side of the wall. One side of the wall is drywall and the other side is plaster. The drywall side is completely dry, but it is probably the easiest way to cut into the wall partition to see what I am dealing with. How would you recommend I go about opening up a small portion of the wall to see what is the source of the leak is?

Answer: First. it is important to remember that where a leak is visible is not necessarily where the leak is originating from.

This said, I would take a careful look in the upstairs bathroom first before cutting any holes in your walls. The likely source of the water leak is probably associated with the upstairs toilet, bathtub or sink. Carefully check these bathroom fixtures for leaks before contemplating cutting into the drywall/plaster. Specifically check around the valves and fittings to make sure they are not leaking. Also, take a careful look around the toilet, as the problem may be associated with a faulty toilet wax ring. If it is a faulty toilet wax ring, you potentially may see water leaking out from around the base of the toilet. If this is the case, you can turn off the toilet supply line and replace the toilet wax ring.

If no leaks are observed, then one simple test you can perform is to leave the faucets off in the upstairs bathroom for awhile and see if the moisture problem dries up. If it does, then the water leak is most likely associated with a waste line or trap. If it was a supply line leak, then the water problem would persist even if the faucets were not used.

If there are no leaks around the supply lines in the upstairs bathroom and the problem dries up when you do not use the upstairs bathroom for a day or so, then yes you will probably need to cut into the drywall.

Finding a Small Water Leak in a Wall

However, just remember the leak is probably higher up, e.g. in the tub or toilet drain trap area for example. Ultimately you may end up having to actually cut into the ceiling to get under a drain trap, as this is most likely the leaking area.

If you are still inclined to punch a hole in the wall to observe the source of the leak, I would certainly go in the drywall side versus the plaster side. However, you may want to go into the drywall near the ceiling. Again, you will probably wind up having to cut into the ceiling to actually get to the problem area. If you can identify the problem bathroom fixture ahead of time, e.g. such as the bathroom tub you may just want to skip cutting a hole in the wall and go right for the ceiling, underneath the tub.

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