2nd floor bathroom sink backup w/no water source

Hi everyone,

We're having a plumbing problem that has us stumped.

We're in the early stages of remodeling a bath on the second floor. The water supply to the sink we're having a problem with is capped off and the sink has not been used for about three months.

Yesterday water started backing up in the sink and overflowed onto the floor. The water was clear and had no odor. We're having no plumbing problems in the rest of the house and so far the sink hasn't backed up again.

Any ideas on what caused the sink to backup?

Thanks for your help,
Lisa

Lisa,
Does this bathroom back up to another bathroom?

Usually the only way to have this happen would be from a sink on the other side of the common wall.

HE

There are two sinks in that bathroom that are on a common water supply but the sink that works wasn't running when the problem sink backed up.

lisa99 said:

There are two sinks in that bathroom that are on a common water supply but the sink that works wasn't running when the problem sink backed up.

Click to expand...

Hi Lisa,
This is not what I was asking.

I asked whether or not there is another whole, or partial, bathroom on the other side of the wall.

HE

sink

Water does not flow uphill, so the water had to come from another sink or source at the same level, or higher than that sink. Without being there we cannot tell you what that source could be, unless it is another sink behind this one.

sorry HJ, I misunderstood.

No, there is no sink on the wall behind this sink and there is no plumbing above the 2nd floor.

I know if it happens again we may have to call a plumber but I was trying to narrow down some causes.

Thanks
Lisa