A trap arm is the length of pipe between the trap and the vent. There is a small bay window on the front of our building and it has a small roof. This roof has a drain. From what our recent probes have revealed, we can tell the drain pipe is made of 3/4″ PVC and has a trap arm length of almost 10′-0″ to the vent.

Now that may not seem like such a great distance for water to travel, but in our case the trap arm is far too long for the size of the pipe. Firstly, a 3/4″ PVC is too small of a diameter for a fixture and PVC piping is only allowed in a one or two-family dwelling in NYC. It should be cast iron, the quiet pipe.

The truth is there was one pipe in the building that we didn’t change. In order to save a few dollars i decided to go against my spidey sense and just let sleeping dogs lie. Unfortunately, this is the outcome of that decision which did not work out so well and has now led to a leaky roof…

3/4″ is just too small of a pipe. The trap arm fills with water and becomes plugged. The water backs up and spills into the ceiling cavity. When it leaks it not only causes damage to the wood paneling, but the floor below as well… Ugh.

The truth is we will now have to open some walls that have been finished and we need to install a proper 3″ cast iron drain line for trap arm to function properly. What a mess…