Expert: Bruce E. Johnson - 1/12/2010

Question
Hi-

I'm adding a bathroom and need to make a run of pipe across (3) 2 x 10 joists for a 3" toilet drain pipe. They are 16" on center.

This will allow me to meet the main drain heading down to the stack, without running a waste pipe in my living room.

Is is better to bore the joists or transfer the load around like you would frame an attic opening, but just wider than the pipe, using double joists.

If it's better to bore, how big should the plywood be or sistered joists run, or, if it's better to frame a slim opening, how far do the joists need to be sistered.  

Answer
Hi David, I would take 3/4" plywood and sandwich it on each side of the joist over a 4' length or a 2x10 4' long on one side of each joist, center it on the hole location and attach with #10 nails or 3" screws. and liquid nail. Drill the hole in the center of the joist after the sistering is complete.  In reality, a hole less than 33% of the joist depth is generally acceptable..But I would feel better if you reinforced the joists..I hope this information helps, please feel free to write again regarding this or other matters, sincerely, bruce e johnson..bejohnsonconsulting.com