How To Deal With Floor Joists That Need To Be Cut Or Moved For Plumbing

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    One of our friends asks about a bathroom remodel they are doing and while replacing their toilet the installation of the new waste line means they need to cut into a joist more than they should. When doing this you will be weakening the structure around an important area that needs to carry some weight. He wants to know what the best way to handle this is.

    Its always disappointing when you run into this type of situation but when you are laying out a bathroom you sometimes don’t have a choice because room to move things a few inches in either direction just might not be available.

    The worst thing is when new homes or poor remodeling jobs end up with joists that are cut to fit the pipes and no repairs are ever made. Things might work out for a while but you  will have a high likelihood that the floor will flex and your tile will crack. Even worse is that over time any rot or decay from moisture that these cut up joists see will result in a dangerous situation where if the job was done right the joists would be able to handle the load.

    How Do You Support Loads When A Joist Must Be Cut For Plumbing

    The idea behind this is pretty simple. Just think of the structure you will be building as if you were opening Joists to allow for a stairway.

    When you build a stairway you decide where it will be located and then support the joists from below with a temporary stud wall while you do the work.

    The next thing you need to do is install double joists on the perimeter of the opening to support the extra load. They need to run the whole length of the joist you are attaching it to and rest on the same beams or outside top plate.

    When you nail headers or double joists you want to nail every 6 inches on center and about 1/3rd of your nails should enter on a 45deg angle to reduce the chance of separation. Liquid Nails is also a good idea but not always required.

    What is different than building a stairway opening?

    After you have doubled your joists you can then install blocking between those joists to allow for attaching your 3/4 plywood subfloor. Depending on the width of the opening you might need to install cross blocking to allow for good attachment.

    When you have an opening at the edge of a building this works fine but what about when you are in the center of the building?

    In this case you need to do the same first step of adding double joists on the first joists that are outside of the cut zone.

    Then you will cross block with a double header on each side of the opening and then attach short joists that will run the distance to the headers at the opening. You should use steel joist hangers when you do this.

    This common opening type is just like when you frame for a fireplace in the center of someone’s home.

    Final Note

    When in doubt get some help from an architect or another professional that understands structure loads and how to frame openings and reinforce joists in these situations.

    The extra time and cost to do things right will pay off because you will have a much stronger floor and your home will be safe.

    In reality the cost is not that much. You are talking about a few extra pieces of joist material and some joist hangers. Unfortunately it can mean that you will have to open the ceiling below the bathroom to install the joists properly.



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