Reinforcing Floor Joists

By Robert Robillard on Bathrooms, Home Repairs And Remodeling

Structural Floor Joist RepairsReinforcing Floor Joists

As a remodeling contractor and carpenter one common structural framing issue I see alot of in bathroom remodeling jobs are  floor joists that were cut, drilled, notched or need to be cut out to accommodate plumbing pipes and drains.  The time to address these structural issues, like reinforcing floor joists, is at the time it was built or during a retrofit or remodel.

Contractors may cut through floor joists to accommodate plumbing or HVAC mechanical equipment as long as they properly transfer the floor load to adjoining joists by adding headers.

Heading Off and Framing Floor Openings:

In a perfect world any floor joist that is cut should have a header installed, preferably two and doubled adjacent joists.  All headers should be nailed to the cut joists and their ends attached to two adjacent, uninterrupted floor joists.   The purpose of the headers are to transfer the floor load to the adjacent trimmer joists.  The use of joist hangers facilitates this repair.

A single header nailed to the cut joist and uninterrupted joists will work for openings less than 4 feet. If the header spans more than 4 feet, both the header and the trimmer joists should be doubled.  We often refer to doubling up of joists or other structural framing as “sistering.”   The doubled trimmer and header joists must be nailed together properly with spaced pairs of 16d nails every 16 inches so that they acReinforcing Floor Joistst like beams.

Reinforcing floor joists is best done by doubling of floor joists.  Doubling of joists is recommended under walls, heavy objects like tubs and at large openings.  Doubling of joists can solve alot of problems like sagging, uneven or undersized floor joists.

Sometimes on a remodel you can’t install a full length floor joist because of plumbing, existing walls or finished floors or ceilings.  In these situations it sometimes makes sense to scab the floor joist or even all of the floor joists.

Steps To Head Off A Floor Joist

  1. Determine the location of your plumbing drain.
  2. Determine the size of your Plumber box.”
  3. For a double header measure back three inches from your “plumber box”  [3″ is the width of a doubled header] and mark a square line on the joist to be cut.
  4. Use a circular saw to start the cut and finish with a reciprocating saw.Reinforcing Floor Joists
  5. Measure the distance between the un-cut trimmer joists and cut four headers.
  6. Place two joist hangers under the first header PRIOR to fastening anything.  This is important if the ceiling below is covered.  If the ceiling is open you can install the hangers after.
  7. Direct nail the first header in to the cut joist and through the trimmer joists in to the header.  Use 16 D common nails and make sure the tops of the joists are flush.
  8. Add the second header and direct nail this header through the trimmer joists.
  9. Nail the double headers to each other three 16 D common nails every foot.
  10. Secure the headers to the trimmer joist with nails of SDS structural fasteners
  11. Repeat on the other side.


Scabbing A Floor Joist or Framing Member

Scabbing is effective technique that allows you to add strength to and  fix minor problems that are affecting floor joists. Scabbing involves taking piece of wood and fastening it over the problem area.   I often use glue and screws to fasten 3/4 inch plywood as scabs to both sides of a joist as far as I can attach to.

Reinforcing Floor Joists

Many times I will apply scabs to floor joists that were previously or will currently be notched or drilled to install plumbing pipes.

Note – the photos show a recent bathroom remodel where we relocated the tub drain across several floor joists.  the floor system is 2×10 joists spaced 12 inches on center.  Prior to drilling holes in these floor joists we installed 3/4 inch plywood to each one.  We cut out and doubled up a header where the tub p-trap was to be installed.

Scabbing is also used alot with rotted rafters and wall studs.  Recently we used a scabbing technique successfully on this roof deck project.

Framing Floor Openings How to Video:

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About the author

Robert Robillard

Carpenter / Remodeler / Editor

Rob Robillard is “The Concord Carpenter” Rob is a builder, general contractor, carpenter, woodworker, and editor of Concord Carpenter and ToolBoxBuzz As a General Contractor and carpenter, Rob owns and operates Concord Carpenter LLC. A full-service remodeling and construction company. Rob is a recognized leader in home building best practices and a source for how-to information for building professionals. On this website, Rob covers all aspects of home construction, building science, home improvement, woodworking, remodeling, and some of the best product and tool reviews. Rob is in charge of our Tool and Product Review series - Concord Carpenter Videos where we post all of our tool reviews and video tutorials. Rob approaches remodeling and building construction with a pragmatic and problem-solving approach. He enjoys using his knowledge and experience to help and educate building professionals as well as DIYers on best practices in the construction and remodeling industry. He's a strong advocate for "raising the bar" in the construction trades and promoting the trades to youth. #BeAMentor #Green2Great Craftsmanship, quality, and pride guide his journey on this channel The Concord Carpenter's motto: "Well done is better than well said!" : Read more about Rob If you have a building or remodeling question you can have rob respond to your answer via video. Click here for more information. Invite code: x22r2

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