Concord Carpenter

How to Fix a Broken Floor Joist

By Robert Robillard on How To

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Making a Jack Post

I made a fast, and simple jack post with two 2×4 screwed together. I was not lifting a lot of weight or lifting and holding for a long period of time so I chose not to make a “strong back” or “T” jack post.  “T posts are stronger because the 2×4 boards oppose each other and counteract any bending force created during jacking.

To make a “T post” nail your 2x4s together perpendicular to each other in the form of a “T.”

Truss Lok Structural LAgs

Installing the Sister Joist

After the new joist was in place; vertical and adjacent to the old joist I used a clamp to hold them together at the chimney end.

I patched in the notch on the old joist, mostly so I had structural bearing onto the joist hanger. I also added a shim board to the width, to keep the hanger from deforming when fastened.

I then relocated the jack under the joist and place a slight amount of pre-stress tension on it, ensuring that I was level, I then installed a double joist hanger.

The joist hanger I used was designed for “old work an fits the wider faming material often found in older homes. I secured the joist hanger into the corbelled beam with 2-1/2 inch Simpson structural screws and into the joists with 1-1/2 inch structural screws.

Then I installed a series of 3” Truss lok structural lag screws, joining the new joist to the older one. Once secured I released and removed the jack.

Note on Structural Engineer Inspection

If you’re concerned about the structural integrity of sagging, cracked or twisted joists in your house, first call in a structural engineer to inspect the floor and pay to get a recommendation for reinforcement and strengthening measures.

This type of inspection typically costs $500 to $700 and will assist with any other remodeling or building permitting projects.