The 2nd story floor on one side of our 1940s lake house renovation had a noticeable low spot in the middle of it. This was due to cracked and sagging floor joists that were exposed when we demoed the interior. While some of the joists were just sagging due to a weak floor system in general, some had holes too close to the edge or a big knot that caused a crack and really weakened the joist, putting even more pressure on the surrounding floor joists. To fix this we needed to sister new joist to the old joist to stiffen and flatten the floor. Read on to see how we did it.
Step by Step Video
Required Tools & Materials
- Bottle Jack
- 4×4 post long enough to reach floor joist
- New joist that is the same size of old floor joist
- Mini sledge hammer
- Clamp
- Framing Nails
- Hammer or Nailer
How to Fix Sagging Floor Joists
- Identify Sagging Joists
Using a level or straight edge, you can press this against the bottom of the joists to figure out which joists are sagging. Also take note of the joist dimensions and purchase the same.
- Jacking the Floor Joist
If your joist has a really bad sag, you may need to jack it up to get the new joist in place. To do so, place a bottle jack under the low spot on the joist and cut a 4×4 to fit. When using the 4×4 to jack up your joist make sure that it is perfectly plumb. With a lot of weight on the jack, any sideways force could cause the jack to kick out.
- Sistering a New Joist
Start by see if there is any crown on your new joist. If so you will want this facing up.
Remove any blocking that may be in your way and place your new joist such that both ends are bearing on a beam or foundation.
Persuade the new joist into place with a mini sledge hammer.
Once you have it in place. Clamp any gaps in the two joists and nail together with 3 framing nails vertically every 16 inches.
- Install Blocking
Cut solid blocking to fit snuggly between the joists in the center of the span and nail in place with framing nails.
