I need to replace 3 joists on the second story of our 2 story + basement 1920s townhouse. The joists are rotted on one side all the way into the cement block wall. There is not enough good wood to sister new joists too. The water problems that caused the rot have been addressed and the subfloor and floor are in good shape despite the 1-1/4" sagging in the floor across the 3 joists.

The area includes a bedroom and a bathroom. The bathroom has recently been remodeled. The area is above the kitchen. We will be eventually remodeling the kitchen and could rip out the ceiling at that point and replace the joists from below, but we would like to be able to continue living in the bedroom and bathroom while the kitchen work is on going.

The house is 18' wide. There are no walls under the rotted joists, but there is a wall perpendicular to the joists between the bedroom and bathroom (this is not in the center of the joists/house and does not appear to be a load bearing wall). There is a load bearing beam in the center of the house in the basement, but it doesn't seem like there is a beam in on the first floor ceiling.

Can we replace joists from below and continue living in the rooms above?

I am assuming that with 1-1/4 of sagging, they will need to jack up the floor over a month plus period, would the kitchen renovation be able to progress while that is happening?

Do they need to get to the basement to support the jacks?

  • Your bathroom remodel is not going to be as pretty as it was. Joints will open up, cracks may occur in tile if that is some of the finishes in the room. Just a heads up....
    – Jack
    Jan 15, 2018 at 19:02
  • @Jack yup, we went with cheap finishes with the intent to redo it again, once we stop the house from flooding and burning down. House has 40+ years of deferred maintenance that we are addressing. Probably would have been cheaper to move out for 6 months and tear it down to the studs
    – StrongBad
    Jan 15, 2018 at 19:10
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    The question is too broad, and without photos it's difficult to give useful advice. The simple answer is "sure, you can do that". At least I could. I'm not sure what your specific challenge is, though.
    – isherwood
    Jan 15, 2018 at 19:11
  • @isherwood my challenge is that I am in way over my head and wasn't sure if joists could be completely replaced from below. What do you want photos of? I have photos of the 6' bathroom span showing severe rot on three joists and others of the 1-1/4 deflection in the floor. They just do not seem helpful.
    – StrongBad
    Jan 15, 2018 at 19:16
  • It would be a hurdle to try and get you through it here, but photos are almost a necessity. The process depends heavily on the layout of the structure, and I can't glean enough from your textual description to get me there. Start with what you think is relevant and we'll go from there if you like. Really, begging or bribing a local professional into a consultation is probably your best approach.
    – isherwood
    Jan 15, 2018 at 19:19

2 Answers 2


Working from the info you proide:

  • Back up 6 feet from the wall.
  • Put a 5 foot chunk of 2x10 against the ceiling spanning the bad joists and good ones on either side.
  • Put another chunk of wood, 3/4 ply or osb on the floor to protect the floor. (If you have ceramic tile on the floor figure out a way to spread the load.)
  • Put in 2 teleposts. and take up the slack. You aren't supporting anything. You are just keeping the teleposts from falling over.
  • On a wall far enough from the problem to be stable, put a shelf, about 18" from the ceiling.
  • Rest a laser level on this shelf.
  • attach 16" chunks of wood to the 3 bad joists and the good one on either side. You want to measure the middle of the board, and the ends that are on good joists. These chunks need to be line of site to the laser level on the shelf.
  • Affix a cheap ruler to each board. Record the ruler marks in your notebook. This is how you are going to check what hapens.

  • Now, in the basement: Set up a similar support beam and pair of teleposts. This might be overkill.

  • Set up a similar shelf and indicator for the joists under the upstairs telepost.
  • Tighten the upstairs teleposts enough to get detectable movement on the center indicator.

  • Go downstairs and see of you are pushing the floor down. If so, tighten up the downstairs ones to bring it back up to the original reading.

  • Back upstairs. Gradually lift the board, until you have motion on the good joists. Back off a bit.

  • Back downstairs. Check that the floor hasn't pushed down.

At this point you have the good part of the joists level with the rest of the ceiling, and the load isn't bending the lower floor out of shape.

  • Now you can cut the bad joist back to good wood, or at around 4 feet.
  • use an angle grinder and take off the ends of nails and screws poking into where the joist was.
  • Clean out the hole.
  • Insert new chunk of joist. If you haven't addressed the problem that caused the rot, use pressure treated wood.

At this point I'm of two minds. You need to splice the new chunk to the old chunk. My gut level feeling is that I want a 6 foot chunk lapping the joint on both sides. There's going to be significant stress on the fasteners. I think I would use both glue and 2.5 inch x 5/15 lag bolts.

This is where you should consult someone wiser than I am. I'm not sure how many bolts you should use or what the spacing should be. Without a consultant I would put two rows of bolts 8 inches apart, alternately 1 inch and 2 inches from the edge. Bolts from the other side have the opposite alternation and are offset 4 inches so that in the middle joist there are no bolts closer than 4 inches, and no two adjacent bolts are on the same grain line.

At this point you should be able to take down the teleposts and not have anything move much.

Caveats: I've not done this. I am not a professional.


Instead of thinking in terms of replacing the joist, I recommend just putting a new identical joist in beside the ones that are giving way. This is a much easier way to achieve similar results. It is called sistering and is a common and accepted practice.

  • I agree Paul but OP said it was not able to ? Maybe they don't realise the need to pull the rotten wood out and re use the original hole?
    – Ed Beal
    Jan 15, 2018 at 22:46
  • @EdBeal I thought sistering was when you nailed the new joist to the old joist. The rot goes into the hole in the block wall which I though meant the entire joist needed to be replaced.
    – StrongBad
    Jan 15, 2018 at 22:56
  • You still need to use the original support hole And enough board to the original where it is no longer rotten, then sister the replaced rotten section with the original.
    – Ed Beal
    Jan 15, 2018 at 23:00

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