Sistering floor joists to flatten subfloor

Old 06-13-14, 02:30 PM
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Question Sistering floor joists to flatten subfloor


I just purchased a home, built circa 1948. The house was moved in the 1970's, and put down on a new foundation.

I have ordered my hardwood flooring, pre-stained ash wood. Beautiful flooring and I can't wait to remove the current linoleum cushion-floor.

The issue I have is the following: in a 12x12 -ish area, the floor has a dip from one edge to the other, that splits the space in half. To properly illustrate what I mean, take two plywood sheets, butt them against eachother on their long edges, and then raise each sheet's exterior long edge, to create a valley that runs along the whole length.

The reason this happened is because a previous owner has, for one reason or another, cut the joists underneath that portion under the floor (reason unknown), and to ensure that the floor doesn't buckle under its own weight, they took what seems to be 2x6's to sister them to the cut beams, giving a four foot or so span on each side if the cut. So as you can imagine, with time, the floor sagged at this point.

I need to repair this before I can install my hardwood. My best idea was to take 2x8's and sister them along with the cut beams, but my issue is that these beams have electrical wires running through them.

If I notch my new 2x8's to accomodate the wires, glue and staple them to the original beams (carrying from one end to the other of the space, not juste 4 feet on each side of the cut) and then insert blocks in the notches I made to close the gaps... will this give me enough structural strength to address my sagging beam issue?

I know that I have to make sure that the crown of my beams is upward, towards the underflooring, and I intend to jack up the beams during installation to raise the overall beam height so they come back to level with the other floors.

If notching my beams is not recommended, then I will have to pull about half a dozen electrical wires from the panel, and re-route them.

I have access to my beams because the concerned floor space is the main entrance to the home, and the basement is unfinished... I have full access to my main floor joists and beams.

Sorry for the long read, and thank you for your help!

Last edited by swordfish71; 06-13-14 at 03:11 PM.
Old 06-19-14, 03:16 PM
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For what's it's worth

I'm not a professional but a DIY'er. If the space under the floor is unfinished and you don't mind how it looks when it's done then I would go for strength instead of aesthetics. I live in earthquake country so safety and strength is paramount. You avoid notching by sistering while leaving even gaps at the top for the wiring. This means the bottom of the new 2x8's will be below the existing but the result be effectively be like 2x10's. You would sister as many joists as possible and as long as possible, preferably the entire span. If you're paranoid, you would add stiffeners below at 90 degrees tying all the joists together in the middle or a pair spaced 4 feet apart. I could go into pre-stressed cables but that's definitely overkill.

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