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First floor bathroom in a Victorian terrace house.

Floor joists run left to right with floorboards laid front to back. Joists are approx 10 foot long.

At some point in the past, the previous owner has cut a 4/5 foot section out of 4 of the 8 joists and replaced the timber using metal straps. As a result, the 4 pieced joists have sunken in the middle and are no longer level (unsurprisingly!).

The ideal option would be to replace the joists but I don't want to ruin the lovely period plaster ceiling attached to the underside of the joists.

My idea was to bolt new joists to the existing joists at the point were they immediately leave the brickwork. Would this work? My aim is to strengthen the joists and level the floor.

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My answer over here is probably better suited to your question than it was there. But that was about sistering, you're talking about scabbing (the other answers there may also be helpful).

What you are talking about doing is scabbing. Sistering is adding the same dimension board for the full length of the joist. Scabbing is acceptable for individual random joist repairs. It would not be acceptable if all 9 are consecutive joists. Scabbing should be done with the same dimension lumber extended 4' past the affected area. Then it should be BOLTED with 1/2 min. diameter lag bolts in a zig-zag pattern to eliminate shear cracks from forming. –oldhouseweb.com

There is no hard and fast rule on scabbing that I've found. The "acceptable" recommendation above is the opinion of that poster. Done correctly there should be no problem, IMO.

30+ yrs as a carpenter and don't know of any real rule of thumb. Call-out varies with severity of notching, joist size and span. I'd recommend qualified contractor or SE and let it go at that.

[scabbing] is installed which extends either 4' beyond the damaged section (or to a bearing point) or 1/3 the length of the framing member plus the length of the damaged section. –homeinspectionforum.net

That seems to be about it for recommendations on scabbing (they may be citing code, but I couldn't find it).

I might do one 2-by-X on each side before I had to match those four-by's. Either way, do a new load calculation and over estimate it: those old joists are now less than helpful. The lag screws I linked to in my other answer are nice (but you need a real impact driver to sink them) but if you go with 4-by's, you'll probably need bolts anyway. And since you're scabbing instead of sistering, I think bolts are in order, at least at the ends.

As for a fastening schedule, other than that sisters need two every 16" max, I'm at a loss. But here's a nice staggered pattern, with notes about distances from the edge, etc. (just ignore that it's a splice). Use four bolts like that at each end, and for simplicity, tack the rest of the old joist up to it with those screws. Again, be aware you'll need a serious impact driver for them or you'll be drilling holes just like for bolts anyway.

The only thing those old joists are going to be doing when you're done is hanging that ceiling.

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  • could someone just use a large number of, say, 20d nails instead of the lag bolts?
    – Phil Esra
    Apr 14, 2016 at 19:45
  • How do you sister joists? @PhilEsra - According to that, 10D would be sufficient. Except with these severed joists, I'd like to see some bolts or threaded lags all the way along them for pullout strength, not shear; the shear load should be all but gone. It will just be themselves and the ceiling, but it should be prepared to take the weight of whatever might be hung from them in the future. Without good pullout strength, those sections may topple under load (unlikely, as they're 4by's, but you never know).
    – Mazura
    Apr 15, 2016 at 1:12
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No matter what method you use to strengthen these joists, you will have to jack them up from down below before adding your sister joists or maybe flat steel to the sides of them.

The floor will have to be level before bolting anything together. This may destroy your plaster ceiling down below anyway. You may be able to use wood and XPS foam to cushion and spread the force of the jacks from below.

Most unfortunate they spliced those with those wimpy metal straps.

Good luck!

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  • I heartily disagree. There are numerous ways to strengthen joists (and flatten floors) without jacking. If the goal is to preserve some plaster, attempts to stabilize it should be made; no adjustments. (In lieu of a DV, I'll just point to my answer here ;)
    – Mazura
    Apr 11, 2016 at 5:12
  • I assumed (possibly errently) that the ceiling below these joists is also sagging. Possibly, they covered that flaw with the plaster. Leaving it sagged and scabbing on more wood is not my idea of an acceptable framing technique.
    – ArchonOSX
    Apr 11, 2016 at 10:31
  • Fair enough, and good point. But the contract goes to the bidder that says they can save the ceiling. (the first draft of my answer had a lot of grumbling about, "you and your damn ceiling," going on ;)
    – Mazura
    Apr 12, 2016 at 23:23

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