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My son recently bought a 100 year old house. We have been working on some framing repairs in the basement, and have some questions about the best way to sister some joists.

This is a balloon framed house. The wall studs rest on the sill plate, and extend all the way up to the 2nd story. The first floor joist rest on the sill plate and were at one time nailed to the studs. The flooring is laid on the joists.

The joists in question are true 2”x10” lumber approximately 16 feet long (for the longest run). They bear on a 10”x10” wood beam that runs down the center of the basement, with a ~1” deep notch cut into the bottom of the joist, and extend to the outside wall. On the outside wall, they were also notched to sit on the sill plate.

Sometime in the past, the sill plate in this part of the basement was replaced. My son discovered, after removing some insulation, that the joists were no longer resting on the new sill plate. In some places the joists seemed to be short of the sill plate. These joists are also supported by a 4”x4” post (used as a beam) at their mid point, which means they are basically a long cantilever. We think this beam was added to support the joists which no longer rest on the sill plate. Oh, and some of the joist are split at the sill plate end. The sketch below tries to show this. The blue beam is being used to raise the joists to their proper height. New sketch showing another beam, in blue

enter image description here

enter image description here

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We want to sister the joists with new lumber, 2x10 or 2x8. Our desire is to use 2x10’s, 16’ long that would sister a joist along its entire length, from the main beam to the sill plate. But, we don’t have room to slide a 16’ 2x10 in place – too many pipes. HVAC ductwork, and other things in the way. We think we can fit a 16’ 2x8 in place. A shorter piece of 2x10, 8’ long would also work (we have two of these dry fitted in place). See the second sketch.

enter image description here

The question is would we better to use a full length (16’) 2x8 as a sister, or an 8’ 2x10 that only sisters the bad end of the joist, the end that would rest on the sill plate? In either case, we want to remove the "added later" beam (shown in the light purple), so that the run from the main beam to the wall is uninterrupted.

Results

Here's what we ended up doing. We sistered ~8' of the damage end of the existing joists so that the sistered joist (2x10), with the notch, would rest at the proper height on the sill place.

enter image description here

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    I wouldn't want to remove the 4"x4" beam without knowing why it was installed - you say you think it was added because the joists are no longer resting on the sill plate, how sure are you of that? Adding a beam in the middle rather than where the joists now end would be a strange solution to that problem, so I would guess there is some other reason for that beam to be there.
    – kaya3
    Oct 8, 2021 at 0:20
  • Because we can see that several of the joists are too short to bear on the sill plate. I tried to capture that with the second picture. If the joists don't bear on something at the far end, the floor will collapse.
    – SteveSh
    Oct 8, 2021 at 0:53
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    Right, but what I'm saying is that the midpoint doesn't seem to be the sensible place to put that beam, if that's the problem the beam is meant to solve. So that suggests to me that the beam could be meant for something else.
    – kaya3
    Oct 8, 2021 at 0:56
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    Agree with 'don't remove the central beam'... Are you trying to eliminate posts? You could install steel with fewer posts and still a relatively low profile. Oct 8, 2021 at 1:07
  • @kaya3 - Agree about the mid point not being the sensible place to put the "helper" beam. I don't why they (previous owners) decided to put it there,which still leaves 3 or 4 feet of some of the joists unsupported and to cantelevered. There are a lot of things we've found in the house that don't make sense to us.
    – SteveSh
    Oct 8, 2021 at 10:28

3 Answers 3

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Yes. I have met your problem. Let's not make a crazy hard job out of this...

The problem is the ends of the floor joists are not properly supported and where they are, the notch at the end creates a place for the wood to split. They don't grow trees like they used to.

Attach a 2x6 flat to the wall below the floor joists to hold them up. If the joists have moved downward (which they probably have), Create a temporary beam and use a jack as needed to lift the floor joists back to where they belong so that the 2x6 can be installed in the proper position, then lower the floor down onto the 2x6.

I hope you can imagine what I am saying...

I am going to assume that the wire in the large picture is attached to the wall and the 'floating' 2x10 is the floor joist:

  • Remove the staples from that hold wire to the wall.
  • Lift the existing floor joists as needed.
  • Install a 2x6 where the wire used to be.
  • Lower the old floor joists onto the 2x6.
  • Support the wire on the new 2x6.

Now the floor joists are sitting on the 2x6. Hopefully, the reason (most likely water entry) that lead to the replacement of the original sill plate has been repaired.

In cases where the floor joist is still short:

If the floor joist is still short, then 'sisters' will have to be used, but they don't actually have to be a full sister joist. They can be rather short.

Consider the cantilever: I was once told that I had to grab twice the meat with secure support as I was extending outward without support. I believe there are a lot more factors involved, but I keep the basic idea in my mind and use it.

Example: If you were to assume that 8 inches remained to reach all the middle of the sill plate, then you would need 16 inches secure grab. Keep in mind that the 16 inches is NOT how long the sister extends against the existing joist, it is the 'actual attachment distance'.

Explanation of 'actual attachment distance': You want a secure hold, so you choose to use bolts and add nails later just to ensure a good overall bond. The distance goes from the sill plate you want to transfer the weight to, until the first bolt. Now, you are no longer 8 inches away, but you are 10 inches away. Therefore, twenty inches later is the closest you need a second bolt. It, the the first, needs to grab the meat of the wood, so the sister needs to be 2 inches longer.

Total length of sister to create cantilever: 8 + 2 + 20 + 2 = 32 inches. Therefore, if you can get a 32 inch 2x10 to sister the joist and attach it well, then you are good enough.

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  • Updated the sketch. We are already doing what you suggested, using the beam shown in blue to raise the joists up to their proper height. That's the 4x4 in the first photo, with the red circle.
    – SteveSh
    Oct 8, 2021 at 2:22
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    "Attach a 2x6 flat to the wall below the floor joists to hold them up". The wall is the foundation wall, right? So like using a ledger board attached to the side of a house for a deck?
    – SteveSh
    Oct 8, 2021 at 10:25
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    I disagree with several assertions you've made about sistering. The first is that a 2:1 cantilever is adequate. It's not. I've seen the results of such a ratio, and it's not good. Tilted joists and humped floors are common outcomes. Levering and shear forces are too high. You need at least 3:1, and 4:1 would be even better.
    – isherwood
    Oct 8, 2021 at 13:55
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    The second is that this is a cantilever situation to begin with. It isn't. You can't apply the same principles to a sistered joist because of the attachment mechanism. It would take quite a few bolts to tie those components together adequately. It would be simpler to just use longer joists, even at today's prices.
    – isherwood
    Oct 8, 2021 at 13:57
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    Every structure is only as strong as its weakest point. The extra length of a long 2x8 adds no extra strength to the weak spot when compared to a shorter 2x8. On the other hand, the extra width of a 2x10 adds extra strength to the weak spot when compared to a 2x8. Every structure is only as strong as its weakest point. Where is the weak point? Make it strong like the rest.
    – Paul
    Oct 21, 2021 at 22:47
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You cannot make a splice in a floor joist in the middle of its span and expect that to be sufficient. The middle of a span is the weakest location in that floor joist. A floor joist can only extend 1/3 of the distance of the total span from beam to the foundation wall. If you want a 2' cantilever then a floor joist needs to be a total length of 8'minimum from bearing point to bearing point.

The area that needs a patch or repair cannot be fixed without providing a new bearing point for the new load distribution. The new floor joist patch will involve new floor joist for the entire span and should coincide with the exterior wall studs from the balloon framed exterior walls. They should be glued and nailed with 4 16d nails vertically at every 12" spaced rows. Before nailing I would draw the joists together with some 1/4" lag bolts every 2'-3' or as needed. Then install the nails. Glue heavily with a polyurethane-based CONSTRUCTION Glue like Liquid Nails construction glue. Do not buy the inexpensive lines of their glues as they will not be designed for your application.

The design of balloon framing vs. platform framing that is used in new construction these days are not designed with the same structural values. Balloon framed walls need to be tied to the floor joists to keep your structural elements locked together as one. When you place a load bearing wall on top of concrete there also needs to be a load bearing footing under the concrete floor to support the new load points. The older homes with the rough sewn lumber will not be the same dimensions from one to another. The carpenters cut those notches accordingly so the top of the floor joist are in a flat plane and the basement ceiling will be irregular. They didn't care because no one used the basements back then as a living area.

You will have to lift each floor joist one at a time because if you get the basement ceiling completely flat, then your surface on the top of the new floor joist will develop a floor that is not flat. Remodeling requires compromising the issues but still completing it so you have a safe structure when you are done.

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    "You cannot make a splice in a floor joist in the middle of it's span and expect that to be sufficient". That's not true. You can splice in the middle, or anyplace else along a joist or beam, so long as 1) the splice member has sufficient strength to withstand the bending moment and 2) it is properly fastened to the existing structural member. Look at road bridges. They have major beams spliced all the time. This Old House had an episode where the main beam of a house was spliced/strengthened with a steel L bracket (similar to a lintel).
    – SteveSh
    Dec 22, 2021 at 13:02
  • Those are structures that have been designed by Structural Engineers and Architects. Those designs have structural elements designed specifically for that application. Engineered lumber has design loads built into them and to achieve the maximum design loads, they have to be assembled so that they meet the design engineer's approval. The floor members you are talking about securing a piece of lumber to, are not structural anymore. Repairing one cracked floor joist with a patch, sistered onto the defect, can be successful but it should not be used on an entire floor system.
    – JimmyB
    Dec 23, 2021 at 4:09
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A recommended splice detail:

enter image description here

For your case, the extension shall be 48" long (16*12/4). To avoid the chance to split the wood, you may look into the splice hardware provided by the vendors such as "Simpson Strong Ties".

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    Please provide the source for your image and the (16*12/4) formula. Where do the 3 numbers in that formula come from? Since the OP isn't looking to splice, but to sister, does this still apply and how? It would be hard to have 24" on one side of the joist, since it only needs about a 4-6" extension to reach the sill.
    – FreeMan
    Nov 8, 2021 at 18:07
  • @FreeMan For the OP's case, the span length is 16', and I suggest placing the splice joint at the quarter-point of the span (see my comment directly under the question). No, for this case I don't like the idea of sistering a less stiff joist to the original, as it creates more problems down the road. Where do you suggest to make the splice otherwise - midspan?
    – r13
    Nov 8, 2021 at 18:30
  • Putting that info into your answer would, IMHO, help a lot. It was not clear to me at all that you were suggesting cutting the existing joist back and splicing in a new piece. I'm not disagreeing or attempting to argue with you, just trying to clarify what you were proposing.
    – FreeMan
    Nov 8, 2021 at 18:31
  • Completely unnecessary. If you wish, you can edit my answer but not to twist the answer. Thanks.
    – r13
    Nov 8, 2021 at 18:35

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