I'd appreciate suggestions on how to reinforce these old floor joists

[MG70]

I hope everyone has a happy and safe 2026!

I'm back to my projects slowly fixing a 1970's Keystone brand modular (prefab) raised rancher with basement. It has roughly a 1000sqft main floor and a little smaller basement. I was rerouting some electrical cables from the open ceiling in the basement, underneath the kitchen, and ran into problems on 3 separate floor joists. Keep in mind the house is from the mid '70's and 2 of these problems have existed since then and I have noticed no symptoms from any of the 3 problems over the last two years. With that said, I'd like to somehow improve or reinforce the joists.

Problems #1 and #2). Two of the joists have a really thin edge on the ends where they rest on the sill plate. It's as if there were knots in the wood that fell apart after milling and for some crazy reason the builder still used them. One has a bad edge that runs about 30" long, with the worst part being where it sits on the sill plate. The other has a bad edge that's about 40" long but again, the worst part is on the end that sits on the sill plate. Most of the joists are 2X10 dimensional lumber that run to the center of the house where it seems they're end nailed to a board that is somehow joined with a similar second half of the house which faces the front.

Problem #3 Someone must've been playing with a circular saw and made a 4" vertical cut on one of the joists (and insulation around it). To make matters worse, this happened on one of a few joists that are shorter in height, about 7 1/2" tall, and sit side by side under the kitchen. I've never seen a mix of 2X10 and 2X8 for the same floor, but I have very little experience with older houses.

NOTE, Unfortunately because this is in the unfinished portion of the basement and there's a garage on one side of the house and the main electric panel on the other, there are many cables going through all of these joists.

I'm wondering if I can sister in portions of same size joists to reinforce them without having to sister the whole joists.

Thanks for any and all help with this.

 

[Sarg]

Decades ago I used lengths of ¾" plywood to sandwich rough cut rafters in my father's home from 1894.

 

[Sponsor]

 

[wwhitney]

On problems 1 and 2, what matters is the contact area between the joist and the sill. If that area is too small, and the load on the joist is high enough, you'll get some localized crushing of the sill. Presumably until the contact area increases sufficiently, as more of the joist comes into contact with the sill.

Without knowing the exact loads involved, a reasonable rule of thumb might be that the contact area is at least 2 square inches. That's what you'd get form 1.5" of 2x joist bearing in an unwaned region, allowing that the edges are eased so you don't get the full 1.5 x 1.5 square contact patch. So if the waned region is say 3/4" wide but the overlap length is 3", still enough contact area.

If you're still concerned, a simple fix would be to take a suitable length of joist and sister it to the side of the waned joist with a suitable number of fasteners. It would take an engineer to determine the suitable values, but as a complete guess that is likely overkill, maybe a 4' length of joist with 3 rows of (7) 3" x 0.131" nails at 6" o.c., one row 1.5" in from each edge, and one row in the middle.

As for the cut joist, you could again sister a suitable length of joist along side it fastened with a suitable number of fasteners, but it would take an engineer to tell you what the suitable values are. As the cut is on just the bottom portion of the joist, you really only need to restore the bottom, tension capacity of the joist, and can rely on the intact portion at the top for the compression side. So it would be enough to sister a 2x4 to the bottom 3-1/2" of the joist, which may let you avoid the utilities going through hole in the joist. Or add a metal strap nailed to the bottom edge of the joist. Here I'm sufficiently uncertain about what suitable lengths and number of fasteners are to not even make a wild guess.

Cheers, Wayne

 

[MG70]

Thank you both for the suggestions! To be honest, I have not measured the width of the joist edges bearing on the sill but the sistering you described is what I had in mind, although I had not been able to find any information for calculating or determining the number of fasteners or length of joist to sister in. Unfortunately the legal system here in the US is such that everyone is afraid to post this type of information for fear it could come back and bite them in the behind. With that said, I had collected some info from various online sources including the use of construction adhesive in addition to the nail patterns. I have a pneumatic palm nailer which is perfect for this kind of job with limited working space for a nail gun.

Initially, I had thought of using plywood for sandwiching around the saw cut, as Sarge described above, in a similar fashion to truss mending plates, but if I buy a 10 or 12 foot length of 2X10 there should be plenty for sistering one side of the truss, which should provide enough reinforcement and leave the benefit of being easier for an inspector to see what's going on with the joist in the future.

Thank you both, again, and if anyone else has any suggestions, I'd love to hear them.

I hope everyone has a safe weekend!

 

[wwhitney]

In re plywood, solid sawn lumber is better. For beam action, you want the grain running the long dimension of the beam/joist. Half +/- of the plies in plywood will be running the wrong way.

Cheers, Wayne

 

[MG70]

In re plywood, solid sawn lumber is better. For beam action, you want the grain running the long dimension of the beam/joist. Half +/- of the plies in plywood will be running the wrong way.

Cheers, Wayne

True, you're absolutely right. I'll go the joist sistering route!

Thank you again!

 

[Reach4]

True, you're absolutely right. I'll go the joist sistering route!

I would glue and screw.

 

[Fitter30]

Had a joist that cracked in half. Bought a ten ton bottle jack from HF. Have a crawl space. Jacked up the pieces used constuction glue in the joint. Then sister 2 - 2X10 either side one at a time with constuction glue and wood screws then went back with 3/8" lag bolts washers and nuts. The joist sitting between the floor and sill was crushed about 5/16". 10 ton jack wouldn't lift it. Soni just left as is. But your don't seem to be crushed. Might with a 5' long 2X10 hold the one up with a scrap board and slege hammer beat it in place then glue and screw or trim the 2X to sister it just a 1 / 16" hammer it straight in then glue and screw.

 

[MG70]

Fitter30, I appreciate the info.! I had seen the bottle jack and 2X4 lift method used on joists before. Luckily, my cut joist is not as bad as I had thought. The cut portion hasn't even compressed onto itself and the joist is only about 8' long because it terminates on the wall that frames the stairs going from the kitchen to basement where it is located.

Having thought about it a bit, I think I'm going to sister in a section of 2X8, that's how high that particular joist is, with about 3' to each side of the cut and glue and nail it in an organized fashion. I'll have to remove some but not all cables. Depending on my mood on that day, I'll end up removing all cables and sistering in the whole joist.

The two narrow ends of the other joists don't appear to be compressing the sill. I was advised elsewhere to leave them alone since they're not posing a problem, but I can't do that, so I will think up a size of a piece of 2X10 (that's how high those two joists are) and sister them in.

As of now, that is the plan.

Thanks to all who pitched in with suggestions and I hope everyone has a safe weekend!