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brokenthumb

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One of the many to do list things here is to beef up my floor joists. My house was built in 74. It's 24 x 48. There were a lot of sins committed when they built her, just one surprise after another. The floor joists are 14 foot 2x8s at 15 inch centers. I have always a bounce, but the other day, my 4 year old was jumping in the middle of the living room floor, and I thought the TV was going to fall over. I thought about sistering the joists with another 2x8. I would have to lay it flat, slide it into position, up over the center beam, then out over the outside wall. I worry about twisting the board into an upright position beside it's sister in case I cause damage to it, or the upstairs drywall. I read an article in the Hammer (gotta get my keyboard fixed) Last letter of the alphabet followed by one. It said when sistering floor joists, it is not always important for the joist to sit on support ends, as the strength is only needed in the center. For example a 14 foot 2x8 spanning 12 feet would only need a 10 foot sister, providing the existing joist is not sagged in the middle.....Thoughts....
 
Sounds right to me. The highest stress is at the mid-point of the length. Also the extreme fiber is where the stress is so a taller beam is stronger than a wider beam. That’s why I beams have a flange top and bottom and not much in the middle. Next time you are in Walmart take a look up. The roof is all bar joists and bar beams. The extreme fiber is spaced apart by a bunch of round rods welded in.

I personally would sister in the full length unless it was a lot of trouble. I would also screw the two together like the screws were free. I might even push up on the joists just a little then screw the sister on. adding a little camber.
 
The trick for rtolling a joist into place is.
Well first the problem is when the joist is on an angle it is to high to fit.
The trick is to cut a bit at each end at 45* on the botton just where it will sit on the wall. You only need about 1/2 angle so that when the joists rolls up it is not taller than the old one.
Hopefully that made some sence.
 
Apply a wavy bead of construction adhesive to the new sistering. Full length and top to bottom as you go from one side to the other. Roll it into place, using a framing hammer if necessary to encourage it into place. You will need to remove wiring that is in the way and pipes too.

To guarantee the new joist is tight to the old one, screw or nail a 2x4 perpendicular to the joists in a few places. You might need a temporary hydraulic jack to get everything level, lifting it about 1/8" a day until you get it right. Then tilt the new joist in place against the glue, clamping if you really want a tight fit. You can nail using 10d nails every 16", staggering the nails top to bottom along the length of the joist. (Some guys want 16d nails but they are long, so you would have to angle them to prevent sharp points sticking out).

Lastly, you could add 2x8 blocking to stiffen the floor too. You will have to toenail them but they can really stop deflection. Best place would be in the center zone where the span is weakest.

Good luck!

JOISTS.jpg
 
Thanks for the prompt responses. I have been gradually replacing the aluminum wiring as I have been doing rooms over. I don't run the new wire runs through the holes in the joists, I prefer to fasten them to the bottom of the joists instead. The plumbing however.....instead of running the pipes below the joists, they drilled, and ran the pipes through the joists...even the drain pipes..including the toilet drain...shakes head. They actually cut 2 joists in half in order to do this. Not sure what's worse, that, or the fact that they all wrote their names on the center beam.
 
Drilling holes in the center of the joist does not weaken them up to a certain size anyway.
Can you post a photo of the two they cut and what the did about them.
Bridgeman is a member here that we don't see very often also claimed that screwing a 2x4 on the flat along each joist made an I beam out of it. Someone here tried it and I think had good results.
There are also metal staps, I'll see if i can find info on that.
 
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