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atmoran 08-10-2011 09:10 AM

Reinforcing joists
I've got old school true 2"x6" Joists 16" OC spanning 11'. I'm already in the process of reinforcing these since obviously these are quite undersized. To make matters worse, I have three that have been cut through to accommodate a 12"x4" air duct. Luckily, directly above is a closet, and one of the joists sits on what is now a load bearing wall. Still, I'd like to support all that dead weight better than what is there right now. How can I reinforce this?

My dad has come up with an idea for it but I'm not sure that we should cut what little remains of the joists since they are still providing tension support, and we should simply bridge around the duct work, similar to what he has drawn:

loftezy 08-10-2011 11:13 AM

The cut joists obviously need to be fixed and your dad is proposing a sensible repair. I wouldn't worry about cutting the remaining portion of the joists since they are hardly providing any support at this point. Just make sure to temporarily support them while you do the repair.

Regarding your 2x6's being over-spanned: what makes you say this? Do you see signs of sagging, cracking, or buckling? Depending on the type of lumber and the loads that they are experiencing, they may only be slightly over-spanned. Unless you are seeing signs of excessive deflection (besides the area around the joists that were cut), it may be a waste of time to sister all of the joists.

atmoran 08-10-2011 11:45 AM

Thanks for the reply, loftezy.

I'm thinking that the joists are over-spanned based on the condition of the ceiling drywall (hidden by mdf and trim when I bought the house) and the floor upstairs being very bouncy. I know for sure that the few joists that are actually 2x4s with shims need to be shored up, but yeah, since this wood is hard as nails I'm not sure that all of it needs sistering. The AHC joist calculator tells me that they are overspanned by about a foot, but thats probably based on current wood specs, and these are from 1922.

Thanks again

BigJim 08-10-2011 11:59 AM

Is there enough room to run the joists above the supply duct? If there is and it doesn't bother you to give up that space you can support both ends of the new joists then tie the old joists to the new ones with 2X4 blocks. Just another option.

I personally wouldn't cut the joists the rest of the way as it could or may cause your walls to spread just a little, Just a thought.

atmoran 08-10-2011 12:43 PM

Unfortunately, no, there is no room to go over the duct, I would have to run through a hallway, good thought though.

Good thinking on watching the walls, I think before cutting I will have to have my bridging in place nearby in order to minimize movement, though both joists run to interior walls

loftezy 08-10-2011 01:05 PM

Jiju1943 made a good point about watching the walls to make sure they don't spread. But, if the other joists are in good shape, and you aren't cutting a majority of them to complete the blocking around register, you should be fine. But please, don't forget to temporarily support the joists before you cut them! Also, the blocking around an opening in joists typically extends to the nearest full length joist, but you are proposing to extend it to a cut joist that rests on the dining room wall. This is fine as long as you are certain that the wall is load bearing and the loads will be transferred all the way to the foundation.

Sistering your joists will help stiffen the floor but you shouldn't expect a huge improvement unless you go with a deeper joist (which I know you can't in this case). There are various ways to sister your joists, including using another 2x6 or using plywood. Here is an example of how to use plywood:,00.html

Just don't skimp on the glue and nails.

TrapperL 08-10-2011 01:25 PM

I like your dads plan but it doesn't say how far across the room is- that's the vertical line in his drawing that is parallel to the duct. Considering the floor joists are now transferring the load to this member, it would be a good idea to double up that part. Since you've now shortened the span by a lot, the current lumber should do it with ease.

atmoran 08-10-2011 02:45 PM

Thanks all, I will post after pictures once we get everything in place!

atmoran 08-14-2011 09:28 PM

As promised, the after pictures:

In progress, with one bridge up and the temporary wall holding up whats left of the joists. I ended up sandwiching a 5/8s strip of plywood between the sisters on the advice of my girlfriend's uncle, as well as the posts in this thread about using plywood in sistering.

The finished product! Thank you to everyone, it feels great walking around upstairs in this area now, and I'm no longer worried about falling through the floor.

Ron6519 08-14-2011 10:00 PM

I would have doubled up the joists on either side of the ductwork. It's the same thing you would do for a stairwell framing.

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