Home Improvement Questions? Ask a Handyman for Answers
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It sounds like a much easier fix would be to install a small beam (4 x 4 or 4 x 6) under the cut joists and put a post in the middle of the beam. Trying to sister a new joist in under these conditions would be a nightmare.
I've done a few in 40+ years in the business. Which is why I made the nightmare comment. Sistering a joist is difficult enough in a full basement with piping to deal with but in a crawl space I doubt I'd even try it. You don't need to pour concrete since the load here is very small and frost heaving wouldn't be an issue since I would expect the crawl space to be dry. You could just set the post on a 2" thick patio block. Another option would be to head off the joists (see attached pic). This what would normally be done when a joist is entirely cut out. This would still be difficult in a crawl space but still much easier that a sister.
This may be a little easier for you, given the location of it all........I had a customer with the same problem and she had a grand piano directly above two beams that were sagging............so I had 2 2 foot x 8 inch x 1 1/2 inch steel plates made up and had them weld it into sort of a 'U' shape to go right under the beam......... and had them drill 8 or 10 1/2 inch bolt holes through.................I jacked up what I could and made a sandwich of the beams ........if you had to put a post on a block like Rick said, it would be easier to do then........you can figure it like this at the least ....it is not going to get any worse and it has been 8 years and my customer said it is holding fast
Again given the location it is going to be a task ......you will have to get that length of timber in there .........if you can take a few photos it would help.......but Rick or I not being there it is a bit more difficult .........you may have to expose the outside to get material and equipment in......something to consider
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