I'm dealing with a minor sag in my floor of about 1/4 to 1/2 inch and I'm considering sistering a couple of floor joists to fix it before I install laminate vinyl plank (LVP) flooring. While it seems like a solid plan, I'm wondering if it's overkill or if it's worth doing at all. I've also thought about just jacking up the joist a little and adding some shorter pieces instead. Any thoughts on whether this could create more problems? I suspect the sag might be partly due to big cracked knots in the wood, and here are some images for reference:
[1st Image](https://imgur.com/4WDVhhN)
[2nd Image](https://imgur.com/5YhdbuL)
[3rd Image](https://imgur.com/Q4VkwXe)
3 Answers
Sistering those joists sounds like a good idea for actual long-term reassurance. I did that where multiple holes were drilled near the bottom edge, even without visible sagging. It was a pain, but for the basement finish I was doing, it felt worth it to have that extra strength there.
Before you make any major changes, what’s the span of those joists? Most likely, they aren’t sagging from weight but because they’ve warped over time. If the floor is otherwise solid and you only need to fix the flatness, I'd suggest sistering just a 2x4 or 2x6 along the top of the joist. That way, you can use it as a straight edge when fastening the subfloor down. Also, it sounds like just adding smaller ties may get you over the sag without too much hassle.
I had a similar issue with some holes close together on my joists and they weren't sagging but I thought it was good peace of mind to sister them. It did take some effort, but I think it’s worth it in the end.
Pictures are super helpful for this! Make sure you share the images of those joists and the sagging area, as it can make a big difference in advice you get. Honestly, the visual context will help others weigh in better. Also, fluctuations in house temperature can lead to some movement as well, so keep that in mind when you assess things.
[Check out these photos!](https://imgur.com/a/yoGv254) I have shots from under the joists and the floor above, noticeable dip included.
It's a 1.5" x 9.5" x 16' joist. You've got a neat idea—would that involve replacing the subfloor? Would I have to offset it so the joints are over where I sister the new top? What if I jacked it up a bit and then sistered a 2x4 to keep it higher? I spotted a big cracked knot near the hole as well; maybe that’s the issue too. I’ll need to check the rest of them.