I am putting an addition on an old house that has 8" floor joists. For various reasons, it's very problematic to change the ceiling height between the original house and the addition. However, the smallest engineered floor joist I can find is 9 1/2".
Code allows me to span 15'7" with 2x8" on 12" center. That's currently the only option I have, however, I've been told my floor will be bouncy.'' Is there anything else I should be considering?
I would recommend, and have, hiring a structural engineer for situations like these. It should only be a few hundred dollars. In my case he was able to give me much simpler solutions than I was expecting to have to install. An engineer stamp may be needed on your plans in any case, when you go to get permits.
You might be able to doubled up 2x8s. That gets you from 2x8 joists to 4x8 joists. Or perhaps add a beam below the new joists.
@Katharine Chartrand I'm a structural engineer, and I strongly recommend against spanning 2x8s at 12" o/c for a 15'-0" span. "Bouncy" would be putting it nicely. I'm assuming you're going with at least Douglas Fir - Larch, grade 2 lumber, and that you will be covering it with at least 15/32" plywood or OSB (and the appropriate fasteners) that is rated for your floor live load and span.
Also, I don't know what floor finish you plan to have on this floor, or what ceiling you plan to have below these joists. If it's a tile floor, you will almost surely have some cracked tiles near the middle of the span. If it's a drywall or plaster ceiling, you're very likely to have cracks on it too.
I'm also assuming this is regular residential live load (not a main corridor in a multifamily).
One option you have is to simply double up the joists. I'm unclear if you currently have any existing 2x8 joists at this location, or if you plan to put all new joists in, but you could simply put (2) 2x8s at 12" o/c (or, put the 2x8 joists at 6" o/c, although that would require more labor for all the connections at the ends). That would allow you to keep your current ceiling height, but still be able to support the live load and not have a bouncy floor (or cracked finishes).
Of course, I don't know your entire framing system, and you should still check out the rest of the framing with an engineer who can see the whole picture. For example, what are your new 2x8 joists being supported by? Can it handle the new load from these new joists? Are you using adequate connections for these new joists? My above response is simply a recommendation based on my limited knowledge of your situation, and does not constitute engineering design or service; you are advised to consult a local engineer.
Hope that helps, good luck!
Edit to add: Looks like Jon got in a response while I was typing. :) Good point regarding possibly needing a stamp from an engineer for this type of reno.
@Katharine Chartrand I was in a similar situation a few months back. I had a span a little over 12' that I needed to bridge with 2"x6" floor joists due to other limiting factors.
I ended up doubling 2x6s, gluing them and nailing them together. (the force working against these two members is shear, so nails are better than screws). I set them 12' on center. I decked it with .75 plywood (not osb) and glued and nailed/screwed it down. It was solid and everyone who walked on it commented on how sturdy the floor felt.
Another option would have been to also deck the bottom (underside) of the floor joists. Sandwiching these joists creates an even stronger floor (think I-beam).
You can also look into using a flitch plate in between your joists. This wouldn't be cheap but it should satisfy any structural engineer while keeping your 8" height.
They also make some LVL beams that are 7.25" deep and would work great, but they are expensive.
As @Kimberly T. mentioned, the load on this space will greatly influence your design.
Thanks. We're going to raise the second floor in the existing building 4". It is possible in this case because we're putting in a new cathedral ceiling.
In any case that is how we will solve this.... allow the 12" joists to stick up into the second floor, then raise the existing second floor.
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