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8" floor joists, 15' span
July 10, 2014 8:26 AM   Subscribe

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?
posted by alcahofa to Home & Garden (13 answers total) 1 user marked this as a favorite
 
You can always use 4x8s or double up the 2x8s, or go with glulam. Modern lumber is often worse than old lumber, so just matching size and spacing of the old joists may result in more flex.

My local lumber yard (not Home Depot, but an actual specialized place) has a set of people who do the beam calculating and engineered wood ordering, and they are who I would be asking this question to. They'd know if there was a source for parallam in exactly that dimension, say, or some other good non-off-the-shelf option.
posted by Dip Flash at 8:52 AM on July 10


Someone else can probably better describe the various engineered lumber solutions available on the market, and what is and isn't allowed by building codes. In general terms, stiffness increases with the cube of depth. But if you can't increase the depth, stiffness also increases proportionally to the width. Doubling up the width (with wider beams or two beams) would double the stiffness.

Steel beams should be way stiffer than anything you can get in wood of the same dimension. Again, you would need to talk to a specialist to find out what kind of beams are available, and what the most cost-effective choice is.
posted by cardboard at 8:59 AM on July 10


I can confirm that at 15' those 8" joists will be quite bouncy - my last house had an addition that was 12' with 8" joists and it was bouncy. I fixed it by running a cross beam in the crawlspace. Can you run a beam halfway or some such ?

Or steel beams. Much more expensive, but a lot more firm for the size.

Otherwise, what I have seen done is you raise the floor on the second level and put in a small stair.

Your plan might be to code, but I don't know anyone who likes a bouncy floor.
posted by Pogo_Fuzzybutt at 9:01 AM on July 10


I'd hire a structural engineer, and just be done with it.
posted by aramaic at 9:05 AM on July 10 [3 favorites]


Consider doubling up the joists and using extra solid blocking (or cross-bracing at minimum)?
posted by BillMcMurdo at 9:08 AM on July 10


Steel beams should be way stiffer than anything you can get in wood of the same dimension.

They can be stiffer than anything in wood, but it depends on what you use. Actual steel "beams", like I-beams, wide flanges, or channels would definitely be stiffer but probably prohibitively expensive. You may be able to get something stiffer with heavier-gauge light metal framing, but lighter gauges aren't necessarily any stronger than a cookie sheet. Any of these options may require engineering to make your building department happy.

Another option would be to use a series of flitch beams or some other kind of composite wood/steel members, but that would definitely have to be engineered.
posted by LionIndex at 10:06 AM on July 10


Adding a beam to cut the 15' in half seems like the most logical option. Is there a reason you're not doing that in the first place?
posted by humboldt32 at 10:15 AM on July 10


Adding a beam to cut the 15' in half seems like the most logical option. Is there a reason you're not doing that in the first place?

If you use lumber, the beam will be deeper than the joists, which may not work the ceiling heights as mentioned in the original question. The whole ceiling doesn't necessarily have to be lowered, but there will at least be a bump where the beam is unless the beam is something more exotic. If there's no attic, the beam may also block extending services from the existing house (ducting, plumbing, electrical, etc.) from reaching the outboard half of the room. So, yeah, it's possible and a simple solution, but brings up some other issues.
posted by LionIndex at 10:52 AM on July 10


I dealt with this in my last house by making a two-inch ramp up to the area that needed 2x10s instead of 2x8s. It was never really noticeable.

But, I would just double up every other 2x8, and put some extra cross-bracing in, which ought to eliminate the bounce.
posted by beagle at 10:56 AM on July 10


What about using a 6" steel I-beam to cut the 15' span in half, with 8" joists borne on the bottom flange and notched to accept the top one?
posted by flabdablet at 12:09 PM on July 10


Thanks. The architect suggested raising the second floor in the existing building 4", which makes sense because we're putting in a new cathedral ceiling.

In any case that is how we will solve this.... allow the joists to stick up into the second floor and raise the existing second floor.
posted by alcahofa at 1:39 PM on July 10


What about using a 6" steel I-beam to cut the 15' span in half, with 8" joists borne on the bottom flange and notched to accept the top one?

This is kind of what would happen even with a wood beam - the joists would be hung on either side of the beam rather than sit on top of it. The only reason the beam would protrude below the bottoms of the joists is just that it would almost certainly have to be deeper to support half the weight of every joist in the room.

What would happen with a steel beam is there would be threaded studs welded to the sides of the beam, and then a wood member would be fitted over those studs (which would basically act as bolts) and fixed with nuts and washers. After that, it's just like a wood beam and you hang joists off it with hangers. Normally, you'd attach a piece of wood to the top of the beam and hang joists off that, but with only 8" available that becomes difficult.
posted by LionIndex at 3:12 PM on July 10


Sounds like you have your solution (it's a good one). For anyone looking in the future who is constrained to the 8" be aware that stiffness is considerably increased using a plywood subfloor that is glued to the joist and applying a solid sheet stock to the underside of the joists (IE: Gpyroc screwed to the underside). One of the reasons that decks feel so bouncy is they are often constructed merely of decking boards screwed (or worse nailed) to the joists with nothing nailed to the bottom of the joists. This allows the joists to squirm sideways under load increasing deflection.

Also if the construction of the house allows for the new joist to slide into the existing joist space then doing so and gluing and screwing the new joists to the existing joists (the more overlap the better) will to some extent convert the new single span into a multi-span which also reduces deflection.
posted by Mitheral at 11:12 PM on July 11 [1 favorite]


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