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SHMBO and I thinking of making our 1.5 story 1920’s house a true 2 story by removing roof and building up. First floor ceiling joists are 2x6x12.5 on each side of load bearing wall down the center. Span table suggests 2×8 to support 2nd floor 12′ span. Obvious approach is to lay the 2×8’s along the existing 2×6’s, but thinking it would be easier to add 2×6 plate around perimeter and down the center and then assemble 2×8 2nd floor joists on top (making the 1st floor ceiling thickness about 14″). Slight increase in materials cost, but won’t have to shim existing 2×6 rim joists up to height of 2×8, would be much easier to run wiring, plumbing, etc. Only drawbacks I could think of are the excessive thickness of first floor ceiling where the stairs penetrate and having to head off the double joist framing for the stair opening. Anyone have experience with this situation and suggestions/opinions? Would like to use wooden I-joists for ease of handling, but supposedly double the cost of 2×8’s. Thanks for all. Don Stephan
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That's a hell of a span for 2x8's. They'd have to be 12" o.c. or less before I'd even consider it. I don't care what that table says, I think it's asking for trouble.
*The magazine "this old house" recently did a similar floor joist restructure and kept floor thickness the same. November issue. While for a differnt purpose something similar might work for your situation.check with your local architect/designer/engineer, or if you have a nice local building inspector he will help. Ours helps builders and homeowners alot, but will not go heavy into engineering in fear of liability issues. I alos agree 2x8 is a push of a span even though it is allowed in some areas if using #1 or structural select
*If you're tearing the roof off, why not take out the current 2x6 rim board as well?Replace it with a 2x10 rim, then either place new 2x10 joists between or sister them to the current 2x6's.TJIs would be much easier to handle, much better quality, much better consistency, and you can get them in lengths to run clear across the main beam of your house from one rim to the other. The difference between stock 2x10 df joists and 10" deep TJIs should only be about 15% +/- a bit. Realize that no TJIs will need to be culled, and that every one is dead-on perfect. That means no waste. Factor in faster layout/installation and it may tip the balance in favor of the TJIs when having an open roof.
*crazy legs is right. 2x8's wont span that far, period. use tji's or at least 2x10's.
*While I don't have experience with your particular situation, I would want to tear off ALL the 2x6, and replace with 2x10. Why? Once the larger joists are in, the 2x6s aren't doing anything, so whay not take them out and build a shed in your backyard with them, and burn the scraps? Plus, you might need to straighten the first floor walls anyway, and that would mean popping the joists. You'd also be able to build a nice flat floor from scratch. I mean, the old joists MIGHT be already layed nice and flat, but that thought makes me smirk.But what do I know? My way is certainly more invasive.
*And the footings need to be checked for size also.You might be better off getting an engineer out to look at the situation and to size all of your memebers.If you use I-Joists, read and follow the directions.
*I personally would use 2x8's which are just marginally acceptable for the span. The floor will be a little springy which I could live with. If I had a paying customer I would use 2x10's.I add one or two layers of 2x blocking to the plates and put the new joists between the ceiling beams. This would allow the wiring to stay in place and not raise the floor too much. The advantage in leaving the ceiling beams in place is that you can also leave the finish ceiling alone. One of my old partners did what you are contemplating and had the drywall taper fix a few popped nails in his old ceiling when he was done with the project. Of course this part of the job is a piece of cake compared to framing the second floor walls and roof and keeping the weather out while you are doing it. He had quite a crew of experienced help to assist him. Good luck.
*George is right , adding a second story often means to build up the footings or por some post supports and run posts down as well. I agree with going to 2x10s. I only suggested beifing up the exsisting to help with your stair issue. I have been into houses wher all the steps are 7 inches and you get to the last one and it is 9 or so, and for the same reason you are looking at. so if you decide to jump up in joists then look at what you need to do to make a setp or two or redo the stairs as well. Within 1/2 inch is ok if you have to,in your situation, but if I was building it I would not want that difference and when I build stairs I dont even want 1/8 difference.
*First question, Is your foundation size properly?? If not, you can run new TJI floor joist [TJI can run up to 30' without a mid-span support] perdendicular to the existing floor joist. Where I come from,[Oregon] a single story foundation[6x12 footing, 6x12 stem wall] carries--one floor & one roof. But over code allows to load-up the other two walls of the foundation with the new second story floor. This eliminates any foundation up grades. Of course you'll need to be above the existing floor joist... contact could create problems such as squeaks and cracks in the existing ceiling. This takes a little creative fudging with the existing non-bearing wall at the plates... but it can be done with out disturbing what's there.Another option, [which I have used] is to build a new crib wall just inside your existing foundation. This will pick up one of the floor loads. You'll need to pour a new strip footing [6x12] just inside the existing footing. **Code required that each new stud in the crib wall line up directly below the existing floor joist. If you have a basement with a concrete floor, you can cut out a strip of concrete along the bearing foundation wall [12 inches wide x 6 inches deep] and then pour a new strip footing [level with existing basement floor] with a new wall on top. Crib wall construction is the same as above.Another option, [which is a pain in the butt] is to pour new concrete pads [at specfic locations] under the existing foundation to create point loads to carry a new beam, which sits on top of existing second story floor joist at the exterior walls. A tip here: existing first story window and door king stud and trimmers can be used as support posts for this method [check with your architect or engineer]. Additional installation of new posts in the first floor walls might be needed. However, if your foundation is OK, then this all water under the bridge. Enjoy your project... there's nothing like a brand new second story addition.
*Thanks for a number of well thought out and imaginative responses. Would never have thought of running new floor joists perpendicular. Think the structural engineer suggestion is no-brainer. Will be next step in the process. Both sets of existing stairs so far off code to be real pain in the ----, new stairways one of the added benefits of the potential addition. Originally wanted to stay with 2x8's to minimize increase, but I think now using 2x10 TJI's will save enough time getting roof back on that worth the cost and height increase. So focused that I never would have thought of replacing rim joist rather than shimming up. Thanks again to all ...
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*Do yourself a favor and use the 2x10's for floor joists. Like someone mentioned earlier, block up the front, center and back walls with one 2x4. I am not a big fan of I joists and never will be. I have used 16" tall x 48' long I joists and I am not impressed at all with how they are built. Open webbed trusses seem to me like they are much better built. Another consideration for your floor joists/ceiling joists. Put a plate down over the 2x6 joists and then stack the 2x8 joists right over the 2x6's. Tie them together in the middle with a plywood gusset. You could take the existing sag out of the ceiling by propping the ceiling with some 2x4's wedged against a 2x on the ceiling. Go easy, if there is a big sag maybe take half of it out.We just did this with a truss roof. We left the 2x4 ceiling members in place and tied them to the new joist. It worked great.