Isolate new 2nd floor joists from existing 1st floor ceiling?
I plan to add a 2nd floor using 2×8 joists for 13′ span. The existing attic joists supporting the 1st floor ceiling are 2×6. Should I tie the new joists to the existing ones and fasten the ceiling to the new joists, or isolate the new joists from the ceiling ? JC in Va
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Replies
I'm guessing you want to save the existing ceiling. I don't see any advantage to tying the former attic joists to the 2x8s; it strikes me as it would just increase the likelihood of cracking the gyprock or making the nailheads pop. No matter how well kiln-dried your new framing lumber is it's not likely it will be as dry as the old attic joists, so there will probably be some uneven expansion going on for a while if you tie the two systems together.
In any event according to my tables, for a 13-foot span you'll need to set the new 2x8s on 12" centers and use bridging. Those requirements alone would complicate tying the two joist systems together.
Dinosaur
How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....
Hey thanks. The ceiling's in good shape and I plan to keep it. And I can use deeper new joists too.
I once heard a suggestion to use plywood shims under new joists to place them a half inch or so off the ceiling to avoid the problems you mentioned, cracking and nail-popping. Whaddya think?
I have always used ply. shims. It works well.
Shimming the new joists up would be fine. Plywood as shims should be good, too.
If you aren't married to using 2x8 joists for other reasons, go for 2x10s; you'll be able to set them at 24" centers, which means half as many joists for only 20% more BF of lumber per joist.
The problem you're going to have is figuring out how to install the bridging. You need to install either bridging or strapping on floor joist systems (or both, if you want to eek out another few inches of span), and with those old ceiling joists in the way, that's going to be complicated, to say the least. It really would be a lot easier and more straightforward if you would just demo the old ceiling. The more I think about it, the more that looks like the best way to go.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Sorry for the delayed reply. And thanks about the joist size. I hadn't looked at the tables in a while. I hadn't considered the instability of the new wood. Makes sense.
Good; glad it helped.
Don't ignore Diesel and Framer's advice either; if you can add that extra riser to your stairs that's about the simplest way anyone's proposed to build it.
If you can't afford the extra height, you're faced with two choices: (a) demo'ing the existing ceiling or (b) futzing around installing bridging around and between two sets of dissimilar joists....
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
What's the spacing on the current ceiling joists?
What's the span of the current 2x6's?
What type of ceiling is there now? i.e. drywall , plaster and lath or plaster and wire lath,?
How old is the building?
I calculate a 2x10 @ 16" O.C. for a 40#live load(10# dead)
Geoff
Thanks Geoff. I hadn't looked at the tables in a while, so yeah, 2X8's are too small. Existing joists are 16" oc. Span is 13'. Built in 1942. Gyprock ceiling.
Why not go on top of the ceiling joist? This way it makes it a lot easier for plumbing, electrical, hvac.
Your already going up a bigger size with the new joists and will be adding a new set of stairs with more risers.
Years ago we always sistered the new joists next to the existing ceiling joists and I always sat them on 2x4 blocks in between the ceiling joists. Doing it like that was always a pain because the electricians had to get there before and disconnect all the wires and then as we were insatlling the new joists they had to be right there drilling holes into the new joists and connecting the lights back up for the first floor.
Now I just cut the existing ceiling joists back 1-1/2" and put a solid box around the perimeter and then add a 2x6 or 2x8 plate flat on top of that and nail the new joists on top. It makes it a lot easier for everyone.
If you are limited to space and can't add more than one riser to your steps, then you have to sister the joists with the new joists and only add one riser.
Joe Carola
Edited 5/28/2007 12:12 pm ET by Framer
Winner Winner, Chicken Dinner!
I do exactly the same as you Joe. There really is no better method assuming the stairs can be made to work (and I've yet to have that problem). It's a tiny bit more lumber, but WAAAAY less work for the framer, the plumbers and the electricians. Makes for a quieter second floor too. I charge more than double the labor for the floor framing part of the job if I have to sister. And that still may not be enough.View Image
Brian,I'm glad someone likes it. I guess jcinva doesn't like the idea though.The last two AAL's I framed we use double plates on the perimeter and center bearing wall. If your not changing the stairs this is the best way to do it. Sometimes the house already had 2x10 joists and we cantilever the front and back or sometimes just the front or the back and we have to use 2x10's and sister them because the stairs aren't getting touched.There are a million different scenarios though.Staying out of trouble?................;-)Joe Carola
Too busy to get in trouble right now Joe... just the way I like it. Been trying to steer the ship away from additions/remodels and towards new work. Bigger crew, more volume.... seems to be about the same money with less headaches and less gambling. But it's a beast to keep feeding this monster with work. Making decent scratch and enjoying myself at work again though.... and that goes a long way.
You?View Image
Glad to hear you're happy and busy. You guys gonna make it to the Fest?
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
I think so man. Still working out the schedule. I'm taking the second week off in July too so I just gotta make sure I've got all my bases covered.View Image
Do it soon, bro. It wouldn't be the same without ya. Besides, I wanna see the neighbours' faces when they see that gorgeous truck of yours parked in my driveway, LOL.
Click the fest logo below my sig and check out the Northfest website. All the info is there that's available right now. I'm gonna post a list of confirmed attendees there in the next day or two.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Brian,Additions, Additions, and Additions............ So many people are adding on, so that keeps me going. More money in additions around here anyway. I'll frame a house for certain people because the money is right when they come up. Keep enjoying yourself brother because you deserve it and work hard and shouldn't have it any other way.Joe Carola
I would probably sister with glue and screws to the existing ceiling joists, given the parameeters you describe.
Here is why - 2x8 @16" )C is not stroang enought for a 13' span.
If that is what you intend to use for some reason, then adding the strength of the existing 2x6s wiull just get you by
Again too, it will be hard to do blocking or bridging if you keep separate.
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I would probably sister with glue and screws to the existing ceiling joists, given the parameeters you describe.
That's what I was thinking as well. Might as well take advantage of the existing joists.
Beer was created so carpenters wouldn't rule the world.
I wouldn't go near the ceiling joist. In fact, put a 2x4 block on the plate between each ceiling joist, then center your floor joist between them making it like a staggered joist floor.
This would eliminate alot of sound issues and you could block with solid 2x4 between the joists.
That has a lot of good benefit, butthen he has to use 2x10 at least instead of 2x8, and he may need one more riser in the staircase which can change a lot in the entire floorplan layout
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Ok! That makes sense. But if the house is anything like mine was, 2x6 over 13' were sagging about 65mm or 2.5 inches in the middle of the room.
How much strength, (and can you calculate it), is added by attaching 2x8?
Do you jack up the center of the room?
And what if he's puttin a pingpong table up there?
Keep going and you can think of another dozen extremem situations outside the norm.. How about a pool hall and dance floor?
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What if you like to dance on your pooltable?
I must have missed the memo on his intentions.
I would think isolating the old ceiling from the new floor joists would be an advantage for sound transmission as well. What about framing a new plate around the perimeter on top of the ceiling joists then setting new floor joists as you normally would. this gives you good nailing at the ends. I would probably go with a matching layout for ease of running plumbing and electric and to access the old stuff in the old ceiling.
"it aint the work I mind,
It's the feeling of falling further behind."
Bozini Latini
Thanks
Hello jc
I like to block in between the existing cj's with 2x's and not atttach them to the existing, that way you don't have to disturb the existing ceiling ( or the sag ), you can isolate from the direct transfer of sound, and sometimes the existing electrical will work without modification. The wire will be able to go under the new joists. ( sometimes, if you are lucky ) This also keeps overall height down. You will, as others have pointed out, have to consider the extra riser in your stairs.
Have fun! RZ