Another collar tie problem, and this one relates to a recently posted thread that I’ve lost. I need to use collar ties as ceiling joists. Here’s the situation:
1. The house is story-and-a-half with everything 16in OC.
2. I’m calling it story-and-a-half because the second floor includes a knee wall which is an extension of the main walls. In other words, it is balloon framed with the second floor joists (2X8) resting on ledgers let into studs which are 2X6X12.
3. 2X8 rafters rest on the top plate, a doubled 2X6. The roof pitch is 12/12.
4. Collar ties are 2X6X14 and are nailed to the rafters. They are 7-6 above the floor and 4-6 above the top plate (=rafter bottom). I would like these to serve as joists to support DW (under) and a PW floor (over).
5. But the span is too great for the ties to serve as joists. SO – I plan to double them, edge-to-edge for a total depth of 12in nominal. I’m hoping this will make them stiff enough for DW and to serve as a floor for access above and light storage.
I’d appreciate advice. (Hope I’ve included enough details. The last poster was demolished for not providing sufficient information.)
Replies
Some engineer will be along promptly with the science, but I don't like the sound or setting one joist, or collar tie, on top of the other, then thinking you've significantly increased the load it can bear.
The thing is, when horizontal framing members fail, the center of them rolls. That's what the cross bridging and midspan blocking we used to put in joist bays was for. How are you going to prevent that rolloing action? If you can't deal with that, I don't see how you're really gaining much span strength.
You might be better served by nailing a second collar tie onto the opposite side of each rafter, then relying on the drywall ceiling and plywood floor to help with this rolling question. You'd save some headroom up there, too.
Right, doubling the members one on top of the other would be not much better than putting them side by side. That's because there's nothing but friction to resist horizontal shear where they meet. To gain an advantage from the depth, it has to be one solid piece. Double the width, double the load. Double the depth, four times the load.
Connection to the rafters is another big issue, just nailing to the sides of them is very unlikely to be good enough. Putting a load on top of the collar ties increases the spreading force they're supposed to resist, and puts a bending load on the rafters, so this isn't so simple. Sorry to say again what is said here so often, but the right thing to do is measure and draw it carefully, and take the papers to an engineer.
-- J.S.
It sounds like they planned the ceiling , In your case. Mine was not like that. My second floor ceiling does not rest on anything. In fact the ceiling isn't level. Because the nailed them up there, without stringing a line, No doubt.
I kid you not this house is not built by any standard framing method. Since you metion it, I wonder if mine is Balloon framed as well . But my Upstairs wall is only 4'high. i can only hope the first floor is supported properly. My thread, On this topic was titled " Roof framing Quandery"
That must be a long rafter you have, with a 14' collar tie. How high is the overhang from the ground?
I contend that a 14' floor joist may serve to hold the rafters, and walls together but if it is 14' and it is resting on the wall. Its not really a collar tie. Its a ceiling joist. It's been a while but I think a collar tie needs to be 2/3rds up. which would be about right in your case. It will carry a load. How much? I can't tell you with out looking it up . A 14 foot 2x6 is the limit for a 2 x 6 ceiling joist. I know that from looking it up. Your wall studs must be about 16' too . I am trying to wrap my mind around the geometry on this one.
That means you have 7 foot hieght in the attic
28 feet on the 2nd floor if all your ceilings are at least 7 foot
42 0n the ground I guestimate,By your 12:12 pitch. Again thats with seven foot ceilings. Thats kinda short.
So you have 2 balloon framed walls going up thru the center of the house 14 feet apart. and 2 shorter walls on the 1st floor 7 feet to either side. thats a narrow room.
How big is the over hang? Your rafters must be about 18' If your collar ties rest on the "knee Wall" Which by the way can't be a knee wall if it's 7 foot high. you got nothing to worry about.
Edited 5/8/2003 6:25:42 PM ET by MuleSkinner
Edited 5/8/2003 6:28:23 PM ET by MuleSkinner
Edited 5/8/2003 6:29:31 PM ET by MuleSkinner
Edited 5/8/2003 6:31:24 PM ET by MuleSkinner
I guess i should spellcheck before I postWhere there's A wheel there's a way, got any wheels?
Thanks to MuleSkinner and all who replied. Everyone seems skeptical about my plan of sistering collar ties on edge. I have to say I don't understand why sistering them face to face (but on opposite sides of the rafter)is any better. Apart from the headroom issue, stiffness is a function of the square of the depth, so it still seems to me I would get more stiffness per board foot of wood from edge-to-edge. Plus I could pack in more insulation. Adding a 2X10 "collar tie" at the same height of the original might be the way to go -- thanks for this suggestion.
The rafters are long -- close to 20 feet. The knee wall I referred to is the continuation of the side wall of the house, not in the center of the house. The headroom is 7-6. (I forgot why I didn't make it 8-0. Probably something I read about second stories.) I forgot to mention that about two-thirds of this potential attic floor would be supported by a wall, so only one-third would be floppy. I have thought about installing a beam to support this third, but it would end right at a window and would (in turn) have to be supported by a window header. That doesn't seem right.
> Apart from the headroom issue, stiffness is a function of the square of the depth, so it still seems to me I would get more stiffness per board foot of wood from edge-to-edge
It's a function of the square of the depth for a single piece. Stack two or more pieces up, and you don't get the square of the total. For instance, lay a piece of 4x10 across a couple saw horses on edge, and next to it stack up seven 2x4's on the flat. The stack of 2x4's will have an inch more depth, but it won't be as stiff because the pieces can slide against each other just a tiny amount.
Of course if we were to stick the 2x4's together really well, we'd re-invent the glu-lam. The potential issue with that idea is how much long-term faith we want to put in the field-made glue joint.
-- J.S.
Jim's right, you won't get much help by doing it the way you suggested, but if you are basically looking at it from the standpoint of providing a less bouncy attic, you could just use 2x10's or 12's as he said on the opposite side of the rafters that the collar ties are on now. Or, you could run a beam across the existing 2x6's and support that with posts on the end walls, or within the finished end wall itself, depending on what they consist of.
Or, if you're mainly just using it for storage, don't do anything, and don't drywall the ceiling below it. Use something else, like t&g boards, and let the floor be a little springy.
Would be nice if those 2x6's happened to be select structural d-fir, but I wouldn't guess they are.
The reason I suggest this is that I just bid on an addition to a manufactured home that had existing 2x6! main floor joists, with the largest span being about 12 feet. I don't remember if they were on 12 or 16" centers but the house has been there for 30 years at least. But obviously they will flex and you are asking for trouble if you put a lot of weight up there and drywall the ceiling underneath.