Okay, our architect convinced us to go from 28 by 30 to 30 by 36 for our garage/mother in law apartment. Two things have sprung up that I am concerned about.
1. I will be using BCI joists that will span 22 feet from an internal bearing wall to the outside wall. I have been told we will need to ballon frame to help reduce rafter spread. (The building is a story and a half cape with a 10/12 pitched roof. I intend to let in a 1 by 6 ledger to catch the floor joists but can I do this with BCIs? (Is balloon framing necessary?)
2. Now that I am spanning 30 feet with the rafters I come up with them being 20 feet long. Am I going to find 20 foot 2 by 12s? And will collar ties every 4 feet be sufficient to prevent rafter spread?
Lots to ponder.
Replies
I don't think a single 1x6 will give you enough bearing surface for the ends of the I-joists. Whoever designs the floor system will be able to tell you for sure. You may be able to go to a wider flanged joist to give you the extra area to spread the load over. Failing that, you can probably nail a 2x6 to the back of the ledger board and toe nail it into the studs to create extra bearing surface.
As far as the roof system goes, your lumber yard will be able to order 20' material no problem. As far as the collar ties go, there are lots of things to consider. First of all is roof load. Is there snow loading in your area? On a steep pitched roof like that, wind loading may also have an impact. Knowing the load will enable you to determine how much lateral force your rafter tails are applying to the top of your walls. Once you know this you'll be able to work out what you need for collar ties.
A note about collar ties: if they're above the lower third of the rafter span they do nothing for lateral restraint. Hopefully this won't be a head room issue. If it is, you'll have to go to a ridge beam to set the rafters on. Either way you would be prudent to consult an engineer.
boz.. i think micster is right... although the balloon framing will HELP , it is still just like a cantilever.. it may not have enough lateral strength to resist the outward push of the rafters.. an engineering solution in the form of gussets or some such may help.. but ultimately the structural ridge may be the easiest solution..
in any case, a good review by a structural engineer.. or some further calculations by your architect may be in orderMike Smith Rhode Island : Design / Build / Repair / Restore
You mention that collar ties do "nothing" if placed higher than 1/3 up. If the building also has kneewalls, is this still true?
Yes, it's still true.
In the balloon framed knee wall scenario, some of the lateral force of the rafters will be absorbed by the floor joists. This tension on the floor joists also translates into a bending moment on the studs which should be considered.
Once you raise the collar ties above the lower third of the rafter they work to counteract the bending stress on the rafters produced by the roof load. They are acting in compression, not tension. While I studied engineering for a couple of years I didn't like it enough to go the distance. The opinions I'm expressing are those of a fairly well-informed amateur. There has been some discussion about collar ties by much better qualified people than I in FHB over the last year or so.
Yes, there is lots to ponder.
You say you have an architect. The pondering I am doing is in wondering why he cannot answer these questions with the drawings sitting in front of him. It's pretty hard for me to analyse a structure with ten percent of the information needed.
A cape often does need ballon frame but in your case, this is probably to keep the roof laod from pushing out on the walls, kbneewalls would deflect worse than taller balloon frames.Another way to suipport that roof laod is with a ridge beam engineered for it or with trusses.
Yes, you can get 2x12x20 here. I don't know about there. But you can always do it with I-Joist rafters too.
It's hard for me to picture this internal support wall at 22' out of 30. or what that does to things.
Excellence is its own reward!
Thanks for your feedback. Yes I too wonder about why our architect didn't give us more information. Part of the package was going to be an engineering analysis which never occurred. We have quit working with him (but that is part of a larger story). The architect also shows the collar ties above the one-third height you mention. I have for years been following the collar tie discussion in FHB and elsewhere and remain somewhat confused and skeptical about what is printed and practiced in the field.
The 22 out of 30 span is a function of a bearing wall at 8 feet that runs the length of the first floor. The 8 feet is an entry/stair/ storage area on one side.
I will seek out the additional analysis before I proceed.
BOZ
Tough question and a lot of good comments. I would add a 2x6 on the inside of the let-in 1x6 for more bearing if you don't mind this sticking into the garage. It will be a lot easier than toe nailing behind the ledger. Long lumber (2x6 and wider) is readily available although few yards will stock all lengths. I would use 2x10's for your span and build down for more insulation.
If you need a structural ridge, you can shorten the length of this if there are walls across the width of the building. If you frame with 2x6 walls this ridge should not be needed.
Engineering analysis on this really is needed because as described, it uses several differing methods of weaving the support together.
The balloon wall ( which would resist the bending outward moire if 2x6)
The structural ridge possibility
The collar ties/rafter ties
Since these each can take up a portion of the load. it is hard to identify how muyc eac would do and certify the design. It's not a straight for ward thing depending on any one of these elements.
Collar ties are placed in the upper third of the roof frame to resist upward lift of wind at connection of rafter to ridge.
Rafter ties is what is being talked about here. They are placed in the lowere third of the assembly to resist spread from roof loads..
Excellence is its own reward!
Something strikes me as odd here. Several people have talked about collar ties in the lower third of the rafter. To me it is intuitive that this would provide the most structural strenght. However, as a practical matter if livingspace is desired under the rafters it would be impossible to place the ties that low. Therefore, the kneewall must be the key to structural strength in the livingspace senerio. (Someone already comments as much).
Any other comments?
A kneewall can only be counted on for structural loading if the joists of the floor system the kneewalls rest on are designed and intended to carry the additional load of that portion of the roof that transfers weight to them. Not likely..
Excellence is its own reward!
Yes, I understand the floor loading issue. However, this answer still does not explain how you can achieve the resistence to the rafter spread forces if you want livingspace under these rafters which requires the collar ties to be higher than the preferable 1/3. ????
You either place that load on a ridge beam or let the balloon framing resist the outward thrust, or a combination of all the above..
Excellence is its own reward!
Thanks for all of the input! I have gone back to the plans. Acouple of things can be added now.
1. At one third of rafter length I get 5 feet of height. This with the knee wall (balloon framed) gives me an 8 foot height. The ceiling can go above the rafter ties. The architect recommended ties every four feet.
2. I don't want to load the knee walls as the BCI are not being designed to carry anything from the roof.
3. A ridge beam is out of the question for the same reason.
At this point it seems that the combination of the the rafter ties and balloon framing is going to give me the structural integretiy needed.
Boz
Probably.
Now, make sure that the floor joists are well secured to the balloon walls. That connection is what prevents to outward thrust on the rafters from pushing the walls out.
I was also a little leery of the 1x6 ledger for that. One that I did had a 2x12 let into the wall by 3/4" with 3/4" protruding. It was srewed and glued into the studs in the same space as the joists and top hanging metal hangers used to attach the floor joists..
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Why not go with a structural ridge?. This load wouldn't be placed on the floor joist, it would go to the exterior walls for support. If possible I would try to go with a intermediate support and bring the load down to a support pad.
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