I am adding a 25 x 37 foot one-floor addition with a garage underneath. The addition will have a cathedral ceiling without structural collar ties or ceiling joists. The roof will have a 12 on 12 pitch. To accomplish this I am planning to use a W18x50 I beam for the ridge. This beam has a 7″ wide flange and is 18″ high. The rafters will be 2 x 12’s 16″ o.c.
How would you recommend that I tie the rafters to the ridge I beam?
And, how would you design the insulation, venting, and vapor bararier configurations so that we would have a R-30 ceiling with the least moisture problems with heating and airconditioning? The roof will be a standing seam metal roof. Would a cold roof make sense? I live in S.E. Pennsylvainia.
Thanks. Bob McCabe
Replies
bob.... did you consider switching to a scissors truss with an energy heel..
say 12/12 pitch top over a 9/12 pitch ceiling , with a 12" high energy heel... and any kind of overhang you want ..
cost less, less labor, more insulation... only draw back is you can't get 12 pitch windows in the gable end ... but that is overdone anyways
Mike Smith Rhode Island : Design / Build / Repair / Restore
Scissor trusses would work well, as Mike sugggested. They might have to be shipped in 2 pieces due to their height - Depends on the fabricator and the equipment they have.
You could probably put a beveled wood plate on top of the beam, and nail the rafters to that. But you would probably need some straps of some sort over the ridge to restrain the rafters.
Or you might be able to put some blocking in the sides of the I-beam and hang the rafters from that.
You're gonna need a heck of a crane to get that beam in place.
For ventilation, you could go with 11 7/8" or 14" I-joists instead of 2X12s. That would give you more depth to work with, so you'd have more room for insulation and ventilation.
Without geometry, life is pointless.
Don't know what your budget is but how about using using SIP panels for the roof structure? Depending on the sheathing you could then probably use rafters on 8'+ centers and instead of trying to make them part of the roof insulation leave them exposed and a design element. A nice hammerbeam design would look real good. I believe there is a couple of companys that will make you up timber frame type rafters that you can set into framed walls. At least one of them advertises regularly in FHB.
Robert
Let's see...37 ft long x 50 lb/lf = 1,850 lbs for the steel. Don't drop it on your toes.
Did similar with a pair of 6x18 timberstrand as the ridge beam and had a 16 /12 pitch with 11-7/8 I joists 24"oc. Collar ties though. (Made them from oak and filled the gap between and it looks real pretty)
As for the top jointing to the beam. 12/12 is a simpson part. (you can buy them there) but in my case there was no fitting for 16/12. I had to rip a fir 4x4 to make an angled reciever that ran the top length of the beam. I spiked the angled fir reciever to the top of the ridge beam and then the bottom of the I joist to the 4x4. The I joists were strapped over the top with 1" by 12" metal (another simpson product.) As additional precaution I used an OSB plate glued and nailed to the top of the webs of the mating I joists at the peak.
Why the I joist for the ridge beam? The timber strand can't be that much more expensive and you can stain it to make it part of the interior woodwork? Otherwise what do you do drywall around it? I imagine the DW joints will crack the 3rd day after paint. I've got some nifty pictures of the lift and the hand assembly. As soon as I get my regular computer hooked up I'll post them.
Oops I didn't realize the shorthand for a steel main beam. I guess there are no interior posts in your structure. I've got one in the middle of my 36 foot span and I know the reason to avoid that one. Still, What are you making? a dance floor?
Edited 1/15/2003 5:42:35 PM ET by Booch
Hi Bob,
I'm going to be like everybody else and try to disuade you from using it. I'm not an engineer but from comparing to what I've done in the past with steel, it sounds like it might be a bit of overkill, structurally. If I'm right there, you might be going with it because you happen to have one laying around cheap. Or maaybe your engineer is more used to steel for longer spans and wasn't creative enough to think wood
But your labor will kill any savings over using wood. You'll have to wrap it to trim. The only way I can see to manage the joins and the insulation/ventilation package would be to run the rafters over the top of it and not try butting to with hangers.
The steel will want to be cold at the very location where your warm moist air normally congregates to deposit those wet little driplets. that is a recipe' for disaster, in terms of satisfying customers.
Good luck. This will be a very complicated job, more so than I would want to do on internet advice only. Not that you haven't chosen a good place to get it...
;)
Simpson does make a couple of different strap systems for this.
Excellence is its own reward!
Sounds like you are committed to the steel ridge, so here goes. Have your fabricator punch the flanges so you can lagscrew with 3/8" x 1-1/2"s some 2x8s onto top and bottom flanges, then treat it like a big wood member. You can wrap sides and bottom with gypboard or finished lumber. Switch your joists from sawn lumber to 11-1/4" I-joists (lightest duty you can buy, they will still be structurally better than 2x12s) and when everything is erected (use Simpson clips where the joists bear on the top 2x8 nailer) rip some 1" rigid foam board to about 15-5/8" widths, and press fit it between joists, up against the bottom sides of the top flanges. Spot tack the foamboard with some finish nails driven on an angle, up into the joist top chords. You have now created a nice 1-1/2" continuous air chute between top of foamboard and roof sheathing so the soffits will vent right up to the ridge. Pack out the rest of the joist cavities with fiberglass. I'll leave out the calculation for insulation value, but I'll bet you will get what you need. You must have two brothers, one in the steel fab biz and the other one doing something with a crane service.
Gene Davis has a good game plan. One little addition, There is a R38 24x10" cathedral ceiling fiberglass insulation batt you can buy. It is an industrial product that is commonly available only at the insulation distributor. The big box stores only look at you with the blank stare if you mention it there.
I speculate you have an indoor tennis court or a gymnasium underway. It sounds cavernous.
Bob?... He reads but he doesn't reply..
Edited 1/16/2003 10:18:30 AM ET by Booch