I'm putting together plans for 12'x16' shed. I'm planning on using an end supported 16' long ridge beam to avoid using rafter ties. Will a
16' 2x12 be sufficient, or should I nail together four overlapped 8' 2x12s? Can I get away with using 2x10s?
If your rafters are 2x4s, then use a 2x6 for the ridge. If your rafters are
2x6s, use a 2x8.I think anything larger is overkill for a shed.
Good luck.
What are you using for rafter material? Pitch of the roof? Will there be overhang? If so, will the rafter act as support of the overhang (no nailed on tails)? Composition shingles, rolled roofing, clay tiles, cedar shingles? Is there a snow load to consider?
Whatever you use for the ridge beam and rafters to support the roof you need to consider that without the rafter ties there will be a lateral force at the top of the walls tending to push out the side walls. To prevent this you need to have a well nailed roof sheathing to act as a diaphragm that takes these forces to the 12' end walls. The diaphragm will also be used for lateral wind forces/seismic depending on the area you are located.
CID...
Not if the ridge beam is supported. That's the whole idea. Supported ridge beam **or** collar ties.
Is there a reason for not using ANY ties? Even 2 or 3 would help a great deal.
It really depends on the loads you expect. For instance, you'll need a bigger beam if you could have a few feet of snow on the roof in the winter.
But, as a rough ballpark, let's assume 40 pounds per square foot (live and dead loads). 12' x 16' x 40 lbs = 7680 pounds total distributed load on the roof.
Half of that is carried by the beam, and the other half is carried by the exterior walls (half of each span rests on the beam). So the beam needs to support at least 3840 pounds.
The size of the beam then depends on the type of wood you use, and the span, but I'll assume a middle of the road 1200 pound fiber stress for the wood type (Doug-fir, or Southern Yellow Pine, should both be adequate for this rating).
According to an old span chart I have, a solid 4x12 only supports 3809 pounds for a 16' span, so I'd probably go with a 6x12 which would support
5781 pounds. If you build that out of three 16 foot 2x12's, the load capacity drops to 5127, still plenty for your needs.Of course, with a 16 foot building, your supports will probably be within that dimension, so the actual "free span" will probably be less, so you might be able to get away with the 4x12 for a shed, especially if you don't expect heavy snow loads.
If you can find a 4x14 beam, that would support 5288 pounds over the 16 foot span. Of course, it's fairly easy to lift 16 foot 2x12's. A 4x14 beam would be a bit more work... :)
Of course, there are other ways to create the beam, laminated beams, LVL lumber, steel beams, plywood box beams, flitch beams, etc. But for cost and simplicity, I'd probably opt for the built-up 6x12 beam.
Remember to tie everything together with strapping, post brackets, etc. so the shed doesn't fall down in strong winds or earthquakes. You'll also have some concentrated point loads under the posts at each end of the beam, so plan your foundation footings accordingly.
Anthony
That's what I was afraid of. That's one hell of a beam for a shed. But the numbers make sense. I think I'd be better off with rafter ties, anyway. At least I have experience with those.
Now, with 2x6 rafters and rafter ties, what size ridge is recommended? Oh, and I think the ground snow load here is around 30 psf. And I'll be using asphalt shingles. No overhang.
Thanks to everyone for your help.
16 feet is one hell of a span for a shed ; )
One ply of 2-by deep enough to abut the rafters.
By "rafter ties" are you meaning collar ties? If you have collar ties, no structural ridge is required. The ridge board is really just there to make it easier to build. I'd consider buying roof trusses or even making roof trusses.
You don't need a ridge at all from a load bearing perspective if you have ties on every rafter pair. The ridge is just a construction convenience at that point. A 1x6 would work fine.
Matt
A 12x16' roof can hold a lot of snow! :)
When you start calculating the loads a structure can support, it's really kind of amazing how much weight they hold without collapsing.
Actually, if you make the end walls with 2x6's, that would put your free span awefully close to a 15 foot span. A couple of 16 foot 2x12's should carry that load OK if you really want the open roof structure.
It's easier, and stronger. That's what I opted for too... :)
You can also gain a little overhead storage if you plan the ceiling joists accordingly.
If you need more headroom, just make the sidewalls taller.
Generally, the ridge is one size larger than the rafters. For 2x6 rafters that would be a 2x8 ridge board. Of course, if the roof slope is really steep, you might need a 2x10. Basically, the ridge should be at least as tall as the vertical cut at the top of the rafter.
On my garage (24'x28') I used 2x6 rafters, a 2x8 ridge board, and 2x12 ceiling joists to clearspan the 24' and provide a usable attic.
However, when I built my shed (8'x12'), I made some simple site built roof trusses. No ridge board needed. You could always order trusses from a truss company too.
For a 12' deep shed, I'd use 2x4 rafters, a 2x6 ridge, and a 2x6 ceiling joist, spaced at 24" OC. If you want to increase the strength, decrease the spacing to 16" OC and/or use 2x6 rafters with a 2x8 ceiling joist.
If you think you might insulate the shed someday, you might want to think ahead and use rafters or joists that will provide space for insulation. For example, I installed 2x12 rafters on the cathedral ceilings of our house, even though I only needed 2x8's for the span. That let me install R30 and still leave airspace under the sheathing.
One final option, you could put a "shed" roof on your shed. Basically, make the front wall taller than the back wall, and set your rafters directly on the walls spanning front to back. Strong, easy to build, inexpensive, and provides high ceilings. You could even install windows in the upper part of the front wall to let in additional light. The biggest downside.... It looks like a shed (or a chicken coop, depending on your viewpoint. :) ).
Anthony
Collar ties in my neck of the woods mean a horizontal tie down from a forth to a third of the rafter. Their primary use is to keep the two sides of roof from blowing apart in a high wind. They will not hold the sides of the building from spreading, if anything make it spread worse as it goes down, ie sags, it spreads the walls out. Now a tie that is equal to a ceiling joist, or in other words down on the plate will hold it forever with absolutely no sag or spread. One every 4 foot is plenty. Then the ridge does nothing. a 1 x 6 just to alien the tops is plenty or you can even forget that and it will hold just as good on that little of roof. a larger roof and I like collar ties down about a third,every 4 feet spaced over the every four foot joists. A 1 x
4 tie would be plenty, because it's holding it together not up, ie it's use would be in tension holding down the reverse side of the wind direction lifting.
Thanks all! You've helped a ton.
I'll definitely be going with the rafter ties now that I think about it. I don't really need vaulted ceilings in my woodshop and I could use some extra storage space above me.
I built a 12' x 16' shed that turned out to be a pool cabana. The 16' foot length has an 8" x 2" ridge beam. The ceiling is a cathedral ceiling with insulation and wood paneling and there are no collar ties. The shed is about 18 years old now and there is no noticeable pushing out of the side walls. The paneling is painted and there are no visible signs of paint cracking on the edges. I live in NY which does get snow falls of 12" to 16" on occasion.
Glen, I don't think he understood what you said. Ceiling joists, yes. Rafter ties, no.
I do think you could do well on your original plan with a manufactured beam. I don't have time to do the numbers, but look at LVL or I joist.
You still have a lateral component reaction at the top of the wall from the rafters if there is no other way to resist it by a tie or diaphragm. Otherwise you have two way bending on the ridge beam to take the lateral component. Then the wall displacement is depending on the ridge beam deflection due to the lateral deflection. You only have three reactions for a determinate condition.
CID...
Ceiling joists and collar ties are the same except the joists have two functions ceiling framing and collar ties for lateral components.
CID...
Is the wood paneling nailed to all rafters? Were the rafters what you call a cathedral ceiling? What thickness is the wood panels? Are they considered as roof sheathing nailed to each rafter? Did you have other sheathing on top of the rafters? I think that you had a internal/external diaphragm on the rafters which resisted the lateral component on the walls and distributed it to the end walls. What were the end walls framed with if plywood they act as shear walls to resist the end reaction of the diaphragm. You had the resistance somewhere if no wall deflection was noticed.
CID...
Not so according to any framing that I have been taught over the last 40 years.
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