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?
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.
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.
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
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.
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.
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.
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.
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.
Keep the whole world singing . . . .
Dan replied to:
"Ceiling joists and collar ties are the same except the joists have two
functions ceiling framing and collar ties for lateral components.
The link you provided says nothing otherwise.
2/3 the way up or 1/3 down from the ridge, those are collar ties. A collar
tie cannot support a ceiling in that location as you indicate that weblink
indicated. A collar tie is usually 1X lumber. Fun attaching sheetrock to
that for ceiling even if possible to act as a ceiling joist.
A collar tie still allows flex where the rafter is nailed to the second top
plate (top plate is attached directly to the wall studs). A ceiling joist
prevents that flex from occurring.
Go Dan, go.
Collar ties can be any thickness you need they don't have to be 1X as you
are indicating and providing the correct connections. You are the designer
and you can size any structural member to the size you think is necessary as
long as you know the functionality. True, if 1/3rd of the vertical
distance it is then basically a collar tie. When the members are considered
ceiling joists that are bearing on the top of stud walls and also attached
to the rafters then they are doing double service.
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