Long Ridge Beam from Shorter 2×12
Hi experts.
I want to make a simple ridge pole with rafters greenhouse roof with clear plastic roof sheets. The ridge is 24 feet long, 2×8 rafter(s) – 8.5 ft long (16′ building width).
Since I was unable to get a single piece ridge pole, I wonder if a sandwich of 2 2x12x16, augmented by 2x12x10 on both ends, all bolted together with 1/2″ bolts, would do without installing a support column in the middle. Rafters are 2 ft apart, with 2×4 purlins, bought bolts for 2 rows 4 ft apart, but also some 3/8 ones to add if necessary.
So the interest is
A.whether such a beam – true 3×11 – is satisfactory at all without a center post, given that I haven’t seen over 2 ft of snow, and
BTW, 2×8 for rafters is only because that lumber is straighter than 2×6, but if too heavy I can switch. There will be ties between rafters on both sides.
Thank you!
Replies
https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction
with rafters on both sides, and proper ties you should not need a ridge beam, a single thickness board will do the trick.
Hi there. Thank you for the reference, I am studying it.
In the meantime, does it make a difference in your suggestion that the eave-side walls are under 5 feet tall, and subsequently not tied to the gable-side by anything other than the roof (so I can walk in)? The eave side is actually 3 6x6 beams buried 3 feet, and connected by a 2x10x12 plate set vertically.
I really took the idea from a metal building and some experience with lumber projects.
So are you saying that the 24 foot long side walls are made up of 6x6 8 foot wood posts/columns set 8 feet apart, and joined at the top by a pair of 2x10 horizontal elements that tie the columns together? (with a second such structure parallel to and 16 feet away)
Are the horizontal pair of 2x10s bolted to the outside of the posts, or inset into them?
you could use rafters joined by a ridge board, with this situation. You would need rafter ties of proper size to keep the roof from pushing the side walls away from each other, but this is explained in the code book. the useful span (you need 8 feet) listed in the table would have to be adjusted by the proper factor if you put rafter ties closer to the peak than ceiling joists which would sit on the walls.
if your wall is at 5 feet and you would be ok with rafter ties at 7 feet, the adjustment would depend on the pitch of the roof.
a steep 12/12 roof would have a ridge height of 8 feet over the top of the wall.
ties placed 2 feet higher than the wall would be 2/8 or 1/4. this would call for multiplying the span capacities in the table by 0.76 several species/grades of rafters would do the trick.
(of course, this would call for longer rafters)
you could play with the roof slope and height of the rafter ties to come up with structurally sound alternatives to choose from.
Thank you, Mike!!!!!
Really learned how much there is to it, and I don't know. Lots to learn, but very, very thankful for pointing me in the right direction. Will spend the winter learning more about carpentry and structural engineering.
From what I understand, the need for a beam depends on how you built the roof:
https://www.finehomebuilding.com/1992/10/01/framing-a-cathedral-ceiling
Superb! This is a great article, making my "design" valid. I frankly chose the ridge beam construction because of simplicity, and having read one of the latest FH articles.
The ridge beam manufacture is then the remaining obstacle.
Thank you!
So, metal (steel) flitch plates, OSB sandwich, shorter metal ties... Is there practical advice for a 26-foot beam, having done so?
I don't see a problem with your concept of bolting pieces together. But I would run it by a
structural engineer. Mine has books with tables and he does not calcs and you get your answer. He bills me for the hour or hour and a half and that's it. If it needs a wet stamp for the city then its a little more but - bottom line the price is worth the piece of mind.
A LVL or Micolam beam would be easier than trying to splice a beam together if the price is acceptable. Alternates would be a box beam, which can be readily assembled on site from readily available material.
All plywood example:
https://www.finehomebuilding.com/1998/03/01/making-plywood-box-beams
Ply and lumber, with span tables for continuous lumber flanges:
https://www.apawood.org/download_pdf.ashx?pubid=a33827f6-cbaa-4092-8fd3-27969cd4aa2e
More detailed design data: From Australia, so data is metric. Principles still apply:
Original site (registration req'd): https://www.woodsolutions.com.au/publications/plywood-box-beam-construction-detached-housing-0
Also freely available here: http://www.5startimbers.com.au/downloads/Design_Guide_07_Plywood_Box_Beams.pdf
Thank you, thank you!!
So much interesting info!
And I thought I got retired, ha ha :)
A double 2x12 is probably not sufficient for a 26 ft ridge beam. Even worse with a splice.
Need to know the actual snow / wind conditions for your area.
Where are you located?
Wow, so much info!!!
I am in the eastern Washington, Tonasket area, but higher up, 4000ft. It hasn't piled over a foot of snow at once since 2016, and such storms are once a winter. It then it melts before the next snow storm.
Wind definitely can be high, never measured though - ordering a meter for potential electricity gen. The plastic panels are without a doubt not a factor, more like the weight of tbe structure itself :)
Just asked the local metal shop for 1/8 and 3/16 metal, then will try to calculate costs against custom laminate solutions as above.
Yeah, I read the most recent salt box article, but am trying to see if I can get away with a longer "freehung" beam.
My pure hunch (can be totally wrong) is that there is no comparison to a deck, but eave walls being almost freestanding is causing doubt.
Thank you!
Big thanks to everyone who helped out!!!
There is so much to learn, and I am very happy there is a wonderful community to help out in a pinch.
As for the green house - I decided to install a center post. Extra money spent on a hangar design is not justified, and a center post might actually be necessary for hanging pipes, etc.