Pergola beam size??

[Dan Beckman]

I'm designing a pergola for my house and could use some insight on the size of beams required for safety and to eliminate sag. I'm planning to use 2x6 rafters spaced 16" OC. Due to the size of the existing concrete patio the 6X6 posts will be 14' apart on the long sides, and 8'on the other sides. What size beam do I need to span the 14' posts. Can I use 2 2X10's, each one bolted to the side of the posts? Would it be better to sandwich the two 2x's together then set it on top of the notched posts?
Thanks.

[rudy de haas]

hi:

Been there, done that, (missing verb) it up.

In my first effort I used 2 x 10 cedar beams glued and bolted together. Looked great - about 20 years later we got home after a night of heavy wet snow to see that the thing had partially collapsed. Turned out the wood had rotted around the bolts ..

When we bought this house in Lethbridge - it had (and still has) a classic open pergola roof sunroom at the back. It has 2 x 10 beams and they're fine but.. the people who built it didn't think about foundations enough. Two years ago I had to shore up the walls, remove the deck, dig out the muck
underneath, and replace the foundations with 6" concrete posts dug and hammered 3 ft down, then put new floor beams and deck. (and now she wants it replaced with a glassed in sun-room :) )

The point is that not thinking about what water and bacteria will do to your design will eventually bite you, or somebody else, where it hurts.

[Rich Riddle]

All new information says you should not bolt beams to the side of your posts. You should place the beam directly on top of your post. So notch the post and set the doubled or sandwiched beam directly on top of the notched post Doubled up 2 x 10 will give you 15'8" according to the span chart I ran. See the photo.

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[Bill Clifton]

Find a real lumber yard and ask them for help. They size the laminated beams you use to carry the load when you eliminate a wall. I am confident they do the same for regular lumber. Good luck.

[Mark Bolton]

All new information says you should not bolt beams to the side of your posts. You should place the beam directly on top of your post. So notch the post and set the doubled or sandwiched beam directly on top of the notched post Doubled up 2 x 10 will give you 15'8" according to the span chart I ran. See the photo.

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That information is a bit skewed. First off its very very rare that you will ever find select structural grade material which is whats called out in that spancalc. Second off, most span calculators will give you little to no relevance with regards to a pergola. Generally speaking the loading on a pergola is very tricky because you have to consider what could potentially be growing on them to create a canopy and of course the geographic region with regards to snow and so on.

More often it seems that pergola member sizing is most always overkill due to proportion and design. For instance many pergola's will have a horizontal member that may be 6x8 or 8x10 solid material for aesthetic's and the member could carry a much larger span that it ever will be expected to but anything smaller would look out of scale.

That said, 14' is a very long span. Very long. Its long from a structural standpoint and long from a design standpoint. That said, I would never trust it to two 2x's notched or bolted as it will more than likely sag over time. If a solid member wasnt an option my approach would be to, at the least, 3x or 4x and notch the posts 3 sides assuming the posts will be 6x6's at the least.

Just my .02

[Dan Beckman]

Thanks for all your replies! The 14' span runs parallel to the house and is on the outside adjacent to the yard, so I could add another post in the middle which would solve the loading issue on that side. But on the side next to the house (3' away) it is all concrete, so I don't know how to support the middle of the span without putting a post in the middle of the concrete patio. I cant support it from the house as there are doors from the house to the patio. Any ideas?

[Mark Bolton]

Are there going to be any knee braces or are you just sitting directly on posts? What size will the posts be? I agree with the other thread that with zero loading (plants to create a canopy and hold snow in winter), and a short rafter run (if your rafters are running the '3 distance) the load will be minimal and the 2 2x10's would likely never sag but I just personally never care to take the chance.

In my experience a lot depends on the design. For instance do the 2x10's compliment the height/scale and so on.

[Dan Beckman]

Mark, just to clarify. The overall dimensions will be 14' on the long side by 8' on the short sides. No connection to the house.
I'm planning to use 6X6 posts. Yes, I plan to use knee braces in all 4 corners, and on the center post on the outside 14' run.

[Mark Bolton]

The knee braces will help considerably because they will support, and effectively shorten, the overall span. Much different than simply carrying the span clear. Given that, I'd have less concern spanning the 14' (which may effectively be reduced by 4-5 feet with the knee braces).

I agree with Rich that notching the 6x6's would make a much better job as compared to simply through bolting. What would be nice may be 6x6 posts with 4x4 knee braces. Notch the posts 1" on each face and simply through bolt or lag the knee braces in place.

[Mark Bolton]

Perhaps something like this

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[Tom Howarth]

A buddy of mine bolted steel lawn edging to replace bouncy floor joists in his crawl space. I think that can be done with your beams. Sandwich the lawn edging between your 2Xs...

[Danny Buie]

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Dan,
Here are a couple of snapshots of a pergola I built about 15 years ago. The posts are about 11 feet apart on the long side. I used two 2 X 8s on each side. I could only get 14 footers so I "spliced" on a 3 foot piece on the right end of the outside beam and to the left end of the inside beam so it looks like the beams are 17 feet long (study the photos). I mortise and tenoned knee braces as suggested by Mark above. They have the effect of reducing the span and also I bolted the tenon between the beams so it makes them stiffer. I drilled the mortises out with a hand drill and finished with a chisel and mallet. It really did not take long and was very enjoyable. The tenons were cut on the table saw and circular saw. I notched the posts for the horizontals. When a 5 1/2 inch post is notched for double horizontal beams there is on 2 1/2 inches left so select your post carefully.

The two knee braces for the long side were cut from a 10 foot 4 x 6, an 8 footer got two on the short side.

This was built in Louisiana so no concern for snow load; hurricanes are a different matter. If I were building this same pergola with 14 foot spacing on the posts I would use double 2 X 10s with knee braces. I would put some spacers (same thickness as the knee brace tenon) between the two beams and bolt them to make the two beams act more like a single composite beam.
Danny Buie

[Jim Andrew]

If you go to a lumberyard that sells laminated beams, the beam companies offer a service of recommending size of beam for your application. So talk to your lumberyard and at least you will get the proper size of beam. If you choose to build it n a different manner, that is your business.

[Jason Roehl]

I think you'll be fine with treated SYP 2x10s doubled up on both long spans. It's a pergola--not much snow load there, not much live load, even with some sort of greenery climbing across it. If you're really concerned, use 2x12s for not much more cost.

[keith micinski]

A 4x4 does make a good knee brace when 6x6 posts are used. Also a 2x10 can span 13'9" when being used as a deck joist so I am not sure why it couldn't easily carry 14 feet on a pergola which is going to halve 75 percent less load on it. Probably more then that.

[Pat Barry]

A 4x4 does make a good knee brace when 6x6 posts are used. Also a 2x10 can span 13'9" when being used as a deck joist so I am not sure why it couldn't easily carry 14 feet on a pergola which is going to halve 75 percent less load on it. Probably more then that.
For a deck joist though, this assumes it is one of several deck joists on 16 in centers so its not really the same as a beam. The beam needs to be sized for a much higher load

[keith micinski]

Everyone keeps talking about load, other then the weight of half of the 10 2x6 rafters themselves what else is this thing going to be carrying? Snow isn't going to factor in and unless he is going to be using this to support some sort of vine from the amazon I'm not sure how much weight from "greenery" needs to be factored in either. It's a 2x10 spanning 14 feet with virtually no load at all on it.

[Jason Roehl]

Everyone keeps talking about load, other then the weight of half of the 10 2x6 rafters themselves what else is this thing going to be carrying? Snow isn't going to factor in and unless he is going to be using this to support some sort of vine from the amazon I'm not sure how much weight from "greenery" needs to be factored in either. It's a 2x10 spanning 14 feet with virtually no load at all on it.

Bingo, Keith.

[Mark Bolton]

Everyone keeps talking about load, other then the weight of half of the 10 2x6 rafters themselves what else is this thing going to be carrying? Snow isn't going to factor in and unless he is going to be using this to support some sort of vine from the amazon I'm not sure how much weight from "greenery" needs to be factored in either. It's a 2x10 spanning 14 feet with virtually no load at all on it.

https://lh6.googleusercontent.com/-tGTAmQekJwQ/TYT_YAQdTxI/AAAAAAAABqw/FiNpRX1iOHE/s1600/Wisteria_Pergola-1.jpg

Do a google for pergola wisteria or just plain ol pergola and you will see endless images of climbimg plants. Now imagine these are year round vines that are never pruned back and eventually become a nearly solid matt. Then of course imagine 20" of wet snow in a cold climate.

The oiginal post was not questioning if the member would carry the span with any acceptable amount of deflection which is the case for a deck floor joist (which by the way this is not, its a header/girder), it was speaking to sag. L/240 deflection or greater in a joist may acceptable and mildly perceptable to the user/customer but 3/4" or mor of sag in this long span is what I assumed the OP was concerned with and as already pointed out a floor joist is part of a system and the spans are in direct correlation to the fact that there are adjacent joists as well as a sheathing surface which transfers load across the system. Calling out a single member using a joist/rafter span table isnt very sound though its understood by anyone that if there is never a planting the load is zero.

May be acceptable to you, but its not to me and I know it wouldnt be to my customers. It wouldnt be acceptable immediately or 5 years from the day I built it.

[Michael Cather]

What seems to be missing from these comments/replies is "Deflection"...one commenter referred to "sag". Code allows for specific limits of deflection. Deflection is critical in as much as the human eye can detect a very small amount of visible deflection and deflection can cause damage to surrounding material. The L stands for length. So if you have a beam 20ft long, the allowable deflection limit is (20ftx12in/ft) /240 = 1 inch if using L/240. Or, (20ftx12in/ft)/360 = 0.67 inch if using L/360. This means that your actual calculated deflection of the beam should not exceed either 1 inch or 0.67 inch, depending on which of the criteria you are using: .67" is visually unacceptable. Deflection may very well meet code criteria but not visual criteria. It is extremely important to review the deflection and determine if it is "visually" acceptable. Have a licensed engineer calculate the correct beam size and review the deflection from his calc. Often just an upgrade of the member can bring the deflection into visually acceptable range.

[David Helm]

Esthetically, if using 6X6 posts, the obvious answer is a 6X beam. Likely 6X8 or 6X10 will do the job.

[John P Clark]

The first questions should be - where do you live? What type of loads will this see? Span length? The consult with an engineer to determine the sizes necessary, just saying I am a engineer. Lumber yards could give you an approximation, but will not be responsible if your structure fails. These types of questions and all of the arm chair quick responses make me laugh - if it was that easy just to,look at a table, why did I spend all the time in school and take 16 hours of tests just saying John Clark, P.E.