We are building a wall pergola that we want wisteria to grow on top of. The pergola will only come about 3.5-4 ft out from the wall (Edit: the columns themselves are less than 2ft from the wall). I have already placed 2 6x6 columns bolted into concrete on both sides. The beams are 16 ft apart.

Are there any wood (preferably cedar) beams that can span that 16' distance without having to place additional support in between? If so, what kind? Is there a chart somewhere with max distances that beams can span with minimal weight on top?

In the picture below, these beams span pretty far, but they are also very thick and I wonder if beams that thick can be mounted to a 6x6.



If worried about the weight, see this wall pergola from Lowes. This would bear the weight with or without the long board in between the columns. This is what I am going to buy or build. I also have 6/6 columns on either side for wisteria to wrap around. I may or may not span a board from one column to the other, depending on if it makes sense to do so.

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    I started an answer, and the numbers got big. Live Load is 5.33 PSF for Wisteria, and snow load in Woodstock Il. is 25 psf. So we're looking at a total live loading of this roof of nearly 2000 pounds. So I'm not going to finish my answer, as the DANGER WILL ROBINSON flag is waving. This is exactly the sort of project that gets amateurs in trouble. It SEEMS simple enough, and then it becomes really complicated. For a couple hundred dollars, consult an engineer, and get a permit drawing done. You're risking losing your insurance. Apr 4, 2016 at 21:05
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    You say you've placed 2 6x6 beams, but do you mean columns? Beams are horizontal members (perhaps you've cantilevered them out from a building?) but it sounds like you mean placed columns, similar in function to the stone columns in your picture. Apr 4, 2016 at 21:08
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    @ChrisCudmore I agree with you. Question about the insurance comment - are ill-conceived building projects executed by homeowners routinely cause for cancellation of homeowners insurance policies or denial of claims? I hear this type of statement often but as far as I know the thing that gets you in trouble is malice, not incompetence. Apr 4, 2016 at 21:22
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    @Dave - seems like a couple complete strangers are trying to keep you from hurting/killing someone and/or having an insurance claim denied because you did something without the proper research and permits. If you want the simple answer: yes, any 16' board will span 16'. If you want to know how long it's going to last and how much weight it can carry you'll need to provide a lot more info: what dimensions are required (16x?x?), how will it be attached, etc.
    – Kyle West
    Apr 5, 2016 at 13:05
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    @ShimonRura If the insurance company doesn't want to pay a claim, they can start asking for permits and inspection reports. Then deny everything for unauthorized work. Apr 5, 2016 at 16:48

1 Answer 1


Here is the answer to my actual question:


Edit: Sorry if I'm not on the same wavelengths as anyone else. I already know about insurance and inspectors, and that was not at all what I was asking about. I just wanted to know what length of wood would span a certain distance given various criteria. I would have expected an answer to that question, instead of all the "your a dumb homeowner who's going to get in trouble" type replies. I question the idea that there will be 50psf of weight from snow given the fact that it is an almost non-solid top, and saw no real attempt to answer my question, so I had to come up with this. It took me awhile to find, and I hope it helps others.

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    Link only answers are not useful, especially when the information in the link does not answer the question.
    – Tester101
    Apr 5, 2016 at 19:15
  • What values did you put in the calculator? I don't see a way to specify a 16 foot span.
    – wallyk
    Apr 5, 2016 at 19:23
  • @wally The span is what it gives you. You specify the type of wood and the board size (2x10, 2x12, etc) and other factors such as load, then it gives you the maximum span.
    – Dave
    Apr 6, 2016 at 13:42
  • Tester, I asked "Is there a chart somewhere with max distances that beams can span with minimal weight on top" and this is a link to a calculator, even better than a table. So it answers my question perfectly.
    – Dave
    Apr 6, 2016 at 13:44

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