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We have a 24'x14' detached pergola that was custom-built for our yard. Every year, we put up a large canvas canopy in the spring, and take it down in the fall. It's a perilous experience each time, and the canvas is starting to show signs of wear.

I'd like to consider putting in a more fixed roof on the pergola. I'm very new at this stuff, and will need to rent/acquire the appropriate saws if I go about this project on my own.

Photos of the column/beam/rafter joints: Columns/Beams/Rafters joint (bottom middle column)

Columns/Beams/Rafters joint (bottom right column)

The layout of the columns, beams and rafters is as follows: Columns layout Support beams Rafters

Can my pergola with a roof support snow weight? I live in Chicago, and it can snow (and collect) up to 36 inches here, for days/weeks at a time. I think the pergola footings are in concrete, but because everything was covered by composite boards, it's difficult to say fo sure.

How to build a slope, easily? I believe, with any solution I go with, I'd want to first create a slope. I have been reading about/watching videos about shimming. It seems like I'll need to create my own shims that run east to west. Not challenging, but it will require more equipment and more time to complete this job.

What material should the roof be? I was looking at corrugated sheets and polycarbonate like Polygal. If others have recommendations, I'm all ears.

Is there anything I'm missing?

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I looked up minimum snow loading in Chicago and for an open air, no snow drift potential, <15deg sloped roof you need to account for 20psf.

I work at an architectural/structural company in Ontario, Canada and our minimum psf for snow loading is 25psf. You would need 4ply 2x8 as the beams since they can span the 137" carrying the required joist span and snow loading.

You can most likely use the 3.25"x3.25" framing as they would be comparable to 2x4s at 1' oc which can span 8.1' max. Your required span is 8'6" minus the size of one post. 16" oc is likely not a problem either due to the fact that each joist is essential 2-2x4s.

As isherwood said though you need to either have the beams resting on top of the posts or they need to use appropriate hangers. You should also add angled bracing at the posts and beams secured 2' away from where the posts and beams meet.

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  • Thank you for your comments and measurements, this is very helpful. Could you explain what this paragraph means, though, as I don't quite understand the terminology. "You can most likely use the 3.25"x3.25" framing as they would be comparable to 2x4s at 1' oc which can span 8.1' max. Your required span is 8'6" minus the size of one post. 16" oc is likely not a problem either due to the fact that each joist is essential 2-2x4s."
    – Jason S.
    Jun 26 at 12:57
  • your dimensions shown look like you measured center to center so for the clear span length it would be your dimension minus the size of one of the posts.
    – Marmiketin
    Jun 28 at 12:41
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    The 3.25"x3.25" joists you have are just a little under the actual height of a 2x4 (3.5") which is the important number. There is no span info for the size you have but the 2x4 at 1' on centre can almost span your requirement so it is safe to say that something double the width and almost the same height will be fine.
    – Marmiketin
    Jun 28 at 12:47
  • @Marmiketon how did you establish "You would need 4ply 2x8 as the beams since they can span the 137" carrying the required joist span and snow loading"? Specifically, what references did you use to get to the recommendation of "4ply 2x8"? And does this recommendation change if the wood type is Cedar or Western Red Cedar?
    – Jason S.
    Oct 5 at 11:40
  • I used the Ontario Building Code 2015. I work for a firm in Ontario, Canada and that is the code I have access to but it is likely safe to say those code requirements are equal or better than that of Chicago. As for the loading information, I got that from the Chicago City By-Law minimum requirements.
    – Marmiketin
    Nov 9 at 20:11
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Probably not.

  • Cross-bolted beam connections aren't considered structural (they aren't even allowed for decks anymore).
  • There's minimal diagonal bracing, which is my primary concern.
  • The rafters may not be up to the task at that span. They're probably strong enough, but they're likely to sag quite a bit.

That sort of makes your questions about slope and material moot (the latter being off-topic anyway as a matter of opinion or style). Slope could be introduced simply by lowering one side of the structure somewhat.

Your best bet would be to no longer consider converting a pergola and just take it all down except the posts. Build up from there with adequate diagonal bracing and appropriate roof framing (or engineered trusses). Chances are you could sell the pergola components for a nice sum in today's market.

I'd probably just be looking for a nice roll-out cover to replace yours.

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