Acrylic covered pergola reassurance

[Ken Mascola]

1st time poster would appreciate some advise from those who have experience with the following type of pergola.
building in clay soil base. Total width 18 ' by 16 ' with double 2x10x16' beams on 6x6 posts with beam span of 14' 10 " . Then 2x8x18' rafters spaced 18" notched into double beams on each side of pergola.total span of 15 ' 10 " . Notched 2x2slats every 8-12 " then on top I have 16 mm acrylic attached to guides screwed into slats. 6x6 posts are sitting on post saddles on . Size concrete column foots not sure size but thinking 12 inch. Concerns are swaying in winds Hamilton area ontario canada as well as rafter/joist spans but believing. Notched slats will help with rafter spanning overall. Thinking will also require 4x8 mid span and angle brace on beam post supports. Your input would be most welcomed. Thanks Ken

[Ben Grunow]

Member sizes sound fine for a pergola to me but the acrylic and the potential snow load (you are up North) concerns me a bit- if you dont keep that roof clear in the winter (or take the acrylic down?) I think you will have issues with the beam/rafter sizes. I might go with a triple beam and 2x10 rafters to avoid too much deflection under larger snow loads. Also going to need some good/bolted diagonal braces from beam/rafter to post to hold it square and resist wind loads if its not attached to a structure.

[Stephen C Cunningham]

That acrylic will make for a pretty big sail.
I had to have a 16" footing on 12" columns here in Niagara for a large elevated deck. Definitely braces on the corners. If you are up the mountain, I would go 8" posts if you are exposed. Not sure if the posts should go in the concrete for more lateral support. I'd talk to an engineer if you aren't going through the municipality. (although they probably would want stamped drawings anyway)

[Peter Quinn]

Man from New England scratching head. 18' sounds like quite a span for 2X8. Then you load it with 16MM of acrylic, which is pretty heavy on its own, also making it a snow bearing roof. If I wanted that kind of span here with zero pitch I'd need something like 2X12 minimum on 16" centers. How does it drain? What is the acrylic for? Ive seen some pretty cool retractable sunbrella systems that pull back for winter and cloudy days. Acrylic sounds like a green house, I'd want some pitch to that roof, maybe a hipped roof rather than a rafter/perlin structure.

[Ben Grunow]

16 mm?!!! I didnt see that. Was thinking 6 (roughly 1/4")...

[Eric Schmid]

We just built one similar to what your describing. Sounds like your roof has a single slope where as the one I build had a gable. Your beams and rafters seem undersized to me. We used a double 2x12 beam and spans were only 10'. We used 6 posts for a 20' roof with 2' of overhang on each side. Do you know what snow loads are up there? You can find lots of beam and rafter span charts segmented by snow load. How many posts? The number of posts determines the pier size. You need to determine the total predicted load (worst case), then size piers from there. The fewer posts the larger the piers need to be. Is one side attaching to the house or is it freestanding. If this is freestanding you will need something to resist racking. Your purlins will help above, but I'd want something between the post and beam. Corbels or braces will work if you can lock them in. We ran the corbels up through the space between the beams and screwed them down. They were mortised 2" deep into the posts and the tenon secured with dowel. Metal bracing would work as well if it's installed correctly to resist lateral and uplift loads. Building a pergola with your lumber sizing might work, but once you add a roof you are dealing with a whole different animal...especially in your area.