I have a couple of questions on designing/building a pergola. I am a woodworker, but I do not have any experience with building construction. One person suggested that I hire an engineer for the design, and another said just to copy other similar structures in the neighborhood. There are two key issues that I have been wrestling with, and would very much appreciate hearing from those more knowledgeable.
The pergola will be detached, and it will be 6 inches from the house (fascia). It will be 17′ x 12′ (cover size) and 10′ tall, and it will have a beam span of 14′. The posts will be 6″ x 6″, and they will be attached to the concrete footings using Simpson connectors. The rafters will be 2″ x 6″ and 16″ oc. And I am proposing using 2″ x 10″ double beams notched into the front and back of the posts, and fastened with carriage bolts. Also, there will be 2 knee braces on each of the two outside posts (away from the house). The posts and beams will be pressure treated, and the rafters will be untreated Douglasfir.
I have two questions:
1) Will the 2 x 10 double beams notched into the front and back side of posts will be adequate for the 14 foot span to keep from bowing? (Some have suggested that I should either use a 4 x 12 beam across the 14 foot span, or use a third post to reduce the span to 7 feet. This later suggestion is not feasible because the beam would fall in the center of a sliding glass door.
2) As we occasionally get wind gusts up to 80 mph, I added knee braces in both directions, but only on the two exterior posts. (Adding them to the posts near the house would create an obstruction, so I would prefer not to do this unless absolutely necessary.) Is it enough to add the knee braces only to the outside posts, or should all the posts have knee braces? And, other than providing lateral support, do these knee braces also work to support the 14 foot beam span?
Thanks for any help you can provide on these questions. Without over-engineering, I am looking for a solution that is structurally sound and will last for years without sagging.
Replies
You're going to put a 17x12 kite out in 80 mph wind with a couple of knee braces to support it??
Big Patio Pergola
Here are a few principles to follow Woodchucks. Don't notch pressure treated where it will trap moisture...leave the posts solid and mount your beams on the outside, or above. I prefer above, that way I can space them 1 1/2 which makes mounting your knee braces much easier, and much more secure.
Your span is fine for a beam size of 2x 2x10 in pt. Blocks every 2'...
Doug fir, though great stuff is heavy... which means you will need more beam and more lateral bracing.
When it comes to braces, rather than using a 2x4 on an angle, which I agree is obstructive, use a larger piece and curve it so that people don't run into it. It will look better too. Braces should go anywhere possible, and if they oppose themselves you should be fine to resist 80mph events. Putting braces on only 2 posts will work great for those posts--so long as you don't mind the rest of the structure near the house wagging like a dog's tail. You should be mounting braces with an exposed face of more than 28". Yes, braces do add support and cut the functional span to about 12" per 24" of bracing.
There's lots of examples on our website... gardenstructure.com
Lawrence
i missed you around the internet
how's your business going ? my customers keep asking for garden structures and exterior pergola type things..
i think i'm re-inventing the wheel with all of the work you've done
i should sign up as your first Rhode Island associate
Pergolas tend to be top heavy. The grid style structure doesn't provide much resistance to racking. I think your span will be fine, you aren't carrying weight. Well done braces will help both issues. At only 6" from the house, I would tie to the house in a couple of places, something that doesn't really show but will add stability. Maybe a heavy H or U connector. I'd probably have a metal shop fabricate something. In my mind I'm seeing a short piece of I beam with the flanges bored for bolts.
This was before your time, but sometime back, McDesign I belive, had come up with a system for anchoring the columns such that no bracing was required. It did require hollow columns however where you have stated that you plan to use solid lumber.
It had rebar extending 2' to 3' out of the pier / footing. The column was set upon the pier / footing with the rebar extending up into the column. Then the column was filled with concrete as required to cover the rebar. Once the concrete set, the column was firmly anchored and ridged.
This is an over simplified description, but I used this on a free standing 12' x 24' pergola with good results. At least for the past 2 years. It is more complex than bracing, but it does away with unsightly bracing.
From the what it's worth department.
Building A Pergola - Thanks
Thank you all very much for your help. Other than the 17x12 kite comment, the responses were very helpful. It sounds like the consensus is that the double 2 x 10s beams are sufficient for the 14 foot span. However, with the wind gusts coming off of the nearby delta, it sounds like everyone agrees that bracing is required on each of the posts to prevent racking. And I've noted the alternate methods of mounting the beams instead of notching pt lumber, suggestions on bracing, possibly tying the structure to the house, and the idea for anchoring the columns so that no bracing would be required. I will give all this further thought and modify my design. This is a great community. Thank you for taking the time to share your thoughts and knowledge!
more support be beddah
Can you extend the 2x10s a few feet past the posts and use cross-braces to the outside the posts instead of the inside? Would that prevent the obstruction near the house?
Structurally the 2x10s will be more than adequate. I'm thinking the 2x6s might look undersized, but that's my eye.
With the 2x6 rafters, consider an overlay. That'll keep the rafters from wandering.
There are pergolas where the posts run a couple of feet taller than the cross beams and cross bracing was done above the beam. An untraditional look, but it works.
more support be beddah
Thanks, Mongo! Some interesting ideas...thinking outside the box!