AboutPaul S. Brown, AIA Expertise How to work with an Architect. What to expect for design fees. What services are normally
included; what are not included? How do I find an Architect in my area? Why do I need an
Architect? How to be clear about your space needs. Why time must be managed properly.
Experience 24 years involvement in the practice of Architecture, including some teaching experience. Most of my work has been within the employment of various firms, but I have also done some small-scale projects on my own.
Particular interests include Environmental Design, "Green" Design, and Appropriate Technologies. LEED (TM) Accredited Professional.
I have some specific experience with Historic Preservation, Medical Facilities, Commercial Interiors, and Public School projects.
Education: Bachelor of Environmental Design Degree (Architecture), 1978, Miami University in Ohio. I am registered to practice professionally in the states of Massachusetts and Maine.
I want to build a pergola over a 13' X 16' patio. Although adjacent to the house, the pergola will have 4x4 posts on each of the four corners and will be free standing. 2) 2x6 will cover the posts to help support the beams. Two perpendicular knee braces will be fitted to each post. There will be 7 pairs of 2x6-with 4x4 spacers between for the rafters. There will be two beams, front and rear. As with the rafters, the beams will be doubled with 4x4 spacers. The beams will(?) be 2x10. I want to cut in the rafters into the beams, which effectively makes the 2x10 a 2x7 (I think). I can send a picture of the model if that would help. Battens will be 2x3, spaced 3" apart.
The question: For the beams, are pairs of 2x10 sufficient for a span of 16 feet?
Thank you.
ANSWER: Hi Michael:
Sounds like a great project. Are there any structural engineers on AllExperts? You might want to check with one of them. If I understand correctly that the effective depth of the 2x10's is going to be reduced to about 7", because you are mortising in the rafters, then no; this sounds like you are pushing the envelope a bit too far. Where are you located? What is the snow load? What pitch for the roof? You would be much wiser to use at least a pair of 2 x 12's, or, better, an engineered lumber beam at each of these locations. If you take your sketch to the local good lumber yard that sells engineered lumber (also called LVL's, laminated veneer lumber), and ask them, they will size the beam for you to meet local codes. Your local building official is also a good source of information. If you don't like the look of the LVL's, you can always box them in pine.
Most important question, how are you going to hold the pergola down in a wind, to keep it from blowing away? This is probably your biggest danger.
Good luck; sounds like a great project.
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QUESTION: Thanks for your reply.
This is located in Southern California. Snow load is not such a concern. Since the pergola is 50% shade providing with a slatted roof, I've also discounted wind lifting. The posts will be fastened to the pad with gal Simpson strong-tie adapters. That'll also keep the wood from touching the concrete. The only load on the "roof" would be several bunches of grapes. It will be essentially flat with no slope. I should mention that the material will be 100% pressure treated lumber. I'd like to stay away from LVLs.
I'm guessing here, but perhaps the use of knee braces might effectively reduce the span length as they extend 2 feet from the posts.
Thanks again.
Answer Hi Again Michael;
OK, yes, you are correct. The knee braces will effectively reduce the span - you are doing the right thing to catch the knee braces on the sistered columns, so they will hold a load. And with no real roof and no snow load, you should be OK. You might find that you get just a bit of sag over time in the long spans, but by that time it should all be covered with grapes anyway! And it is good that you are anchoring it properly; many people neglect this. Enjoy the shade, I hope it goes well.
PB
