Building Homes or Extensions/Large Patio cover

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Expert: - 2/17/2009


Question

Diagram of patio cover
Thanks for taking the time to look at my question. Ok so here’s the scenario…I extended my current concrete pad to be about 20’W x 18’D.  I want to build a fairly large patio cover covering this pad. My homes roof slopes down along the length of where the patio cover is to be. I attached a make shift diagram to better illustrate what I am trying to explain.  Anyway I was planning on having a total of 5 posts around the perimeter and one a couple feet away from the house in the center, out from the house as far as the fireplace, to help bear the large span. (figures I could frame it that small area in as storage).  I plan on using pine for all posts, beams and rafters. I want them all to be seen from underneath. On top of the rafters I was thinking of using 1”x whatever to cover the top. Shingles are what i currently have on my roof so want to continue that on to the patio cover.  So my first question is on size of timbers. Here’s what I was thinking:

Posts:  (6) 8” x”8” x whatever height they need to be.

Perimeter beams: (4)  8”x 10” x  9’ and (2) 8” x 10” x 10’

Rafters  4” x 8” x however long they need to be.

Center beam: (1)  ?????? was told around 8” x 12” x 24’  or 10” x 12” x 24’  What do you think?

So I need to know if these sizes are adequate given my design? Is it overkill or do I need even larger?  Remember I was going to use pine (for cost reasons) but is Douglus fur a better choice? Maybe just for the large center beam?
 
Thanks for any help you can give thanks.


Answer
Eric,

The basic design is sound, and the sizes are good, and pine will work just fine.

But I would approach the design a bit differently, using full-length horizontal beams, which makes for a much stronger structure and is easier to build and raise. The only reason to use shorter pieces is if the longer ones are not available.

I suggest that you have full-length (18') rafter plates, rather than dividing them in two. I would also lower the 20' tie beam on the gable end so that you have staggered joinery.

The center post on the gable end is not necessary if you make the 20' tie beam about an 8x12.  You can stand a king post in the center to hold up the ridge beam, and insert that middle post beneath if you want it for looks.

The ridge beam doesn't need to be that large, since it is supporting only half the weight of the roof, at most. If you make bird's mouth rafter feet, then it will be supporting mainly itself. I would make it a 4x12, or whatever looks good with your 4x8 rafters.

I hope this helps. Write back if you want to discuss the design some more.

--Daniel  

Daniel Humphrey

Expertise

I can answer questions about designing and building homes and outbuildings, especially for colder climates. I am expert in timber framing, but of course I also know about other kinds of residential construction techniques and materials. Because of my emphasis on using local materials and organic alternatives, I know quite about about Green Building and am willing to do the research to find out more.

Experience

I've been a timber framer for 20 years, working as a general contractor who designs and builds custom timberframe homes, working as much as possible with local organic materials, from foundation to finish. I also have expertise in designing, building, and setting up pre-fab remote camp facilities on terra firma or temperate glaciers for research and expeditions, using helicopters for support.

Education/Credentials
B.A., M.F.A.

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