On Saturday, October 25, 2014 4:27:56 PM UTC-7, John Sheppard wrote:
Hey all,
Just wanted to run a design for a timber framed structure that I am planning on building and wanted to appeal to those with more experience than this weekend warrior for feedback.

I plan on building an outdoor kitchen. I will excavate and pour a 20' x 20' 4" slab with the appropriate turn down on which I want to place timber framed structure. The structure will use 4 12" x 12" Douglas Fir posts set at 16' spacing to support the roof. These will be affixed to the slab using a 18" x 18" x 1/4" steel plate with a schedule 80 1 1/2" x 12" pipe welded to it that will be tapped into the bottom of the posts. These plates will be affixed to the posts with 4 - 18" timber screws. They will be affixed to the slab using 4 -3/4" x 4" concrete anchors embedded in the concrete slab when it is poured. This will handle the uplift forces on the structure, or at least that is the intent, as well as any shear at the base of the posts.

The roofs gable ends will use 6" x 8" beams for the king post truss's principle rafters and tie beams as well as for two struts. The tie beams for the gable end rafters will be framed to the posts using a 2" thick x 4" high x 6" wide tenon on the post and the appropriate mortise on the tie beams. They will also be supported by a 2' x 2' x 3' knee braces between the tie beam and post using the same 6" x 8 " Douglas Fir.

The tie beams between the two posts in the other direction will be 6" x 8" Douglas Fir as well. They will be framed to the posts using 2" thick x 4" high x 12" wide tenons on the posts and the appropriate mortise on the tie beams. These tie beams will be 6" x 8" x 20' and will support 6" x 8" principal rafters that are 4' on center. They will also have 2' x 2' x 3' knee braces between the post and tie beam. These principal rafters on the non-gable ends will also have a 6" x 8" collar beam 2' down from the peak. I will also tie all the rafters together using 4" x 4" beam running the length of the roof halfway up the principal rafters and at the peak.

On top of the rafters I will put 1 1/2" x 6" T & G Western Red Cedar car decking on which a standard felt / asphalt shingle roof will be placed.

So with that context I want to ask a couple of questions. 1.) I want to make sure that the tie beams on the non-gable ends are sufficient to support principal rafters that are on 4' on center as well as the rest of the roof. I had someone say that the span for a 6" x 8" beam shouldn't be any more than 12'. These will span 14' but will have additional knee braces that will reduce the span to 10'. Is this sufficient? 2.) Is the 4' spacing between the principal rafters too large?

Thanks for any advice,

John

Point of clarity, the knee braces will be 3' long. Not 2' x 2' x 3'. That is the triangle they will describe...