White Oak is a good choice. Very strong and works nice...when green. Will you have a loft for storage or maybe an office?

I am planing to build a shop for myself one day.

Send us some pictures when you are raising please.

I just finished the roof on my timberframed workshop. 4x6 common rafters 46" on center with 1x12" rough cut that I shiplapped. I decided to insulate with 2" polyiso then OSB, 15# felt and metalroofing. Went together well. Glad now I didnt cheap out and just put metal down. Winters are long here! What ever you do I would put felt under the metal.

BB

Yes, I will have a second story loft for both an office and storage.

I plan on taking a lot of pictures during the construction and raising, I will post some. It will be next spring though.

How did you cut your shiplap? I have heard suggestions from table saw to router.

Rookie I have misquoted myself. Those rafters are set at 33" on center.
The shiplap was cut with a 2hp tablesaw,feather board and dado blade. Make sure to use the cheaper adjustable dado with one blade. Learned this the hard way after buying the expensive big stack. The single blade works much better for this sort of thing.
Worked great with green pine and hemlock.
BB

R:

Not having done any of the math for you, I'd agree with your initial intuition: that the purlin spacing is too wide. Of course I live in snow country, and my intuition is biased. CB.

Don't know if this will work but I'll try.

http://www.davisframe.com/Images/Product/lg/Saw%20Mill-GR.jpg

If you can see this picture it is VERY close to what I have talked about. Note the large span between the roof joists and the large span of the purlins with the "subfloor" boards going up the roof slope. It seems like this is possible based on my original concept but the roof pitch in the piture is obviously much greater than mine would be and I do not know how thick his roof material is.

Thoughts?

That's right. And you don't know his snow load, or if he is using SIPs. SIPs are very stiff, and can span a dozen feet or more on their own if properly supported at the ends. CB.

The pitch of a roof makes a big difference. Even without snow on the roof you have the weight of the rafters, purlins, decking and steel or shingles. It will be the forces perpendicular to the beams that count. Gravity wants to pull everything straight down so if a beam is horizontal then you have 100 percent of the force acting against the beam. If a beam is straight up like a wall then there is (for this exercise)no force acting on the beams. The roof in the picture looks like a 12/12 pitch roof which will have 50 percent of the load acting against the beams.
You will have to brush up on your Trig. & Geometry to calculate the actual forces acting against your roof members.
There are likely some tables kicking around as well.

Fun with math

Cheers

The codebook doesn't alter sheathing, rafters or purlins based on pitch. The horizontal clear span from plumb to plumb would be the way the NDS measures the span. Roughly then, 4' centers on an 8/12 with 4" wide purlins is going to give a clear horizontal span of about 40".

Your building department can give you the snow load for your area, I'm guessing it will be 30psf. Add 10psf for dead load so 40psf total load.

White oak base design values for #2 are;
Fb-850
E-.9
Fv-220
I would bump up the Fb by 15% for short term loading(snow), 15% for repetitive member use, and you could take 15% more for flat use but I don't think I would. I'd call the adjusted Fb-1125psi

Assume a 6" wide board spanning the 40"=1.66 square feet X40psf=67lbs load

I looked at it as a simple beam in flatways bending;
http://www.windyhilllogworks.com/Calcs/beamcalc.htm
I entered;
Load-67lbs
span-40"
width-6"
depth-1"
Fb-1125psi
E-.9
Fv-220psi
Click "enter"
the fail in deflection shows it would make for a bouncy floor but a passable roof board.

BUT,
Let's look at those purlins..
40square feet of roof bearing on a purlin X 40lbs per square foot= 1600lbs load bearing on each 4x6 purlin. I also changed the span in the calc to 140" then entered this info assuming the same species. It's too small, it might squeek in at a 6x6.