20x20 timber frame pergola

[tnbison]

Brand new member, glad to be here.

My wife wants a pergola and I've put it off for a number of years.  Well this spring/summer is the time.  I poured a couple of 25' square concrete pads in the backyard so this will be going atop one of them.  After looking at kits and dimensional lumber pergolas I think I will build a timber pergola. 

The kits that are at westerntimberframe.com are very nice looking and I'd like to emulate something like that.  I believe I've read on this board the possible dangers of using their dovetail style joints and they are very expensive so that route is out.

So my plan is to build a 20x20 pergola on 6x6 or 8x8 posts. I can find cypress from a mill nearby.  I need to figure out what size beams will be able to span 18'(1' overhang of beams/rafters).  I'd like 3x10 rafters atop the beams spaced roughly 16" o.c. and 2"x4" or 2"x6" shad slats on top of the rafters.

What size cypress beams would support that sort of load?  I can't find a simple span table for cypress.  Aesthetically I was picturing a 6"x10" or 6"x12" beam sitting on top of the 6"x6" or 8"x8" posts.  If treated with linseed oil annually how long could one expect a structure like this to last in the elements.  I'm in the west TN region.

Great site.  I look forward to soaking up a lot of knowledge here.  Thanks

[Don P]

Welcome to the forum 

See if this will get you there, from the calcs in the red toolbox at the bottom of the lower left column, inside the toolbox scroll to the bottom under Don P's calcs, (great guy, humblest man I know  :D) "beam and column calcs";
Design for Bending
Bald cypress is at the top of the menu.