Expert: Stephen Major (Owner--Major Design Group) - 4/15/2007

Question
I want to span 20 feet between columns on one area of my pergola.  Can this be done?

Answer
Linda,

Linda,

Thanks for forwarding the weblinks.  This looks like an interesting project.  To span 20 feet will require a long and fairly substantial beam that will not sag under it's own weight or the weight of the joists.  You might consider a flitch plate arrangement, where a steel plate is sandwiched between two outers layers of wood to make a strong beam.

My other concern is: what are your provisions for racking (the columns tipping sideways under lateral load such as wind)?  Being so open, pergola structures rely on three things to stay up: tight and firm joinery, the sheer weight of the components, and the base dimensions of the columns.  Or, you could add some triangulation into the mix.

You may want to consult an architect or engineer for guidance before constructing this.

I forwarded you a photo of a pergola I did.  The columns are drawn tightly to the deck using internal tension rods.  The structure is attached to the house with a half beam on half columns.  The timbers are painted redwood.  I've done a few similarly, some much larger.  

Good luck on this project!

--Steve Major