Expert: Steve Holleran - 9/8/2008

Question
Mr. Holleran, I'm an old timer who has forgotten most of what I learned in high school. Here is my question.
I am building an attached pergola on the rear of my house. The top edge of the ledger board (2x6)is 88 inches high from patio floor and attached to the house. I want to drop the rafter pieces 5 degrees from ledger board to front cross member, which will be attached to two posts. Distance
from house wall to posts is 9 feet 6 inches.  How high will
the front of the pergola be from bottom of cross member to patio floor where the cross member is the same size as the ledger (2x6).I would need at least 6feet 3 inches so that I
don't hit my noggin.  Is there a formula you can advise me
of for future needs or do you know of a pamphlet or book that gives simple formulas for different angles. Thank you
a million for your help. John  

Answer
Hi John,

I'm really struggling with a diagram with this one, John, but I tried my best to get an idea of what you want to do.


If you run the rafters from the top of the 2 by 6 header, a 5 degree drop on a 9'6" run will mean you come down 9.97" at the cross member

(If you run a horizontal from the top of the header and make the angle 5 degrees, you have a right triangle and can use

            tan 5 = opp / adj = y / 9'6" = y / 114

so then      y = 114 tan 5 =9.97 so call it 10")

This brings you to 78" clearance, and if you run them to the top of the cross member, the 6" of the front cross member brings the clearance down to 72"  , which is 3" lower than what you want,  (I think).


I hope this makes sense to you.
It's tough without a diagram.

In any case, I hope your pergola works out ok !!

Steve