Expert: Daniel Humphrey - 9/23/2010

Question
Can a 6"X12"-20'long (treated #2 pine) span 20'with no support in the middle.  There will be 4"x12"-8'long rafters on 5' centers.  The Pergola dimensions are 12'wide x 20'long x 8'tall post.  The rafters are on a 4/12 pitch to a 6"x12" ridge joist.

Answer
Darrell,

I'm assuming this is a wall plate and not the ridge beam. The specific design of the frame has everything to do with how much stress is on the beam and in which direction it is being pushed. In some frames, there is force pushing out on the plate as well as down. Generally, in a frame that has a supported ridge beam, there is no outward force on the wall plates; however, if the ridge beam deflects at all, the wall plates will be subject to outward forces as the ridge sags.

Ridge beams hold up half the weight of the roof, while wall plates hold up only 1/4 the weight of the roof. The ridge beam is normally only subject to downward forces, unless the roof is unevenly loaded, and so it can be narrow but deep.


As a 20'wall plate, the 6x12 is marginal in areas that have a snow load and will not do if the ridge is not fully supported. The 12'end wall beam must also be substantial if it holds up the ridge beam at midspan. I would use at least 10x12s for wall plates and the end wall beams. If you live somewhere you don't have a snow load, then you can easily use the 6x12s.

Without working with the frame plan and knowing the snow load, I can only offer you this general advice.

Daniel