Expert: Stephen Major (Owner--Major Design Group) - 5/31/2007

Question
QUESTION: Hi.. I'm planning on building a pergola.  If the beam span is 18' and the rafter span is 18', what size should the rafters be?

ANSWER: Rich,

I need more information to answer this question accurately.  I'll make several assumptions,  and you can let me know if they need correction.

I assume that the pergola will not be supporting any live loads (people, snow, objects).  If this is the case then the rafter (technically a joist if it is not sloping) just needs to be deep enough to not sag under it's own weight, and rafter spacing is not an issue either.

I would opt for either 2x8 or 2x10 material.  Your design should include a grid of crosspieces fastened to the top of the rafters to hold them in position laterally and to counter their tendency to twist.

However, if your design calls for thicker rafters, then you can reduce their depth.

What are your beam dimensions, out of curiosity?

Steve Major

---------- FOLLOW-UP ----------

QUESTION: Correct.. No live loads on the pergola.

Regarding the beam dimensions.. I meant to ask about that as well.. Can you tell me what size beam should be used?  Two 2x10s?  Also, is it best to cariage bolt each beam onto each side of the posts OR to put the beams on top of the posts?

Also, there will be one beam and one ledger board (pergola is being attached to the house).  Can you tell me what size ledger board should be used?.. 2x10?. AND how do the rafters typically connect to the ledger board.. Do they rest on top or are they attached to the front of it with brackets?

Answer
Rich,

Have you considered adding a third post at midway on the beam?  This would probably be more visually pleasing.  18' is a long span.  

I would use 2 or 3 2x12s for the beam.  Connecting them with carriage bolts is nice, as it adds stability to the beam and also makes it appear larger.  If your posts are 4x4, then simply bolt directly using three 1/2" bolts at each end.  If your posts are 6x6, then you can notch them with, say, 1" bearing surfaces.  The remaining post material will continue up through the beam space.

At the house you have several options.  The one I like best is to use a doubled ledger of the same dimensions as the beam, buried into the siding.  Lag-screw the first member securely to the house, being sure to penetrate studs or other solid structure at least 2-3".  Stagger  3/8" or 1/2" lags at 24" to 36".  The second 2x member is nailed or screwed to the first, to conceal the lag heads (drill out clearance holes in the back side).  Flash the top and rest the joists on it.   Secure them with carefully driven toe-nails or toe-screws, through clearance holes (to avoid splitting the joist).

You can view a similar pergola at our website gallery at:
http://www.stephenmajor.com/gallery.html

Alternatively you could lag a single wide ledger to the house, and a second 2x2 bearing ledger at the bottom, upon which the joists will rest.  Use toe-nails or screws, angle brackets, top straps, or a similar method of positive secure attachment.

Best of luck with this project.  

--Steve Major