Expert: Richard Taylor, AIA - 3/24/2009

Question
I'm planning a pergola for my backyard.   I'm planning 18' X 18' in size.  I would prefer not to use 6X6 timbers for the posts.   I would like to use 2X6 lumber but I'm not sure how to laminate 3 boards together for each post?   Do I nail and glue it or bolt and glue or what?   I'm trying to eliminate the problem of warping and twisting of 6X6 posts.

Answer
John - thanks for your question.

Yes, you can laminate the 2 x 6s if you want.  Since it's an outdoor application you must use marine-grade structural adhesive.  Any nails or screws you'd use would just be to hold the pieces together while the adhesive cures - you could clamp the pieces if you want and eliminate the fasteners altogether.

But this doesn't sound like a very attractive solution.  Warping and twisting of lumber is a function of the quality and species of the wood, and of the amount of moisture it holds.

Many people mistakenly assume "pressure-treated" lumber is the only choice in such situations, and most consumer home improvement stores perpetuate that myth.

Actually PT lumber is a poor choice whenever appearances are important - for just the reason you're concerned about.  The "green" boards you see in the store have much higher than normal moisture levels (ever notice how heavy they are?) because the copper chromate used as rot and insect treatment is forced into the wood under pressure with water - lots of water.

When the boards are removed from the pressure vessel they're like big green sponges that begin drying out on the way to the store.  When you install them they're still quite wet.  And the lumber used for pressure treating is not highly graded.

But they eventually dry (unevenly) while they're in your backyard and that's when they split, twist, crack and warp.  Not a good look!

A much better choice is to use one of several naturally rot and insect resistant wood species - redwood and cedar are the two most common choices.  If you buy the highest grade you can afford, and get kiln-dried wood, you won't see the problems that you've described, and you'll have a much more attractive pergola.

Both species are expensive, but if you just use it for the posts you'll keep costs minimal.

Depending on your region you can get other species that have the same properties - check with your local commercial lumberyard, or ask a landscape design firm in your area.

Finally, be sure you install each post with a base that holds it up off the ground a bit (don't set the posts directly in concrete) to keep moisture out of them.  Simpson makes lots of different bases for just this purpose (www.strongtie.com) and most are available from lumberyards and home improvement stores.

Hope this helps!

Richard Taylor, AIA
www.rtastudio.com
www.rtaplans.com