I use premium pine dimensional lumber for my L-girders, risers, gussets and braces. I use 3/4" birch plywood for the module ends. I use 2x2 clear redwood balusters for my legs and some risers, braces, skyboard and fascia mounts.
I haven't had any problems whatsoever with warpage, and I don't use joiner tracks.
Some of my modules are over 15 years old, and are just fine.
At one time I was worried about warpage, so I sprayed my basic module benchwork with white sealer. Worked okay, but my other modules which mate up to the ones I sealed haven't warped either.
Finding straight lumber is becoming more and more difficult. I buy mine from HD, and I check every piece I buy for straightness. Since I buy premium pine, I don't worry about knots.
My benchwork for 6' modules, with folding legs, quality hardware, nice adjusting glides on the folding legs, laminated Masonite subroadbed, Masonite skyboard and fascia and 2" Styrofoam scenery base runs me about $100 bucks, which I don't consider to be exorbitant.
However, I live in a desert State, but the humidity here varies because I live in the mountains near three ski resorts, where there's lots of snow.
Maybe warpage would be more of a problem in locations where there's high humidity.
Here's a pic of my pine benchwork:
As an added bonus, premium pine dimensional lumber is considerably lighter than plywood. In my own module construction, I use plywood only on benchwork parts that get a lot of stress, like the ends.
I use premium yellow carpenter's glue, I pre-drill clearance holes for the screws which hold things together while the glue dries, and I countersink 99% of the holes. I also use either square drive screws or star drive screws, which greatly speed things up.
I also build my modules on an old institutional solid core door (3'X9') which provides a very straight surface, so my modules are pretty square.
You won't have a problem finding the correct length premium pine boards at HD.
Good luck!
