Another lesson in durable exterior trim
A few years ago, I wrote an article on Caulk-Free Column Wrap—a column that can be installed without having to come back in six months and caulk the corners or the trim. In this article I’ll take a second step in the same direction, and add some serious and much-needed improvements.
I pinch myself a lot. I’m always amazed at how lucky I am.
Fifteen years ago, I used to run to a pay phone when I got a page with a “911.” Now I send email from my iPhone—with spreadsheets and PDF files attached.
Ten years ago, I used slide film in my cameras. What a bear. And I carried a lot of gear: lights, stands, cords, scrims, flags. Sometimes sandbags! Now I use digital cards in my video camera, and can almost shoot film in the dark. What a great time to be a photographer—to learn the hard way; and be able to work the easy way.
What a great time to be a carpenter, too.
Sure, the economy hasn’t been this bad since the 1930s (and back then, they said the country would never recover!), but there’s still a lot to be thankful for. Here’s just one example:
About ten years ago, after seeing my work get torn apart by wood movement and early decay, I started using engineered wood and PVC trim. Now, like a lot of quick-thinking craftsmen, I’m using both materials together. I’m glad I learned the hard way; but I’m happier working the easy way.
Historic Design with Contemporary Tools & Materials
In my other article on caulk-free column wrap, rather than wrapping the rough post first, then installing the plinth or base detail on top of the column wrap (where wood movement tears up all the joinery!), I described a method for building columns from the plinth up. Let’s start right there.
So that the plinth will be proud of the column wrap, back out the bottom of the column, then install the plinth section. Because the plinth always rests on grade—on concrete, stone, brick, or a deck—use PVC trim for the plinth.
The Base Cap
Baseboard mimics the plinth on a classical column, that’s why base cap molding usually has a torus shape. But craftsman-style homes rarely incorporate that profile; square edges are more common. This is a detail that you can not do with wood because there is no pitch for drainage on this square base cap. Use PVC trim because it won’t be damaged by sprinklers, standing water, or even a pile of snow.
The Column Wrap
I wanted to demonstrate how to mix materials, so I used wood for the wrap on this column—though PVC could have been used, too. For a stain-grade newel, I might still opt for a paint-grade plinth—especially in this style, with so many flat surfaces that collect standing water.
The cap must be made from PVC, too, because it is dead flat. Had I used wood, the cap would need to be pitched like a hip in all four directions.
Cutting the Flutes & Tapers
I used a Domino to reinforce the butt joints on the column wrap. First, I eased all the edges with a 1/16-in. radius bit so the joints would be emphasized—in other words, so you’d see them immediately, which is a common feature on Greene & Greene Homes. Emphasized joints are also neat because any cracks in the paint at the butt joint are completely hidden.
|I used the Dominos to align the emphasized corners, not the top and bottom, so I cut wider mortises for each domino, which made it a lot easier to assemble the sides for glue-up.|
Expansion & Contraction
Wood and PVC move at different rates. Wood moves because of moisture content; PVC moves because of temperature. So the joint between the column wrap and the plinth and top cap must allow for expansion and contraction. But even if the whole column was made from wood, I’d want to include a caulk joint at the top and bottom, because the wood grain would be running in different directions—and that means different rates of movement.
Using backer rod is the only way to create a durable caulk joint, one that allows for expansion and contraction between dissimilar or similar materials. Here’s why:
If you don’t use backer rod, the center of the caulk joint is always too thick and deep, which means it won’t be flexible (see below)—it’s like a rock of caulking. And the edges, where you want the caulking to adhere, will be thinner than the center, weakening the adhesive bond right where you want it to be strong. That’s why caulking always cracks along the edges, sometimes splintering the wood fibers as the wood moves.
Not only is backer rod cheap, but it saves on caulking! Plus it ensures that the center of the caulk joint will be thin and flexible, and the outer edges will be much wider than the center. That’s the right recipe for maximum flexibility and maximum adhesion.
|Before installing the column wrap, I pin small 1/4-in. standoffs on top of the base cap—small chips of 1/4-in. plywood worked for me. Those spacers allow just enough room for 3/8 in. backer rod.|