January, 2007

Why Miters Move

Using new-growth wood for millwork means finding ways to avoid future joint failure.

By Gary Katz

My father used to buy his lumber on the street, literally. Right after the war, during the San Fernando Valley building boom of the early 1950s, he’d stand out on Ventura Boulevard and wave down lumber trucks coming into town from Oregon and Washington. If the truck had a load of 2x6s, that’s what he’d frame the roof and floor joists with; if the trucks was loaded with 2x8s, so be it. I wish I could get my hands on a truck load of that wood today. It was all old growth Douglas fir. His carpenters made everything from it, including the the trim on the outside and sometimes the inside of his homes. The growth rings were so tight the wood shed water as if it were waxed, so it didn’t swell or shrink much at all. And it held paint for years—the final test of durability.

Why Wood Moves

Why does wood movement matter to lumberyards and builders?

Wood movement matters because reputations depend on durability, and product failure claims cut dealer profit margins.

Until we change our building traditions and habits, until we stop installing exterior trim as if it were old-growth wood, we’ll continue to experience product failures, high claims rates, and damaged reputations.

To achieve the durability of old growth wood while using new growth lumber and millwork, builders must adopt new installation practices. And dealers can help.

Old growth wood, made up mostly of dense winter growth rings, doesn’t absorb nearly as much moisture as new growth wood. But soft summer growth rings, which make up the majority of new growth wood, absorb moisture like a dry sponge. Any carpenter who has been splashed in the face while cutting into a new 6x6, or has seen water rise around a framing nail, knows that new growth wood can hold a lot of water.

We don’t have to get into the science of hydroscopic materials to understand wood movement. The problem facing our industry is simple: new growth wood moves more than old growth: It swells as it absorbs moisture; it shrinks as it dries out. During summer months in the Northeast, wood swells from increased humidity, while in the cold winter, wood shrinks, especially indoors when heating systems dry the air even more. Conversely, in the Southwest where humidity and temperature swings are mild, wood swells during wet winter months, and shrinks while drying during warm summers.

Why Miters Open

When wood swells and shrinks, it moves mostly against the grain, not nearly as much with the grain. Movement along the length of boards, especially where exterior trim boards are joined over longer spans, creates unique problems for trim carpenters, and we’ll review those issues in a future article. For now, let’s concentrate on solving wood movement at joints where boards and moldings change direction: miters. And there’s no better example of miter movement than wide casing.

A miter cuts diagonally across the grain. At the toe of the miter—the long point—there is no cross grain, just a sharp tip. All the cross grain material is at the heel of the miter—the short point. No matter how perfectly a miter might be cut, if the casing is installed with a low moisture content and swells after installation, the angle of the miter will decrease and the heels will press against each other, forcing the miter open at the toe or long point. Conversely, if the casing is installed with a high moisture content and dries out after installation, the angle of the miter will increase as the material shrinks, and the toes will press against each other, forcing the miter open at the heel or short point.

Moisture Meters

For decades I’ve heard contractors say that moldings and flooring material—all finish wood products, must “acclimate” to the job. I’ve heard some pretty specific timetables for acclimation, too: Some people say acclimation requires 24 hours, though I’ve also heard 48 hours. I’ve been told one week for acclimation by some authorities and two weeks by others; I’ve even heard a few people say 28 days, though I think they’re confused with the time it takes concrete to cure!

The truth is, acclimation can be double-edged: Most wood trim is shipped to the job site with a moisture content of approximately 8-10%.

If the job site environment has high humidity (freshly poured concrete, rain-soaked rough framing, new lightweight floors, recently finished plaster walls, and so on), the trim material will “acclimate” to the site by absorbing moisture and swelling prior to installation. Once the season changes, especially when the heat comes on in the winter, the trim material will shrink, sometimes dramatically, resulting in miters opening at their short points.

Conversely, exterior trim material shipped to the site and installed with an 8-10% moisture content will swell after installation; few climates, except the Southwest, have year-round humidity levels low enough to maintain that moisture content—which explains why miters on exterior trim always open on the long point.

So yes, trim material should acclimate but not always to the condition of the job site at the time of installation. For instance, carpenters installing exterior trim in Louisiana should allow material with a moisture content of 10% to acclimate and rise to a moisture reading of 14-18% before installation; carpenters installing interior trim on the East Coast during the summer should be sure the job site has a working dehumidifier prior to stocking material in the home.

Because wood movement and joinery failure result from the relationship of pre-installation and post-installation moisture content, using a moisture meter and knowing your environment is now a prerequisite for designing and installing durable trim applications.

Reinforcing Interior Miters

Jim Chestnut, a seasoned finish carpenter with experience all along the East Coast, likes to tell this story: “How many times has a pop or a snap awakened you suddenly in the middle of a cold winter night? You always lay there listening to the downstairs, thinking someone has just taken a step in your living room, wondering if you’ll hear the next step on the stairs.” Jim always saves the punch line for last: “That’s just a glued miter joint snapping open.”

Because we’re using new-growth lumber for all our millwork, we can no longer install casing, baseboard, chair rail, and crown the way my father’s carpenters did—gluing each corner and nailing the molding in place. Today, miters must be reinforced, and most methods actually speed production time.

Several ways to reinforce miter joints exist so that they can withstand most wood movement in the interior of a home. But first of all, carpenters must recognize that glue, by itself, won’t reinforce a miter joint. Glue reaches its full strength only when the joint is clamped under pressure. So always clamp your miters.

Exterior Miters

As for using miters for exterior trim work…just say no. Read the installation instructions for every new exterior trim product—from compressed fiber to engineered wood—and you’ll notice that miters are not ‘recommended.’ That means manufacturers won’t warranty miters on exterior trim. Sure, there are times when stain grade work requires a miter, but specialized joints, like lock miters, should be used for reinforcement. Look for more on that subject in my next article on Caulk-Free Columns, where I’ll talk about the five straight D’s that ensure a passing grade in construction today: Design, Deflection, Drainage,  and Diffusion—all of which create Durable Exterior Trim. Those five D’s will also ensure your company a long-lasting reputation for craftsmanship and care, and they’ll prevent costly product claims, too!

(Featured in this article was Information from many specialists, including Bill Robinson, Norm Slavik, Steve Easley, Joe Lstiburek, Jim Chestnut, and others. For additional information see: www.durable-wood.com, the web site for the Canadian Wood Council.)