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 2x6's, that's what he'd frame the roof and floor joists with; if the truck 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 trim on the outside and sometimes the inside of his homes. The growth rings were so tight the wood shed water as if it was 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 lumber yards 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's growth wood, we'll continue to experience product failures, high clams 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.

	Growth rings tell the age of a tree, but they also reveal how durable the wood will be. Soft summer growth rings, which make up the majority of the new-growth wood pictured above, absorbs more moisture than dense winter growth. Almost all the wood we use today requires more protection than the old-growth lumber our fathers' used.

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 hygroscopic 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 when 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 gain 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 time-tables for acclimation, too: some people say acclimation requires 24 hours, though I've also heard 48 hours; I've been told one week by some authorities and two weeks by others; I've even heard a few people say twenty-eight days, though I think they're confused with the time it takes concrete to cure!

The truth of the matter is, acclimation can be a double-edged sword. Most wood trim is shipped to the jobsite with a moisture content of approximately 8-10%. Fig. 3: photo courtesy of WindsorOne) If the jobsite environment has high humidity (freshly poured concrete, rain-soaked rough framing, new lightweight floors, recently finished plaster walls, etc.), the trim material will "acclimate" to the jobsite by absorbing moisture and swelling prior to installation. Once the season changes, especially when the heat comes on in the winter time, the trim material will shrink, sometimes dramatically, resulting in miters opening at their short points.

Conversely, exterior trim material shipped to the jobsite 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.

Today's carpenters and builders work with new-growth lumber, and moisture meters are as important as tape measures. Always read the moisture content of trim material before installing it; read the moisture content of the jobsite, too. Don't let material acclimate to a high-humidity jobsite where winter-time heating will dry out the interior. Before installing any trim, condition the air and allow the trim to acclimate to the environment of the finished home, not the jobsite.

So yes, trim material should acclimate but not always to the condition of the jobsite 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 jobsite has a working dehumidifier prior to stocking material in the home. Because wood movement and joinery failure results 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 finished carpenter with experience all along the east coast, likes to tell this story: "How many times has a pop or a snap woken you up 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.

There are several ways to reinforce miter joints so that they can withstand most wood movement on 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.

For small moldings, Collins Spring Clamps work well and should be left on until the glue has set—about ten to fifteen minutes.

Clam Clamps are the industry favorite for large casings, where more pressure is required. Pressure helps the glue set faster. On warm days, I've removed Clamp Clamps in little more than five minutes without joint failure.

One of the easiest and quickest ways to reinforce miters is with pocket-hole screws, applied from the back of the casing (www.kregtool.com). This process is easy with flat casing, a little bit harder with profiled moldings, but if the material is thick enough, pocket screws provide both clamping and fastening reinforcement for wide casings.

	Look also for David Collins' soon-to-be-released new clamping system for casing—Miter Works, an ideal platform for small and medium-sized moldings. With this tool, you can drill a pocket hole in the rear edge of the casing, drive a pocket screw, and drill plugs, too! (early prototype pictured).

Biscuits and splines are another method for reinforcing miters, and they provide additional gluing surface, too, which strengthens a joint. Biscuits can be installed using a biscuit jointer, while splines are easy to install with a router and 1/4-in. slot cutter, using 1/4-in. mdf for spline stock

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 notice that miters are not 'recommended.' That means manufacturers won't warrantee 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, 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!

(Portions of this article will appear in the new WindsorOne Exterior Trim Installation Guidelines, to be released in 2007. In the writing of this article I was assisted by 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 website for the Canadian Wood Council.)