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  Miter Joint Movement  
How to Succeed (or Fail) with Exterior and Interior Trim

Simple, un-reinforced miter joints are the bane of carpenters everywhere, especially where humidity levels vary dramatically from winter (dry cold air—particularly with the forced-air furnace on) to summer (hot moist air…at least until the air conditioning is turned on). Understanding seasonal wood movement, and moisture content, is the key to saving your hard work from ruin. Here’s the reason why miters open and several methods for installing long-lasting, durable, interior and exterior trim.
Always check the moisure content of the material before installing it. As Jim Chestnut writes: "In a New England home, with dry winters and no humidifier, an 11% moisture content is at least 3% too wet to install 4 1/2" casing without biscuited or splined and clamped joints. And it would be really pushing the limits with the biscuits." If the moisture content of the material is over 15% at the time of installation, even miter joints reinforced with biscuits or splines might snap open in the winter.
Interior miters rarely open on the long point unless the humidity inside a home rises above dangerous levels, for instance, when you see water leaking down the drywall from an upper roof deck and it's dripping off the door or window casing, chances are the long points of the miters are open. But for exterior trim, high-moisture levels are the number one cause of joint, paint, and wood failure.
Miters on columns and exterior trim are particularly susceptible to movement. Rather than drying out and shrinking like interior casing, exterior trim often takes on moisture and swells. You’ve all seen this happen. Preventing moisture damage and installing long-lasting exterior trim requires careful design and construction techniques.
The long point of the miter on this deck wrap is just beginning to open up, but the moisture damage is already irrevocable. The rising grain in this swollen material can not be repaired.
Even though the raw edges of this finger-joint pine exterior column have NEVER been primed, the material has held up pretty well to six months of alternating sprinklers and sun (other than a few end-grain cracks on the plinth), testifying to the benifits of incorporating a good drain-plane design in all exterior trim applications. Had the carpenter painted the trim properly before moving on to some other project (another article), the column would have withstood the sprinklers, the sun, and his dog, Whitney, too.
Sandwiching two pieces of material together traps moisture and causes rapid material failure. Always separate exterior finish material from any rough framing. Exterior kiln dried trim is shipped with a 10-12% moisture content, while rough framing and pressure-treated lumber is often saturated with water. Without a barrier between the two materials, moisture in the rough framing will be drawn into the drier surface trim. The sun speeds that process, forcing the moisture through the surface material and breaking down the primer and paint, then the joinery, and finally the material itself. Because of wood movement, try to avoid miter joints in exterior trim. Butt joins are more durable, which is why exterior trim manufacturers recommend butt joints over miter joints. However, if miter joints are required, take adequate precautions against wood movement (see future article on this site). GreenGuard RainDrop is good choice for creating a drain plane. An all-in-one house-wrap and drain-plane channel, RainDrop provides air circulation without the need of a separate house-wrap or bug screen. Home Slicker Plus Typar is another alternative for exterior housewrap. This product combines Home Slicker with Typar housewrap in a single application. The material is 1/4 in. thick and exposed areas (bottom of siding, joise wrap, etc.) require a bug screen (shown above).

And finally, always protect the top of the trim and prevent water intrusion between the two materials by flashing over joists and rim joists with a self-healing membrane. Cut the membrane flush with the face of the trim and the back side of the joists so that it's not visible.
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