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Miter Joint Movement |
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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. |
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| 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. |
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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. |
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| 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. |
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| 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. |
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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. |
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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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Supported by corporations who care about education in the construction industry. |
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