A.Paul Fisette, a wood
technologist at the University of Massachusetts at
Amherst, responds: There is one thing that
will make wood swell, and that is adsorption of
moisture. By contrast, expansion of wood resulting
from an increase in temperature is insignificant.
Your boards were installed flat and are now
buckling. That tells us that they have gained
moisture since they were installed. The question is
You have ruled out roof leaks. One possible
scenario is that warm interior air, which contains
more moisture than cold outside air, is somehow
leaking into the soffit cavities and condensing on
the cold boards. The moisture content of the boards
would rise, and they could swell as a result of
increased relative humidity or the presence of
But a second scenario seems more likely. The
boards are installed on the underside of protected
gable overhangs and an attached porch, parts of the
house that are disconnected from the interior air.
It seems most likely that the boards gained
moisture through exposure to water vapor in the
outdoor air. Though you bought the boards
kiln-dried, you probably don't know what their
original moisture content was.
My guess is that they were installed too dry for
the current ambient conditions. Wood typically
equilibrates to around 12% moisture content for
outdoor applications. Perhaps the manufacturer
dried the boards to 8% MC, making them more
suitable for interior use. Or perhaps the boards
were mistakenly over-dried to an even lower
moisture content. That happens.
Another factor may have contributed to the
buckling. T&G boards up to 6 inches wide can
be properly blind-nailed as you did. However,
8-inch-wide T&G should be face-nailed twice
per bearing point. Using a tighter nail schedule
would not stop swelling, but it would restrict
movement and help minimize buckling.