Q. I have been a contractor for over 25 years, but I recently ran into a new problem. We installed a 1x8 T&G pine soffit at the eaves and under the gable overhang on a new home here in Washington State last summer. The material is select tight-knot Ponderosa pine, kiln-dried and of good quality; it's coated on both sides with Messmer's stain. The work was done with summer temperatures ranging between 85° and 100°F. The pine was very dry. The boards were blind-nailed by hand with 6d galvanized shake nails. This winter, the boards have somehow expanded and buckled to the point that they're about to fall down in several places. There is no possibility of roof leaks. The house has vaulted ceilings throughout, but some of the worst areas are in the roof over the entry deck. The roof is vented at both the top with a ridge vent, and the bottom with a continuous vent. What is happening?
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 how.
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 water.
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.