Ask the Builder: Wood needs room to swell and shrink as moisture level changes

Tim Carter, Tribune Content Agency on Mar 29, 2024

The cause was lumber shrinkage. My 2x12 and 2x10 floor joists from the lumber mill were no longer as wide as they had been when I nailed them all together. It turns out that lumber shrinks or swells the most across the width, or face, of a board. It shrinks the least along its length.

Just days ago I employed this knowledge to help a woman in Ohio. She purchased one of my phone coaching calls. She called me because her beautiful new sunroom addition had a serious problem with the ceiling made from real wood. The room has a steep vaulted ceiling faced with tongue-in-groove maple beadboard.

She sent me photos of the buckled wood up near the peak of the vaulted ceiling. It was a huge mess. I requested a photo of the floor of the room. I suspected that the homeowner, much as my wife would do, had filled the room with many large plants. My hunch was right.

I asked about the roof ventilation and the insulation used in the ceiling. It turns out the builder made two huge mistakes. First, he used closed-cell spray insulation. This product does not allow water vapor to pass through it. Second, he hadn't created a pathway at the peak of the roof for air to escape out to the roof ridge vent.

The houseplants in the room were belching water vapor into the air. This water was concentrated in a layer of very warm air at the top of the vaulted ceiling. I’m sure you remember from your high school physics class that warm air can hold more moisture.

The wood at the top of the ceiling was absorbing all of this water and expanding. It’s easy to solve the problem. She just has to remove the last row of beadboard on each side of the vaulted ceiling. To hide this gap, she can nail the same stained beadboard to a piece of plywood that will form a flat surface up at the peak of the ceiling.

This flat piece will have beveled edges and it will be secured to the roof ridge board. It’s vital this flat board not be secured to the beadboard. The beadboard ceiling will now have a generous 3-inch space on each edge of the ceiling to expand and contract as the seasons change.

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