Wood swells, shrinks as moisture changes
Wood is a true gift from God. We tend not to think of trees as an agricultural crop like tomatoes, wheat or corn, but in many cases they’re not much different.
Any farmer who is lucky enough to have a stand of maple trees, aka a “sugar bush,” will tell you that the first crop of the year is maple syrup. Then, of course, there’s the lumber value. Trees are routinely harvested here in New Hampshire, but the growing season is not 120 days like most fruits and vegetables; it’s decades.
We are lucky in this country to have abundant forests. The lumber harvested from them allows us to build durable homes, outdoor decks, pergolas, play sets, forts, furniture and countless other projects.
While wood has many positive attributes, it has one negative quality that can cause homeowners enormous frustration and expense. Wood happens to be hygroscopic. This means it can absorb liquid water or water vapor. It can also release both, much like you squeeze water from a sponge.
The issue with wood is that when it absorbs or releases water in either state, it can swell or shrink. As if this movement is not bad enough, the swelling or shrinkage is not equal across the length, thickness and width of any given piece of lumber.
I’ll share two stories to illustrate this point. Almost 40 years ago, I built a stunning three-story Queen Anne Victorian replica home for my family. The joists used for the first and second floors were 2-by-12s cut from Southern Yellow Pine trees. The third floor was framed with 2-by-10s.
I also had two very tall chimneys that extended from the basement up through the roof. When we roofed the house, I had to install tin-coated steel flashing to prevent leaks. Some of this metal was
embedded into the mortar joints of the chimney, and the rest of the flashing was attached to the wooden roof sheathing.
The metal counterflashing that was attached to the chimney was cut so a quarter-inch gap remained above the step flashings on the roof. Ten years later, I was up on the roof installing a solar-powered attic fan that ended up being a waste of time and effort. I happened to look at the chimney flashing and was stunned.
The gap between the two flashings was now 2 inches! You’ll hear builders and remodelers blame cracks in houses all the time on settlement, making you think that your entire
house is sinking into the soil.
If this had been the case at my home, there would have been no gap because the chimney footing and the house footing would have both dropped into the soil. Yes, I know as a college-trained geologist that some houses can sink into the soil, but that’s not the cause of most cracks, and it most certainly was not the cause of the huge gap between my flashings.
The cause was lumber shrinkage. My 2-by-12 and 2-by-10 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 roofridge 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. As we learned in high school physics class, 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 that 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.