Mind the Gap

Jeff bought his home in the middle of summer several years ago. The first winter, a gap opened in the drywall between the gable-end wall and the vaulted ceiling. The gap seems to close every summer and open again every winter. Jeff hired PERCH to diagnose the problem and recommend solutions.

What on Earth was going on here? My initial investigation ruled out several possibilities. The gap is seasonal, not progressive (although Jeff does think it gets worse each year), so it doesn’t indicate a problem with the superstructure but something environmental. Frost heave seems unlikely, as Jeff has no uneven floors or major cracks elsewhere in the house.

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The gap between gable-end wall and ceiling.

For a little while I was wooed by the idea of truss lift. Roof trusses are known for expanding and contracting in cold climates: in winter, the bottom chord stays warm and damp because of its exposure to inside air, while the top chord and webbing gets cold and dry right below the roof. The top chord and webbing contract and pull up the bottom chord. But Jeff with his vaulted ceiling clearly doesn’t have roof trusses. They must be rafters, no deeper than 2×6, for which any differential contraction would be barely visible.

The best-fitting explanation was not frost heave, but a different kind of heave. Certain soils (typically soils with a lot of clay) are known as expansive soils because they collect groundwater and expand during wet seasons, then lose the groundwater and contract during dry seasons. If the gable end wall was built on an expansive soil, it would tend to drop in the winter and rise in the summer.

But wait. Wouldn’t the rest of the house move, too? I found evidence to the contrary during my site visit. On the first floor, a wall adjacent to the offending wall had a few minor cracks in the drywall. Directly below, in the basement, a crack ran across the plaster covering the concrete foundation. Aha – the walk-out side of the basement is framed by a stud wall, which is much lighter than the concrete walls on the other three sides. The soil under the stud wall hasn’t compacted as much as in the other locations, so it’s more susceptible to subsidence when the soil contracts.

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Cracking in the first-floor wall, about 6 feet from the corner.

Sometimes structural engineering is like solving a mystery. I search for clues and weigh possibilities against the evidence, and hitting upon the right answer is very rewarding.

“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”

Sir Arthur Conan Doyle

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