Thirty Inches of Difference: Rainfall and Clay

Most foundation advice treats rainfall as a background condition. On soil like this county’s it is closer to a load case.

The numbers

NOAA’s 1991 to 2020 climate normals for the Murfreesboro 5 N station, an official Historical Climatology Network site five miles north of town, put annual precipitation at 55.92 inches. That is well above the national average, and it arrives across roughly 134 days a year with measurable rain, 16.6 of them exceeding an inch.

The monthly distribution matters more than the total:

Month	Precipitation
June	5.68 in (wettest)
December	5.54 in
March	5.29 in
January	4.66 in
August	3.61 in
October	3.46 in (driest)

Then the annual figures. The same station recorded 40.36 inches in 2007 and 70.72 inches in 2020, repeated almost exactly in 2021. Roughly a thirty-inch spread in annual moisture, falling on the same soils, under the same houses, within a fifteen-year window.

Why that matters here specifically

It would matter less on sand. On the clay mapped across Rutherford County it matters a great deal, because these soils change volume with moisture content.

The USDA measures this as linear extensibility, defined as the change in length of an unconfined clod as its moisture content drops from moist to oven dry, reported as a percentage for the whole soil. Rutherford County series include Ashwood at 9.6 percent, Mimosa at 9.2 percent with clay content over sixty percent, and several others above seven percent. Three county series carry the taxonomic prefix Vertic, which USDA soil classification applies specifically to soils that shrink, swell, and crack.

A soil with that behaviour, cycled through a thirty-inch difference in annual moisture, is doing real work on whatever is bearing on it.

What it looks like in a house

Two patterns, and they are worth telling apart.

The annual cycle. Doors bind through a wet winter and swing freely again by late summer. Hairline cracks open slightly and close. A tiled floor develops a faint line that is more visible in February than in September. This is the soil breathing, and if nothing is cumulatively worse year on year, that is generally what it is.

The step change. A crack appears after a particularly wet stretch and does not close again the following summer. A wall that was plumb is now leaning. A door that used to bind now no longer closes at all. That is movement rather than a cycle, and the giveaway is that the dry season no longer undoes it.

The way to distinguish them is to record widths and dates. On this ground, a single observation in February tells you almost nothing; the same crack measured in February and again in September tells you a great deal.

The dry-down is the underrated half

Most people associate foundation problems with rain, and lateral pressure on a wall is indeed a wet-weather load. But settlement on expansive clay is frequently a dry-weather event.

Look at the normals again. October averages 3.46 inches and August 3.61, and both of those months carry mean temperatures in the seventies, so evaporation is high exactly when rainfall is low. Clay under and around a foundation gives up moisture, contracts, and the support beneath a footing reduces. Add a mature tree drawing water from the same soil and the effect concentrates on one side of a house.

That is why a homeowner will sometimes report that everything was fine all winter and something appeared in September.

What to do about it

You cannot change the soil, the rock, or the weather. You can change how much of that 55.92 inches reaches the ground next to your foundation, and stabilising soil moisture is the whole game.

Roof water first: an inch of rain on a modest roof is well over a thousand gallons, and a downspout discharging at the corner is putting a large share of it into exactly the soil you want to keep stable. Extensions carrying it clear of the wall, then a ground surface that falls away from the house, do more than anything else available at that price.

The water management guide works down from the roof, and the soils and bedrock guide covers what the survey records under this county.