Monsoon Season Is When Grand Valley Foundations Move

If you keep dated records of a crack on a Grand Valley house, the readings that change tend to be the ones taken in late summer. There is a reason for that, and it has a start date.

When the monsoon arrives here

The National Weather Service office in Grand Junction states that for western Colorado and eastern Utah the southwest monsoon generally begins around the second week of July, and that in most years it is over by the end of August, though it can last as late as October.

That is a narrower and later window than the July-to-September framing people often use, and the precision matters if you are trying to understand what your house does and when.

Why a few weeks of rain matters so much

Grand Junction receives about 9 inches of precipitation in an average year and about 18 inches of snow, according to NOAA 1991 to 2020 normals for the Walker Field station. In a place with that little water, the monsoon is not a modest seasonal uptick, it is a substantial share of the year’s total delivered in a handful of convective events.

Expansive clay does not care about the annual average. It responds to change in moisture content, and the sharper the change the more the soil moves. The monsoon delivers the sharpest change in the calendar, and it delivers it at the worst possible moment: onto soil that has just spent June, the hottest and driest stretch, drying out and shrinking.

So the sequence over a Grand Valley summer runs roughly:

1. May and June. Hot, dry, high evaporation. Surface soil desiccates and shrinks. Cracks in bare ground open. Foundations lose some support at the perimeter as the clay pulls back.
2. Mid-July onward. Intense storm cells. Runoff hits ground that has shrunk and cracked, so water finds those cracks and reaches depth quickly rather than soaking evenly.
3. Late July and August. The clay takes on water and swells, sometimes rapidly and unevenly, because water arrived through cracks rather than through the whole profile.
4. September onward. Drawdown begins again, and irrigation shuts off for the season on top of it.

Step 2 is the one that does damage. Water entering desiccation cracks reaches deeper soil far faster than uniform infiltration would, which produces localised swelling at depth rather than a gentle general wetting.

Why the shrinkage cracks make it worse

Bare ground in this valley develops a polygonal crack pattern in high summer. Those cracks are not cosmetic; they are a plumbing system. A storm that would otherwise wet the top few inches instead pours water directly into the soil profile through them, and it does so in whatever pattern the cracks happen to take, which is rarely symmetrical around a house.

That is one of several reasons a house can end up with one wall on swelling soil and another on soil that stayed dry. Differential is what racks a frame.

What to look for in the weeks after

If you have been keeping a crack log, late August through September is the highest-value time to take a reading. Compare it against a reading from May or June. Movement that shows up in that comparison and reverses again by the following spring is seasonal cycling on expansive soil. Movement that shows up and stays is progressive.

Other things worth noting during a monsoon storm, since they are only visible while it is happening:

• Where water stands against the foundation, which no amount of dry-day inspection reveals.
• Whether any downspout is overwhelmed or discharging at the wall.
• Whether water sheets off a slope toward the uphill side of the house, which is the classic Redlands and Orchard Mesa configuration.
• Whether a window well takes water and how quickly it clears.

Ten minutes outside during a storm produces better diagnostic information than an hour of walking the perimeter in dry weather.

What reduces the swing

The objective is not to keep soil dry, and it is not to keep it wet. It is to reduce the amplitude of the annual cycle.

• Keep watering steady through June rather than letting the ground desiccate. Counter-intuitive, but a soil profile that has not shrunk and cracked absorbs monsoon rain far more evenly than one that has.
• Get roof water clear of the house before the monsoon starts, not after. A downspout extension installed in June is worth more than the same extension installed in September.
• Even out irrigation rather than running long infrequent cycles. Frequent light watering holds moisture content steadier than deep soaking every few days.
• Check window wells and drains before mid-July. Debris that accumulates over spring turns a drained well into a container.

None of that is expensive and all of it is more effective than reacting after the fact.

Freeze on the other end

The monsoon is one half of the annual cycle. The other is winter. NOAA normals record around 137 days a year with a minimum temperature at or below freezing here, which is a considerable amount of freeze-thaw for a place with this little precipitation. That cycling acts on saturated near-surface soil, on flatwork joints holding water, and on concrete already suffering sulfate attack from gypsum-bearing ground.

A Grand Valley foundation therefore gets a wetting event in late summer, a drying stretch in autumn as irrigation shuts down, and a long freeze-thaw season after that. Three separate mechanisms, all annual, all repeating.

More on the moisture story in the irrigation and foundations guide, and on setting up a crack baseline in the crack types guide.