Monsoon to freeze: what a Prescott building actually goes through.

A building in Prescott gets tested twice a year, and the two tests are connected. First the monsoon soaks it. Then winter freezes what the monsoon left behind. Neither one alone would do much. Together they are the reason old stucco spalls, mortar crumbles and paint lets go.

The monsoon

The National Weather Service defines Arizona’s monsoon as June 15 through September 30, and calls it the most dangerous time of year weather-wise in the state. For a building the hazard is not drama, it is volume and direction: a large share of the year’s rain arrives in two months, often driven sideways by thunderstorm winds.

Per the Arizona State Climate Office, using NOAA 1991–2020 normals, Prescott averages about 12.9 inches of precipitation a year — and July and August together account for roughly a third of it. Rain that arrives that fast finds every open joint, failed sealant and blocked scupper you have.

Then the freeze

Prescott sits at about 5,200 feet and runs roughly 120 to 130 nights a year at or below freezing on current NOAA normals. What that does to a wet wall is mechanical, and the NIH Office of Research Facilities describes it plainly:

Freeze-thaw damage occurs when water fills the voids of a rigid, porous material and then freezes and expands. The volume of frozen water is 9% greater than liquid water, so when water freezes pressure is exerted on the surrounding material, and when the pressure exceeds the tensile strength of the material, cracks will result.

NIH Office of Research Facilities, Technical Bulletin 84, January 2019

Two failure modes follow. Surface spalling, where the outer layer fractures to the depth the water reached and falls away — that is the flaking you can see. And internal cracking, which as the same bulletin notes “starts on the interior, which may not be evident to visual inspection.” The damage you can see is the smaller half.

When to look at what

Rather than invent a schedule, the sensible starting point is the inspection frequency chart published by the National Park Service in Preservation Brief 47, which is written for exactly this — keeping the exterior of a modest building sound. Its guidance:

The May list

Before the monsoon, the job is to make sure water that lands on the building leaves it:

Clear gutters, scuppers and downspouts and flush them through. Check that gutters still fall toward the drains and do not pond. Look at roof-to-wall junctions, parapet caps and flashings. Check sealant joints at windows, doors and any penetration — pipes, vents, conduit, light fixtures. Confirm grade still slopes away from the building.

The October list

After the monsoon and before the freeze, the job is to close what the summer opened:

Re-check the drainage you cleared in May, because monsoon debris is exactly what blocks it. Seal open cracks in stucco and masonry. Replace failed caulking at trim, corners and penetrations. Repair damaged siding, fascia and eaves. Look for staining or streaking below gutters and valleys, which marks where water has been running where it should not.

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