How Southwest Florida’s Rainy Season, Salt and Humidity Wear on Commercial Buildings

What the weather and the coast do to concrete, roofs and walls in Sarasota and Manatee County, using government and university sources. Where we could not verify a claim, we left it out.

The rainy season, in the National Weather Service’s words

The NWS defines the Southwest Florida rainy season as May 15 to October 15^[1]. It describes three phases: late May to June, when the most severe storms are likely; July to early September, the peak, with heavy rain; and mid-September to early October, when it winds down^[1]. Thunderstorm season is usually firmly established by the second or third week of June^[2].

Sarasota-Bradenton’s normal annual rainfall is 49.05 inches (1991-2020)^[4]. Here are the monthly normals in inches^[3]:

Source: NOAA NCEI 1991-2020 monthly normals, Sarasota-Bradenton airport. June through September add up to 29.55 inches, roughly 60 percent of the year. That total is our arithmetic from the NOAA figures. 2024 was far above normal at 78.07 inches^[4].

Warm, wet and shaded is what mildew wants

UF/IFAS Extension says mildew requires moist, warm air, grows best between 77 and 86 degrees, survives from 32 to 95 degrees, and flourishes in dark areas with poor air circulation^[5]. In a Florida summer, that describes the shaded side of a plaza, the base of a garage wall, a covered walkway and a flat roof that stays damp. It is what we see in our own photos: green growth along the base of a garage wall, and a green-gray flat roof.

Before

After

Left: before. Right: after. Two spots in a parking garage, both photographed after dark.

Salt and concrete

FDOT classifies a structure as being in a marine environment if it is over or within 2,500 feet of a body of water with chloride above 2,000 ppm^[6]. That is a design classification for structures, not a measure of how far salt spray travels, and we did not find an authoritative figure for that, so we do not quote one. FHWA explains the mechanism: the high pH of concrete pore water forms a protective film on the steel, chlorides can break that film down and start active corrosion, and the corrosion products then crack and spall the concrete^[7]. That is a reason for building owners to have reinforced concrete inspected. Pressure washing does not repair corroded steel, and we will tell you if what we see needs an engineer rather than a wash.

Hurricane season and debris

The Atlantic hurricane season runs June 1 to November 30, and the peak is around September 10^[8]. After Hurricane Ian, about 113,647 cubic yards of debris were removed from roads and public property in the City of Sarasota^[9]. Storm years leave leaves, debris and residue on roofs, decks and walkways.

What we could not verify

• How far salt spray travels inland. We found only vendor claims, so we do not repeat them.
• Humidity normals for Sarasota-Bradenton. The NOAA fields were empty for this station.
• An authoritative source on how sand accumulates on and wears hardscape.

What to do with this

• Plan big exterior washes around the rainy season, and expect growth to return fastest on shaded, damp surfaces.
• Look low and look shaded: garage wall bases, north-facing walls, walkways under overhangs and flat roof drains.
• Have reinforced concrete near salt water inspected by an engineer. Wash it to keep it clean and visible, not to fix it.

Property managers can see our commercial pressure washing and the areas we serve. Where the wash water goes is covered in our wash water guide.

Sources

1. NWS Tampa Bay: the rainy season
2. NWS Tampa Bay: summer rainy season summary (PDF)
3. NOAA NCEI 1991-2020 monthly precipitation normals, Sarasota-Bradenton airport
4. NWS Sarasota-Bradenton climate data (2024 total and annual normal)
5. UF/IFAS Extension: mildew removal
6. FDOT Structures Design Guidelines (marine environment classification)
7. FHWA: corrosion of steel in concrete
8. NOAA National Hurricane Center: hurricane climatology
9. FEMA: Hurricane Ian debris removal, City of Sarasota