Humidity, Clay, Post-Tensioned Slabs and Floods Under a Fort Bend Floor Coating

A floor coating in Fort Bend County has four local conditions to answer: wet Gulf air, clay that cracks as it dries, slab designs built for that clay, and floodwater. Each one has a public record. NOAA's humidity table has no Sugar Land row, but for Houston Intercontinental it gives average morning relative humidity of 85 percent in January and 93 in both July and August. The USDA Natural Resources Conservation Service types its Lake Charles clay in Fort Bend County and records that it cracks when dry. The City of Sugar Land requires post-tension designs for new houses to carry an engineer's seal. And the Brazos River at Richmond set its record crest in Hurricane Harvey, 55.19 feet by the National Weather Service's preliminary figure.

This guide sets each of those records beside the limits printed on coating data sheets and in the ASTM moisture methods, with the source named in the sentence that uses it. Airport readings describe outdoor air, not the air in your garage or the surface of your slab, so treat them as the conditions a job is planned around rather than a reading of your floor. The station tables, house ages and flood figures sit on our Sugar Land floor data page. On a job, Epoxy Flooring Sugar Land Co. takes its own readings: slab moisture by whichever method the product sheet names, then the air and slab temperatures, the relative humidity and the dew point, all before any resin is mixed.

Gulf humidity and the dew point against the data sheets

Humidity limits when a coat can go down because the data sheets set both a humidity ceiling and a dew point margin. The Sherwin-Williams sheet for ArmorSeal 1000 HS, a two-component polyamide epoxy (revised August 10, 2021), prints "Relative humidity: 85% maximum" and asks for a surface "At least 5 degrees F (2.8 degrees C) above dew point". The National Weather Service glossary entry for dew point defines it as "the temperature to which air must be cooled in order to reach saturation", and dew as moisture condensed on objects that have cooled below it. A slab below the dew point is a wet slab, however the air feels.

In NOAA's record, Houston's morning air sits at or above that ceiling in every month. The Comparative Climatic Data table for Houston Intercontinental (WBAN 12960, record 1969-2018) puts average morning relative humidity at 85 percent in January, its lowest month, and 93 in July and August, with afternoon averages between 56 and 62 percent; NOAA does not print the hours of either reading, and neither Sugar Land nor Hobby has a row. NCEI's 2023 Local Climatological Data summary for the same airport puts July's mean dew point at 74.5 degrees F and January's at 45.7 degrees F. These are outdoor readings at an airport, so the reading that decides a coat is the one taken in the garage.

The margin matters because of amine blush. Sherwin-Williams' technical article on it calls blush "a chemical reaction that occurs between water, carbon dioxide, and the amine component of epoxy coatings", one that can surface "When some epoxies are applied under low temperature and/or high humidity conditions", and warns that it "can act as a bond breaker between the coating layers". The article's rule is to stay "at least 5 degrees F above the dew point prior to and throughout the application and cure," and it says a fuel-burning heater "will also introduce moisture and carbon dioxide into the area, increasing the opportunity for blushing." Before every mix, our crews write down the dew point, the relative humidity and the temperatures of both the air and the slab surface, then time each coat inside the product sheet's window.

• ArmorSeal 1000 HS epoxy (Sherwin-Williams, revised August 10, 2021): "Relative humidity: 85% maximum"; "At least 5 degrees F (2.8 degrees C) above dew point"; application from 50 to 120 degrees F.
• Rust-Oleum one-part floor paint, technical data ROC-109 (Rev. 030325): "Apply only when air, material, and surface temperatures are between 50-90 degrees F (10-32 degrees C) and the relative humidity is below 85%."
• Spartacote Flex SB polyaspartic (Laticrete DS-864-0822): "Working properties based on 70 degrees F & 50% RH. Changes in ambient conditions may cause times to vary."

The clays under Fort Bend slabs, in the NRCS soil descriptions

The soil descriptions the USDA Natural Resources Conservation Service ties to Fort Bend County describe clays that pass water very slowly, and some of them crack as they dry. Its Official Soil Series Description for Lake Charles places the type location of that coastal prairie clay near Guy, in Fort Bend County, and records: "When dry, the soil has cracks 1 to 5 cm (1/2 to 2 in) wide at the surface", extending to a depth of 30 cm (12 in) or more, and "Cracks remain open for 60 to 90 cumulative days in most years." NRCS classes Lake Charles as a Vertisol formed in clayey deposits of the Beaumont Formation, rates it moderately well drained with very slow permeability, and calls the series extensive.

Bernard, a second prairie clay, was established as a series in Fort Bend County in 1952, with its type location near Tavener, which NRCS places about 10 miles west of Rosenberg; NRCS rates it somewhat poorly drained with very slow permeability and puts its extent at about 500,000 acres. In the Brazos floodplain, the Pledger and Brazoria clays formed in river alluvium. Pledger's description says "When dry, cracks more than 1 cm (0.4 in) wide extend from the surface", and Brazoria's records slickensides, which NRCS lists as vertic features, from 14 to 203 cm (5 to 80 in) down. Asa and Norwood, the loamy floodplain soils, carry no cracking or shrink-swell sentence at all.

Two things follow for a coated floor, both in the sources' own terms. The Post-Tensioning Institute writes of soil movement putting tension stresses into a slab-on-ground, so a crack in a Fort Bend slab is judged by whether it still moves, as section 3 explains. And Stonhard, a resin floor maker, says the ground beneath a slab "is almost always damp with a relative humidity of 100%"; over clay that NRCS rates very slowly permeable, that dampness is where the moisture tests in section 4 begin. None of these descriptions says which series lies under a particular house; they describe each soil and where it was typed.

Post-tensioned slab-on-ground foundations: what they change for floor work

A post-tensioned slab-on-ground is a foundation slab held in compression, which the Post-Tensioning Institute's slab-on-ground page presents as a solution for "problems associated with ground-supported residential foundations on shrink-swell soils." PTI says the compressive stresses "resist the anticipated tension stresses induced by the soil movements", that a uniform-thickness foundation is used on less expansive soils, and that post-tensioned foundations are used on stable soils too, "to reduce cracking." In Sugar Land the design is an engineer's work by rule: the City's residential permit page says "Post tension designs must be signed and sealed by a professional engineer."

According to PTI, the 2024 International Residential Code added a section for these slabs: the 2021 code's Section R506.1 referred only to R506 and ACI 332 for the design and construction of slab-on-ground floors, and the 2024 edition adds Section R506.2, under which post-tensioned slabs on expansive or stable soils "shall be designed in accordance with PTI DC10.5." The City of Sugar Land's adoption of the 2024 International Code Council codes, the residential code among them, took effect on January 21, 2026, with local amendments in Ordinance 2395; applications filed before that date are reviewed under the 2021 codes. The Foundation Performance Association, a Texas nonprofit for Houston-area engineers, lists papers such as "Guidelines for the Evaluation of Foundation Movement for Residential and Other Low-Rise Buildings" (FPA-SC-13-1).

Surface work and slab work are separate questions on a post-tensioned slab. Preparing for a coating works on the surface: ICRI Guideline 310.2R defines it as producing "a sound, clean and suitably roughened surface" and captions grinding as profile CSP 2. Cracks are judged by movement: the International Concrete Repair Institute's terminology calls a crack "that is still moving" active, ACI's FAQ on overlays says active cracks "need to be stabilized to avoid being reflected through the topping," and PTI's own technical notes include one titled "Cracking in Ground Supported PT Slabs." No title on PTI's FAQ and technical notes page mentions drilling, coring or cutting, so anchors for a lift or racking, and any cut, are questions for the engineer who sealed the design, asked before work starts.

Moisture rising through a slab: the three ASTM tests and their limits

Water vapor rising from the ground through a slab-on-grade can lift an impermeable coating, so the slab is tested before a primer is chosen. Sherwin-Williams describes moisture vapor transmission as vapor that "moves upward through the concrete slab from the ground," and Sika warns that when it is trapped beneath an impermeable film the pressure "can be sufficient to blow sections off or cause other forms of damage like blisters." Stonhard adds: "With concrete slabs that are over 25 years old, there is a risk that a moisture barrier was not installed." The median housing unit in Sugar Land was built in 1994 (ACS 2020-2024, table B25035), so a slab of the city's median age is past that mark.

Three ASTM methods do the measuring. ASTM F1869, the calcium chloride method, yields a moisture vapor emission rate "measured in pounds of moisture over a 1000 ft2 area during a 24-h period", and ASTM notes that the rate "only reflects the condition of the concrete floor at the time of the test." Under ASTM F2170, in situ probes set in test holes report the relative humidity inside the slab; Wagner Meters, a probe maker, places them at 40 percent of the slab's thickness when it dries from one side. ASTM D4263, the plastic sheet test, only indicates moisture, and Sherwin-Williams calls it qualitative. Simiron's Polyaspartic HS sheet warns that "seasonal variations in ground moisture can cause excessive moisture vapor transmission (MVT) regardless of results measured prior to coating application."

The pass mark belongs to the product, not the test, and the sheets disagree. Sherwin-Williams' flooring preparation guide, Form G-1, sets the F1869 limit at "not more than 3 pounds of moisture per 1,000 square feet per 24 hours" and calls probe readings of "80% or lower" acceptable under ASTM F2170. Laticrete's Spartacote Flex SB sheet caps slab humidity at 75 percent; Sika's Sikafloor-264 epoxy sheet allows 85 percent; Stonhard says its liquid-rich (non-mortar) systems need a moisture mitigation system above 80 percent. Sherwin-Williams' blistering article separates standard epoxies, which it calls not moisture-tolerant, from formulas and primers built for high vapor drive. When a primer of that kind is needed, our written quote lists it as its own line.

• ASTM F1869 (calcium chloride): KTA-Tator describes bare concrete "under a sealed dome for 60-72 hours"; the result is an emission rate in pounds for each 1,000 square feet over a 24-hour span.
• ASTM F2170 (in situ probes): Wagner Meters calls for "Three test holes for the first 1,000 ft2 and at least one additional hole for each additional 1,000 ft2", and a 24-hour wait before a reading is documented.
• ASTM D4263 (plastic sheet): KTA-Tator describes a sheet at least 18 by 18 inches of polyethylene, 4 mil or thicker, taped at its edges and left at least 16 hours; it shows moisture moving, not an amount.
• New concrete: Form G-1 says "the industry standard for curing concrete is 28 days" and that this "is not always sufficient time to allow excess moisture to leave a concrete slab."

Floods: Harvey, the Brazos crests, Atlas 14 and the floor elevation rules

Floodwater reaches a Fort Bend floor twice: once when it soaks the slab, and again through the rules that decide how high a rebuilt floor must sit. At the Brazos River gauge at Richmond, the National Weather Service sets major flood stage at 50 feet, the level at which it says "homes in Richmond flood," and expects FM 359 between US 90A and Pecan Grove to take on water at 52 feet. The record crest, 55.19 feet on 1 Sep 2017 in Hurricane Harvey, is still marked preliminary; the river reached 54.74 feet on 2 Jun 2016. The NWS Houston/Galveston Hurricane Harvey page lists storm totals, in inches, of 39.17 at 2 SE First Colony and 36.36 at 0.5 SE Sugar Land.

The rain behind the rules has been revised upward. NOAA Atlas 14 Volume 11, released as final data in September 2018, superseded older federal rainfall estimates for Texas, and the City of Sugar Land's Atlas 14 page says the City adopted it on April 2, 2019, and that under it a 24-hour rain of 12.5 inches went from "a 1 percent chance" to "a 4 percent chance". Fort Bend County's interim criteria, effective January 1, 2020, set the 100-year, 24-hour rainfall depth for county drainage design at 16.50 inches. Levee building here is not new: the Handbook of Texas records that in 1913 the sugar company built 8 1/2 miles of levee, and the County's Drainage District now maps the county's levee districts. The City's floor rules, listed below, treat land inside a levee improvement district separately.

After water, two rules and two checks apply. The City of Sugar Land page on substantial improvement says buildings put up before December 31, 1974 were grandfathered into the flood insurance program at their current finished floor elevation until repaired or remodeled to 50 percent or more of the building's current market value, with the work of the past five years added together; past that threshold the building usually has to be raised to the City's current elevation requirements. Unincorporated land, including the area on our Pecan Grove floor page, is under Fort Bend County's flood regulations instead. On the floor itself, an old coating is checked for the signs of disbondment Sherwin-Williams lists, "hollow-sounding areas, raised bubbles or dull patches," and the slab is moisture-tested again before anything new goes down.

• City of Sugar Land, outside levee districts: 2.0 feet above the 100-year Atlas 14 base flood elevation, or 2.0 feet above the FEMA 500-year elevation where Atlas 14 data is not available.
• City of Sugar Land, inside a levee improvement district: 2.0 feet above the district's coincidental ponding elevation.
• In both cases: also 1.5 feet above the highest adjacent natural ground and 1.0 foot above the top of curb, with the highest test governing.
• Mechanical and electrical equipment: at least 1.0 foot above the base flood elevation.

Heat, sun and the topcoat on a Fort Bend garage floor

Heat and sun work on a coated floor in three ways: hot tires mark some finishes, heat shortens the working time of mixed resin, and sunlight discolors many epoxies. At Houston Hobby the 1991-2020 normal high reaches 93.5 degrees F in August, and in the National Weather Service's 2025 annual summary the 1991-2020 normal for days whose maximum reaches 90 degrees F is 100.5 a year; 2025 had 136. NWS Houston/Galveston called 2023 Hobby's hottest summer on record, and its heat sheet lists a run of 69 straight ninety-degree days there that summer, from early July to mid-September. These are outdoor air temperatures at an airport, not the temperature of a slab.

Hot-tire pickup, which Euclid Chemical's bulletin CP-22 treats together with tire marking, starts in the rubber. Euclid writes that "When the car is driven, the tires heat up, causing the plasticizers to soften and leach out of the rubber," that most marking happens on acrylic cure-and-seal products "because the acrylic polymers have minimal 'crosslinking'," and that "Epoxy and urethane sealers and coatings are much more crosslinked than cure & seals and are therefore less likely to tire mark." Sherwin-Williams says its one-part acrylic ArmorSeal Tread-Plex is "Not recommended for areas subject to hot tire pickup," while Laticrete lists "Resistant to hot-tire pickup" among the advantages of its Spartacote Flex SB. Heat also cuts pot life, the working window of a mixed kit: ArmorSeal 1000 HS gives 4 hours at 77 degrees F and 2 hours at 120 degrees F.

Sunlight brings color change. The ArmorSeal 1000 HS sheet says "Epoxy coatings may darken or yellow following application and curing" and limits its clear to interior use, and Sika's Sikafloor-264 sheet says that epoxy "Will discolor over time when exposed to sunlight (UV)." Laticrete describes Spartacote Flex SB as an aliphatic polyaspartic and lists "UV Stable; retains optical clarity" among its advantages. The EPA's UV Index puts readings of 8 and above in its "Very High to Extreme" band, and NOAA publishes its Houston UV index only as graphs. For a garage whose door opens to the afternoon sun, read the topcoat's sheet first; our polyaspartic floor coating page sets the systems side by side.