Weathering (Building Materials)
How sun, water and temperature change exposed materials, including weathering steel.
Quick Answer
Weathering is the gradual change in exposed building materials from sun, rain, wind, temperature cycling and pollutants. Some materials are chosen for it: weathering steel forms a protective rust layer instead of needing paint. Weathering has no takeoff unit of its own, but it drives finish choices, maintenance cost and exterior material quantities.
The Full Picture
Weathering is what outdoor conditions do to a material over time: ultraviolet light fades and chalks coatings, water and freeze-thaw cycles crack and stain porous materials, and wind, heat and pollutants wear on surfaces. Different materials respond differently, so specifications name the expected exposure and the finish that goes with it. Wood grays unless it is finished, copper darkens and then develops a patina, and exterior coatings erode. For estimators, weathering is not a measured quantity. It affects which material and finish are specified, the number of coats, the allowance for maintenance and the owner's long-term cost, so it matters when comparing alternates.
Weathering steel is the best-known case where weathering is deliberate. It is a family of steels with alloying elements that, with repeated wet and dry cycles, form a tightly adherent rust layer that slows further corrosion, so the steel may be left unpainted. Grades such as ASTM A588 are used for weathering steel in structures. FHWA's reference on corrosion-resistant steel says weathering steels do not develop a protective rust patina in the presence of high-salt exposure, so they must be painted in those environments. That is why site conditions matter for the spec.
The risk is in the environment and the detailing. FHWA's Technical Advisory T 5140.22 says that if the yearly average time of wetness exceeds 60 percent, caution should be used, and that where serious doubt remains, engineering judgment should lean toward coated steel. Water that stays on the steel, trapped debris, ponding and runoff are all problems, and runoff can also stain concrete, masonry and paving below, which is a cost on neighboring scope. Details are normally shown on structural and architectural sections, and the spec, typically in Division 05 for structural steel and metal fabrications or Division 07 for metal panels, lists the grade, finish, fasteners and any pre-weathered or sealed treatment.
For takeoff, treat weathering as a spec driver and a risk allowance. Weathering steel plate, shapes or fabrications are taken off like other steel by weight in lb or tons, and weathering steel cladding or panels are measured in SF with LF of trim, as for any metal panel. The extra cost items are the weathering-grade fasteners, the sealing or pre-weathering requirement and the protection of adjacent materials during the weathering period, plus lifecycle maintenance such as inspection and cleaning. The mistakes are pricing weathering steel as ordinary painted steel, omitting runoff protection, and assuming it never needs inspection.
Real Examples
Common Misconceptions
People assume: Weathering steel is maintenance-free.
Actually: It needs inspection and care. After the Fern Hollow Bridge collapse, FHWA's July 2023 memo asked state agencies to identify bridges with uncoated weathering steel and to confirm that preventative maintenance from Technical Advisory 5140.22 is identified and communicated.
People assume: Weathering steel works in any environment.
Actually: It depends on the site. Under FHWA's technical advisory, a yearly average time of wetness above 60 percent calls for caution, and high salt exposure prevents the protective layer from forming.
People assume: Weathering is only a durability issue, not an estimating one.
Actually: It changes the specified finish, coats, fasteners, detailing and maintenance allowance, so it affects both first cost and lifecycle cost.
Frequently Asked Questions
What is weathering in building materials?
It is the gradual change in exposed materials from sun, rain, wind, temperature cycling and pollutants. Some materials are chosen because they weather in a useful way, such as weathering steel.
What is weathering steel?
It is steel with alloying elements that form a tightly adherent rust layer under repeated wet and dry cycles, which slows further corrosion so it can be left unpainted. It is commonly specified to standards such as ASTM A588.
Where should weathering steel not be used?
FHWA guidance cautions against it in prolonged wetness and says it does not develop a protective patina under high-salt exposure. Coastal and deicing salt sites, and places with trapped moisture, need special care or coated steel.
How is weathering steel taken off?
Structural shapes and plate are taken off by weight in lb or tons, and panels or cladding are taken off in SF with LF of trim. Fasteners, sealing or pre-weathering and protection of adjacent materials are separate items.
Does weathering affect maintenance cost?
Yes. Exposed materials need inspection, cleaning and sometimes recoating, and weathering steel needs inspection and detailing that keeps water from sitting on it. The spec and the owner's maintenance plan decide the allowance.