Most calls I get about commercial window cleaning in Charleston sound the same. The glass looks bad, a tenant said something, and the last cleaning did not fix it.
Answer up front. Cleaning commercial glass more often is cheaper than rescuing it later, and the industry says so in its own documents. The National Glass Association's technical paper on cleaning architectural glass warns that glass products "can be permanently damaged if infrequently or improperly cleaned," and sets a baseline of twice per year, more where environmental factors and atmospheric pollutants are working on the building. Vitro publishes a higher floor for coated glass installed with the coating facing outdoors: a minimum of three to four times per year.
Charleston does not have hard water, and that changes the argument
Charleston Water System publishes its hardness on its FAQ: moderately soft, averaging 58.4 ppm, or 3.4 grains per gallon. The utility treats surface water, typically 90 percent Bushy Park Reservoir and 10 percent Edisto River, not limestone groundwater. A contractor blaming the city supply for your glass has not looked it up.
Tap water is still not what you want drying on your windows. For the pure water pole technique the IWCA recognizes, where water runs up a specialized extension pole and the glass gets a final pure water rinse, the NGA paper specifies 20 parts per million total dissolved solids or less to prevent spotting and streaking, and says "the use of tap water is not acceptable." Hardness and total dissolved solids are different measurements, so I will not do arithmetic between those two numbers. Whatever is dissolved in a hose rinse stays on the glass after the water leaves.
Deposits and corrosion are two different problems
Deposits sit on top of the glass. A joint bulletin from the Glass Association of North America and the International Window Cleaning Association (GANA has since merged into the NGA; this is the 2010 edition) names the condition exactly: "Stubborn, tenacious surface deposits from concrete and masonry runoff, as well as hard water spotting." For installed glass, the cause it lists is "absence of prompt, interim cleaning of installed glass during construction." Removable. Just harder the longer they sit.
Corrosion is chemistry inside the glass surface. Vitro's technical document TD-105 lays it out: water leaches sodium out of soda-lime-silica glass, and once the solution touching it reaches pH 9.0 or greater, silicon-oxygen bonds start getting severed. Microscopic pitting first, then widespread iridescence or a dense translucent haze. Vitro's summary: "the optical quality of the glass is destroyed even though overall mechanical integrity of the glass is maintained." The window still works. It just looks permanently bad.
Here is the correction I owe you, because our own trade gets this wrong. That same document says corrosion "is not usually observed on fabricated glass products" once they are installed, because moisture on an installed unit evaporates or gets diluted and never reaches the pH the damaging stage needs. So I will not tell you your irrigation heads etched your windows. Repeated overspray lays deposits on the same panes, and deposits left alone get tenacious.
What actually wrecks glass on buildings here
Residence time. Vitro says rundown from metals and masonry should come off "as soon as they occur," because if it is not removed quickly, "permanent staining and/or glass damage may occur."
Construction is the other one, and it hits newer buildings hardest. The GANA and IWCA bulletin ties chemical attack, surface pitting and hard-to-clean deposits to "overspray and runoff of chemicals from sealing/cleaning of concrete, masonry, roofing." A Daniel Island building with trades still on site needs interim cleaning during the work, not one cleanup after it.
Salt I will not claim. People ask every spring whether the air off Shem Creek bonds salt to their glass. I looked for a manufacturer or a standards body saying salt spray etches or bonds to glass and could not find one. Cleaning company blogs say it. That is not a source.
An honest office window cleaning schedule
Two published numbers exist and that is all of them. Twice per year from the NGA, more in some regions. Three to four times per year minimum from Vitro, for coated glass with the coating facing out. The NGA adds that frequencies "should be tailored to the individual characteristics inherent to the site conditions."
What follows is how our crew builds schedules, not a standard. Nobody publishes a per-building-type interval.
Restaurant and retail storefront glass on King Street or Coleman Boulevard: monthly or every other month. Fingerprinted at eye level, and the first thing a customer looks at. It usually rides along with the Storefront & Sidewalk Cleaning visit, since the concrete under the glass ages at the same rate.
Office and medical exteriors: quarterly is where I start, then adjust after a year of watching one elevation.
Multifamily and hotels: lobby glass faster than the unit exteriors, which can live near the twice-a-year floor.
Active construction next door: interim cleaning while it is happening.
Retail tenants, read your lease first, because the glass and the sidewalk in front of it often land on different parties. We covered that in the post on whether the tenant or the landlord cleans the storefront sidewalk. If yours is ambiguous, ask your attorney or your property manager, not me.
How not to wreck your own glass, whether you hire us or not
Razor blades never go on coated glass. The NGA states it flatly, twice.
Low-E coatings are "typically neutral in color and very difficult to see," which is why untrained hands damage them. The NGA gives a pencil test, though it scopes that test to locating a reflective coating: touch a pencil point to the glass, and if the reflection of the pencil point meets the real pencil, the coating is exposed on that side. For Low-E, the NGA says to consult the glass manufacturer.
No acids. The NGA says solutions "should not include acids, especially muriatic / hydrochloric and Hydroflouric," and Vitro adds abrasive cleaners, fluoride salts and hydrogen producing compounds. Plenty of product sold as a hard water stain remover for commercial glass is acid based. Read the safety data sheet first.
Test clean one small area, stop, and examine it before doing the building. Some damage only shows in certain light.
Do not clean tinted or reflective glass in direct sunlight. The surface runs too hot to clean well.
Work top down, so runoff does not land on glass that is already done.
The limit, and it is ours
Here is where I lose bids. We clean glass. We do not restore it.
Vitro on severely corroded glass: restoring it is "at best a formidable task," and grinding and polishing "is not practical nor economical; it is simply easier to discard all heavily corroded glass." Lightly corroded glass, it says, can often be cleaned with acidic detergents or abrasives well enough "that it can be used in less critical applications where no coatings are involved." Read that scope. Less critical applications. No coatings. Not the same as restored.
Both bodies warn the rescue can be the damage. GANA and the IWCA: "The use of a more aggressive procedure may itself damage the glass." The NGA: "any non-routine cleaning carries a risk of irreparable damage to glass products."
So if deposits on your building will not come off with normal professional Commercial Window Cleaning, we say so and decline, which sometimes means walking away from work we drove out to bid. Our re-do guarantee covers our cleaning, not what was in the glass before we got there.
A next step that costs nothing
Walk one elevation on a bright morning in raking light and photograph the three worst panes plus the two beside them. Note what sits above that glass: a metal coping, a masonry band, a roof somebody sealed last year. That tells you whether you have an interval problem or a rundown problem, and those get fixed differently.
Send the photos through our quote form or call 843-696-4739. We run plant-safe surfactants and biodegradable detergents, carry $2,000,000 in general liability, and send the certificate of insurance before the crew shows up, with your ownership entity named additional insured when a management agreement requires it.
