Row houses in Baltimore. Black roofs above bedrooms get as hot as 150 degrees.
People who sleep in a top-floor bedroom under a black asphalt roof are likely to lose sleep on summer nights due to heat from the roof, with serious health consequences. Technically, the fix is simple—coat every black asphalt roof with a white coating—but progress toward that goal is slow. The answer: a public health law requiring white coating of hot roofs in every city with this problem.
The issue: black asphalt roofs get really hot, keeping people from sleeping well, causing dangerous health problems
Black asphalt roofs, which are common on Northeast urban row houses (townhouses), “can reach temperatures of 150°F or more in the summer sun,” reports the U.S. Department of Energy. With or without air conditioning, a roof that hot above your bedroom will radiate heat at you all night long.
And people can’t sleep well when it’s too hot. A research study using data reported by 765,000 people over 10 years found that “increases in nighttime temperatures amplify self-reported nights of insufficient sleep.”
Losing sleep is far worse than a nuisance. “People make cognitive errors that matter when they sleep badly, whether crashing vehicles or making poor decisions in the workplace,” said UC Berkeley professor Solomon Hsiang in response to the study, as reported by Bloomberg. He added, “Students learn poorly when they don’t sleep, and consistent lack of sleep harms people’s health.”
And in a heat wave, hot roofs can result in death: in Chicago’s 1995 heat wave, those at increased risk included those “who lived on the top floor of a building (odds ratio, 4.7),” according to a New England Journal of Medicine article.
The technically easy solution: white roof coating of asphalt roofs
Any roofing firm can apply an “elastomeric” white roof coating to a black asphalt roof, which can reduce a roof’s temperature on a 90-degree day from 150°F to 95°F. (“Elastomeric” means the coating will stretch with the roof on hot days, and contract with the roof on cold days.) A homeowner who can safely get up on their own roof, with tools and supplies, can also do the job. The cost is modest, since the job is relatively small: clean, patch, prime, and apply the finish coat.
But progress in getting black asphalt roofs coated white is slow. For example, people in Philadelphia have been talking for a decade about this problem, but the Google Maps satellite view of the city shows mostly black roofs, while the roofs that aren’t black are generally gray—a color that provides only about half the cooling benefit of a white roof.
How to persuade landlords to apply white roof coating to black asphalt roofs?
Landlords are under no pressure to apply a white roof coating to their properties to improve their tenants’ health. Cities could institute a public health measure limiting the temperature of bedroom ceilings, and requiring white roof coating if that temperature is exceeded. Such a law could presume that black roofs exceed that temperature (and gray roofs as well, if data showed that to be the case). Then, if a landlord failed to coat the roof white—or provide data proving the bedroom ceilings remained cool enough—the city could do it for the landlord and bill the landlord via the property tax bill. (In my city, letting your grass grow tall is considered a public health problem, and if you don’t cut your grass the city will cut it for you, and bill you for the work. So a public health law to prevent sleep deprivation is a no-brainer.)
Air conditioning is not enough
With or without air conditioning, bedroom ceiling temperatures reached 90 degrees and higher in row houses during a Philadelphia heat wave. Any ceiling above 91 degrees (human skin temperature) will radiate heat to humans, and any ceiling only several degrees below that temperature will make it difficult for humans to radiate heat to the ceiling.
Bedroom air temperatures measured in that study climbed to the 80s/90s in all houses with black tar roofs, with or without air conditioning.
A precedent—recognizing heat as a public health hazard
In Maryland, Montgomery County Councilman Tom Hucker has introduced a bill to make working air conditioning a requirement for all rental units in the county (Bill 24-19). This proposal recognizes that heat is a public health hazard. Even with air conditioning, a hot tar roof over a row house will radiate heat into bedrooms all night long—and some tenants will limit their use of air conditioning because they can’t afford the bills. Cooling the roof will dramatically reduce heat exposure.
