Roof Rocks Help Cool Hot House
Excerpt from Valley 101: A Slightly Skewed Guide to Living in Arizona, a collection of Clay Thompson’s columns for The Arizona Republic. (Originally published May 28, 2000.)
Q: Recently, I saw a house in Tempe that had large rocks on the roof. I mentioned this to a friend who had grown up here, and he said it used to be quite common, but he didn’t know why. Can you help?
A: Of course, sir. Help is our middle name.
Which is not to say we actually had ever heard of such a thing and, we blush to admit, when we first read your question we decided you perhaps had ingested some controlled substance. We felt much the same way early one recent morning when we groggily caught a glimpse of the haircut we had gotten the previous evening and, for an instant, wondered whether we had been eating peyote before telling the barber to cut it real short. We had not.
But we digress.
Example of rocks placed on roof. Photo taken by Nick Perez in 2010, posted on Flickr.
Rocks on the roof? Who to call but the estimable Max Underwood, professor of architecture at Arizona State University, a specialist in Valley architecture, and hail-fellow-well-met.
Underwood says your native friend is correct. Rocks on the roof can be traced to the Native Americans who lived here long before anyone dreamed of professional hockey and were adopted by homesteaders.
The idea is thus: During the day the rocks soak up the sun’s heat. In the cool of the night the rocks give off the heat and because hot air rises, the escaping heat would cause a kind of updraft that would help cool off the house.
There still are a few older houses around town, said Underwood, with good-sized rocks on their roofs, notably the Mitchell Park area of Tempe. And a colleague tells us she recently saw a new house, near 56th Street and Shea Boulevard, that had adapted this ancient technology.
We would have gone by to see it, but we were too occupied looking for a lawyer willing to bring suit for wrongful barbering.
Hope you won’t mind if I disagree on your assessment of how the roof rocks work. You said that roof rocks give off heat and this causes a kind of updraft that helps cool off the house. Gotta say that the kind of updraft this creates would only cool off the rocks, unless there are large holes in the roof to allow the air inside the house to rise. That would unfortunately allow the rain to come in though! I live in a house that has 12 inch thick solid concrete walls on the first floor and the second floor is made of standard frame construction. On a hot day, the upstairs is boiling hot and the downstairs is so much cooler you have to see it to believe it. In fact, I can get by with very little air conditioning. The reason is the thickness of the walls is excellent insulation and also there is the principle of thermal mass. During the night the walls cool down and when the day comes and it gets hot, the walls absorb the heat slowly, which is totally different from wood walls, which heat up very rapidly. Plus, the R value of the wall’s insulation is very high. So, putting rocks on the roof does the same thing, high insulation from the outside heat; then the rocks absorb a lot of heat into themselves and give it back off to the air after the sun goes down, rising heat as does moves it away from the house… Another way to look at it, would be to imagine a house completely incased in rocks, think how great the insulation would be!
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My mom who lives in Colorado said the rocks heat up and help melt snow.