The House is a-Rockin'

June 28, 2009 12:09 PM

Why are there big rocks on this roof?

Though the paint on the building appears to be new, the strange roof texture has been there for years. The pitch of the roof is low, so I can't see what kind of material is under the rocks. I don't see any obvious asphalt or wood shingles, corrugated metal, fiberglass, or tiles.

A great deal of moss has accumulated between the stones. I assume this is unintentional, but I don't know for sure. I've driven by this building for years, and the roof has always been like this.

I wonder if tar paper was put down, then never finished with shingles. Perhaps the rocks were to prevent the tar paper from blowing away. Yet there seems to be an awful lot of rather large rocks; more than enough to simply hold tar paper down, I would think.

For what it's worth, this is a commercial building, not a residence.

This is the only roof like this I've ever seen. Is it simply an unusual style? Do the rocks serve a function I'm unfamiliar with?

posted by Tube to technology (9 comments total)

I'd guess the building has a tin/metal roof and the rocks were added as a noise damping device during rain storms.
posted by torquemaniac at 12:20 PM on June 28

Apparently tar/rock roofs aren't all that unusual so it could be that, too.
posted by torquemaniac at 12:24 PM on June 28

It looks like just a standard asphalt roof where they decided to go with a larger stone size for ballast (keeps the roof material on the roof during windy times - the forces involved can actuall be a bit greater than just the force of the wind. Since the roof is pitched, it actually acts something like the top of an airfoil, and gets a significant amount of lift, especially on the leeward side), just because they felt like doing it/liked it aesthetically. I've seen things like that before.

I'd guess the building has a tin/metal roof and the rocks were added as a noise damping device during rain storms.

No, if it was a metal roof, you'd probably see some seams protruding and there'd be some different edge details.
posted by LionIndex at 12:27 PM on June 28 [1 favorite]

That's what is known as a tar and gravel roof, common for low slope applications. I had it on my roof until I replaced it with standing seam metal.

Rocks/stone/gravel on a flat/low slope roof do three things:

• Absorb UV to prevent the degradation of the waterproof membrane underneathwhether that is a builtup tar/felt or a butyl sheet.
• Reduce deck temperatures. The rocks are light coloured while the membranes are almost always black and therefor reflect away more heat.
• Protect the membrane from mechanical damage from hail or people walking on the roof

I guess they could be a measure against uplift but that has got to be pretty far down the list, more of a bonus I'd think. The primary purpose is to extend the life of the waterproof layer.
posted by Mitheral at 12:42 PM on June 28

It sure looks like a water gathering roof to me.

A friend of mine who lives in the Caribbean has a roof like this. His house is not near municipal water, and the ground water is saline. So he had a roof built similar to the one in your photo, only with smaller rocks. It collects rainwater during the rainy season and stores it in a 25K gallon cistern that is built under the house. This provides eenough water to get him, his wife and their garden through the dry season. Under the rocks is a white rubberized roof material, that is useful to keep the heat down.
posted by kuujjuarapik at 1:39 PM on June 28

Apparently tar/rock roofs aren't all that unusual so it could be that, too.

Yeah, there are several around where I live (Billings, Montana). Some actually have MUCH bigger rocks interspersed with the gravel and smaller stones. They are always light colored stones, no doubt to keep inside temperatures down during our hot summers.
posted by The Deej at 1:41 PM on June 28

I grew up in a house with that kind of roof. It's just a tar-and-gravel roof with big rocks instead of small gravel (or more likely, a mix). You get tired of small pebbles blowing off the roof, so you go with bigger rocks less likely to be moved by gusts of wind.

In my house, eventually, Grandad got tired of that, too, and replaced it all with asphalt shingles.
posted by Cool Papa Bell at 4:13 PM on June 28

I lived in a house with that kind of roof. I think it's just cheaper (it wasn't a very nice house).
posted by KokuRyu at 10:25 PM on June 28

I think what puzzled me from the start was that I didn't know asphalt roofs could be anything but flat.
posted by Tube at 5:27 PM on June 29