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Purpose of rocks on 1960's style roof

We have a 1960's style home with a tar and rock roof. There is quite a bit of moss on the roof, but the roof doesn't need replacing right now. We would like to know if we can just clean off the moss and rocks and not replace the rocks, or is there a real purpose for replacing the rocks. Also, what is the best way to clean the moss off the roof.

So now, if we are thinking of replacing the tar and rock roof, would it be a good idea to replace it with shingles?

A. Spruce
Re: Purpose of rocks on 1960's style roof

Flat roofs, or nearly flat, use a tar coating to seal the surface, the rock protects the coating from the sun, which literally burns through it. Do not remove the rock. There are a number of moss killing agents that can be used on roofs to control moss problems. I believe they have a zinc base, but don't quote me on that.

Re: Purpose of rocks on 1960's style roof

What size rocks are they?

If the rocks start falling off the roof you know it's a sign that the tar under them is dry and brittle, and could be time to re-roof. If you have gutters, check them - they are probably filled with rocks. Your roof i's probably more than 50 years old - start saving for a new roof.

Years ago, there were a lot of homes with rock roofs around my city. Today there are none.

I've replaced a few rock roofs in my life, and I was shaking my head every time, like "what were they thinking, putting this crap up there".

By the way, the moss is probably the result of excessive moisture.

Re: Purpose of rocks on 1960's style roof

A built-up roof (typically used on flat roofs) is typically made up of three components:

  • A fabric base layer, which may be made of asphalt-impregnated paper, fiberglass, or other fibrous material. This layer provides mechanical strength to span joints in the roof sheathing and also provides a substrate for the next layer:
  • A layer of liquid asphalt. It's typically heated to soften it for application; afterwards it cools to form a thick, gel-like layer. This is the waterproofing layer.
  • The previous layers may be repeated several times. This enhances the strength and durability of the roof.
  • A final layer of aggregate -- finely crushed rock or fine round rock. This provides a barrier to ultraviolet light, which would otherwise degrade the asphalt. By using aggregate that is light in color, you can reflect a significant amount of infrared radiation, which would otherwise increase your cooling load and would speed up the loss of VOCs in the asphalt.

The thing to remember is that the asphalt is not a solid, but a very thick, viscous liquid made up of several different types of hydrocarbons. Some of these hydrocarbons are volatile, and over time they escape the mixture. As more volatile organic compounds are lost, the asphalt gets harder and more brittle. When the asphalt loses too much VOCs, it loses adhesion with the aggregate, it begins to crack and shrink, and the roofing fails. This is why there are several layers: to add additional time until the next reroof job is necessary.

For newer flat roofs, vulcanized rubber sheeting (really, it's a form of asphalt!) or some other asphaltic membrane sheeting is used, and the seams are "torched" or melted together to form a seamless, continuous barrier. It goes down faster with less mess, and should last just as long as if not longer than a traditional built-up roof. But it, too, will suffer the effects of environmental exposure and will need to be replaced eventually.

Regardless of what material is used on a roof, you should minimize foot traffic as that will break aggregate loose. If you want to use the roof regularly or frequently need to service equipment on the roof, you should build a sundeck that elevates foot traffic above the aggregate.

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