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.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.