We have a pending roofing project. In selecting colors, black is very hot. A new color is on the market -- a dark blue green. Would this be much better? It's not a light color.
While a black roof is hot, there are a few advantages.
The heat helps to seal them quickly when they are new.
When the sun comes out, they dry quickly.
When they get old and the tar has been bleeding down the roof for many years, you can't hardly see it on a black roof.
Not every roof looks like it is bleeding tar in streaks down the roof. White seems to show them the most. I used to think that trees caused the streaks, until I put a white roof on a house with no trees around it. The streaks seem worse when the attic space is not properly vented. But even when the attic is vented very well, sometimes I still see streaks on the roof. That blue-green color would look pretty bad with a lot of streaks or other discoloration.
My theory about the streaks is this, petroleum products are continuously off-gasing. These vapors, I assume, are carrying enough black petroleum product to gather on the surface and when it rains, they are washed into a nasty pattern of streaky lines. You may have already seen what I'm talking about, if not, pay attention to some of the white roofs in your area. White isn't the only color affected, but it's the most obvious. It could be a defect in the shingles. I have seen different brands of shingles do the same thing though.
Unfortunately, with new technology, it isn't always easy to tell which color roof is going to keep your attic space the coolest. Probably the two most important engineering factors for this are absorption or relectivity. The absorption coefficient will tell you how much heat energy the shingles will absorb and the reflectivity will tell you how much sunlight the shingles will reflect. Low absorption and high reflectivity would seem to be what you want in the ideal situation. In the real world, with a properly ventilated attic, it really doesn't make a lot of difference. With small ventilated spaces, like cathedral ceilings, it could make a difference.
Thanks very much for the input. The people making the selections don't want black because they say it's hot. They have selected the dark blue-green instead. Doesn't seem that much lighter though. I will check into the factors you mentioned on this particular roofing to see how it compares. Our attics get very hot and humid. You'd think there would be ample ventilation since the side gable on my particular unit is just aluminum siding on studs????
I have got roofing on the brain right now. People make some really bad color selections for their homes. I have noticed most of the colors seem to have streaking and that some don't have trees around. That's interesting about the tar and black roofing.
Hey Square Eye..That is an interesting theory.
Sometimes the stains you see can be controled or minimized by installing ZINC STRIPS. These strips kill any mold or liechen, that grow on the shingles.The no tree thing with the mold still on the roof is basically airborn stuff floating around until it lands on your roof. The pollen is basically everywhere and it eventually sticks around and multiplies.
This does not typically do any harm to the roof ,it just looks terrible.
Install these zinc strips and I guarrantee they will eliminate most of the issue.
I like the lighter roofs to keep down the attic temps.
They also do tend to last longer in my experience on inspections.
The ventilating of the attic is the key factor as Craig said, You sound as though your attic needs more ventilation.
As far as how much longer a roof will last I could not say. Mabey a few more years, Tops.
Also depends on orientation. A southern V/S northern.
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