What type of paint and preparations would be appropriate for painting a mural on a rooftop?

Question

My house has a modified bitumen membrane roof, at a very low pitch/nearly flat, which was repainted with aluminum asphalt paint a couple years ago. My friend is considering painting a mural on the roof itself (if you must know, one draft is: a giant medusa head over an ocean, with the roof ventilator as a third eye, the snake hair forming a walking meditation labyrinth, etc.) Ideally we'd like the resulting painted mural to:

• be hard-wearing enough to support occasional foot traffic (we hang out on the roof sometimes)
• be long lasting (This is in New York City, so we're dealing with a wide range of temperatures/weather conditions, from below freezing and mounds of snow to swelteringly hot 100% humidity torrential rain)
• provide reasonably decent reflective insulation and protect the roof against UV damage as compared to the current aluminium coating (if possible - I know it won't be as good though)

I'm not sure where to begin in selecting paint for this. Acrylic paints are generally used for wall murals. I expect we might have to mix our own colors which is ok.

Also not sure what preparations would make sense apart from thoroughly cleaning the roof. Glossy aluminum paint maybe means a new coat wouldn't stick well, so we could dull it by using an abrasive cleaner.

What do you guys think?

Answer 1

As far as UV resistance, you're in luck. Multiple coatings stack, so the silver coating you'll be overtopping will still be 100% effective on whatever UV (if any) gets through your next layers.

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As far as reflective insulation, you have one choice: White with a 90%+ albedo. If your hope is to both have the roof reflect solar heat and also present multi-colored artwork to the eyes, light does not work that way. You are sacrificing one property to gain another.

There might be a paint formula that presents near 100% reflectivity in the UV and IR spectra, but desired reflectivity in the visible spectra. I have never heard of it, and I would think it would be a top seller.

The closest you can do is put a platform above the roof surface high enough air can roll under it, thus effectively putting the roof in shade. Shade is 100% albedo. The air needs to roll under it so it can remove the heat of solar gain.

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As far as paint durability, that cannot be better than your substrate. Aluminum asphalt paint is not made to be walked on. Your best bet is an outdoor latex deck paint, which will at least flex with the aluminum asphalt (which epoxy won't do). Pigment durability is a question of pigment quality; old fashioned lead, chromium and cadmium mineral pigments are toxic and no longer allowed, which leaves a variety of organic pigments which do fade from UV. Your friend shouldn't even think about squeezing pigments from artists' tubes, beacause many of them are still made with the nasty stuff.