Bidding a stucco building today that also needs the roof cleaned. Roof is aluminum. Light house wash mix with a 12v should be fine, right?
Is the aluminum painted or bare?
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Pretty sure it’s bare.
You will be brushing that roof or at least the two I have done, just spraying a mix didn’t do it. I had to get a soft bristle brush and run it up and down each section. If you can walk it, then light to med pressure works as well.
Did a bank 3 months ago and 1/2 the roof I could walk and spray that other half was on a ladder with a 20 ft extension pole and nylon soft brush working it.
Note - if you walk it, it will be slick when wet. I had on non-slip shoes (not Cougar Paws) and it was slick.
I was thinking of getting what I think is called a Spray Bar - it has wheels on each end with 3 or 4 nozzles with a pole. I believe you connect your hose to the pole and then you can run it up and down the roof from a ladder to “wash” the roof. I am sure I am butchering the name or how it works but I have seen videos just for this purpose when the pitch is too steep to walk.
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That roof is probably not aluminum it’s probably GALVALUME .I -2 percent sh is all that’s needed.
DO NOT BRUSH OR PRESSURE WASH. The galvalume is just a coating process. It is extremely easy to scratch and it will rust if the galvalume is gets exposed.
I spent 4 hours on the phone with the manufacturer about 5 years ago before cleaning my first one. Treat it like a painted surface not a roof. It is very corrosion resistant that’s why 1-2 percent will not harm it . Make sure to rince it well once it’s clean or the hardware will rust . If you do need to brush it use a car wash brush nothing stiffer.
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Definitely steel if it has any color to it. Galv is normally bare and silver in color.
Two most popular are Colorbond and Zincalume.
I’d say at least half of the homes here in Australia have it. (85% of new builds go this way too)
Piece of cake to clean. You can use pressure, and it’s proven (here is Aus) that it’s best to clean with a surface cleaner at low pressure. I have a small Mosmatic one just for this type of roof.
Obviously use a floaty and not a wheeled one 
Heavy mould makes it VERY slippery, but light stuff like that is no problem.
I guess it looks like we have variations of the roof. The ones I did, I started with 1-2%, dwell and rinse and it was still dirty. Did nothing. I then got on the low pitch side and gently applied medium pressure (maybe 800 psi - similar to cleaning pine wood) to small area rinsing. Let it dry and then inspected. Looked great, with no damage. I proceeded to do entire roof that way.
On another with a higher pitch, I used a soft nylon brush that wouldn’t scratch a thing as their was no way I was going to try and wash while on the roof. Again, it came out great with no issues.
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I was told by the manufacturer Soft brush and super low pressure are exceptable in extreme cases . But not advisable. I’ve never had to use anything except 2 percent.
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Interesting. I don’t doubt you one bit and prior to the first one I did, I thought I was going to hit just like you said - 1-2%, dwell and rinse. After I did that and nothing happened, that is when I started looking at other options. I believe that was the day I bought a 20’ extension pole and a soft brush.
