What is the correct way to clean a metal roof?
January 24, 2018 10:23 AM   Subscribe

My house has a relatively new metal roof, installed by previous owner. We have been told by multiple realtors and home inspectors that pressure washing will damage/cut years off a metal roof. Is this actually true? I am currently in the market for a roof cleaner and all the ones I'm finding are proposing pressure washing.

Prior to buying this house we looked at 3 dozen homes on the market, worked with 2 different realtors and at least a half dozen inspection companies. All of them said that pressure washing metal roofs was no bueno. In a couple of home inspections it was specifically called out when the roof of the house had obviously been pressure washed and the inspector would estimate the number of years lost on it due to that. The house we bought has a metal roof but had no such history/prior damage. I'd like to keep it that way.

What should I be asking for when hiring someone to clean my roof? In addition to leaves, pine needles and evergreen tree branches there is moss to be cleaned off (yay Pacific NW).
posted by joan_holloway to Home & Garden (8 answers total) 5 users marked this as a favorite
I am speaking from zero experience about metal roofs, but for unpainted metals designed for exposure to the elements, the surface oxidizes (rusts) in a way that protects the underlying non-oxidized metal. Removing this oxide through scrubbing removes this protection, and the metal underneath must be consumed by re-oxidation.

Perhaps the people you’ve talked to are concerned that power washing would remove this layer.
posted by zippy at 10:40 AM on January 24, 2018 [1 favorite]

I don't think the problem is washing it with water so much as washing it with a 3000psi stream of water that would easily punch a hole in the metal anywhere it isn't laying perfectly flat against the underlayment among many other bad things I can think of happening by doing that.

I'd think garden hose sprayer level pressure and a light brushing with a broom would easily remove all the crap from the roof. Just don't spray up the pitch of the roof. I doubt it would cause permanent damage but could cause leaks at the seams similar to a wind driven rain. If you have to clean from ground level, arc the stream up and let it "rain" on the roof from above. The concentrated volume of water should help wash off the debris even without high pressure.
posted by wierdo at 10:45 AM on January 24, 2018 [2 favorites]

Response by poster: To clarify and then I will stop threadsitting: I will hire a professional to do this work, not do it myself. I just want to make sure I understand what is acceptable for them to do vs not, since the first quote I have received included pressure washing, which I thought was a no-no.
posted by joan_holloway at 10:54 AM on January 24, 2018

I own 3 houses with metal roofs (in the PNW), and I've built 2 others with metal roofs. Every couple of years I pressure wash these roofs with a prosumer type unit like this. Works great. Has had no observable or long term effect on the roof.
posted by humboldt32 at 11:05 AM on January 24, 2018

it's a metal roof, it should last 100+ years. wash away!
posted by patnok at 1:17 PM on January 24, 2018 [1 favorite]

Can you reach out to the previous owners (maybe through the realtors) or pull a copy of the installation permit from the city, in order to find out who installed the roof? They should be able to provide maintenance instructions. This info is usually conveyed together with the warranty info (although the warranty probably doesn't transfer to subsequent owners, but you could also ask about that).
posted by vignettist at 2:20 PM on January 24, 2018 [1 favorite]

Best answer: I assume this is a "standing seam" metal roof. The reason that you're not supposed to pressure-wash it is because it's coated metal, probably—unless it's sheet copper, which you'd know pretty easily—steel with either paint or a more sophisticated metallic or polyester coating (ex. "Galvalume"). The underlying metal is not stainless steel, it's just regular carbon steel, so once that paint (or other coating) is damaged, the metal will start to rust, and then it will eventually fail. It is very easy to take paint/topcoating off of a metal surface with a pressure washer. It's not just the pressure, but also because the pressure will vibrate the metal like a drumhead and cause it to separate.

The phrase you probably want to Google or search for locally in some other way is "standing seam roof cleaning", and I wouldn't work with a contractor that doesn't specifically advertise this service. If they mention using a pressure washer, or seem at all confused about how to clean a metal roof, I wouldn't hire them. And I'd be especially careful if such roofs aren't common in your area. (They're pretty common in far northern New England, so up there I'd have a much different comfort level than, say, suburban New Jersey, where you rarely see them.)

Here's a cleaning guide from a manufacturer of standing seam roof products, just as a sample—but really you need to find out exactly what's on your house. Basically you put on various cleaning products via a garden hose sprayer, scrub lightly with a soft brush (presumably on a long pole) if needed, then rinse off with an excess of water. To remove algae they recommend dilute ammonia. Again that's just an example, which is specific to that product and the coating it uses. (Though I think in general using a dilute alkaline is probably going to be involved for a lot of products, because they assume the slight acidity of most rainwater will neutralize it if you miss anything; I doubt anyone is going to suggest using an acid wash.)

I would approach a roofing company, not a "roof cleaning" company, and see if they will do the job. If you know the type of roofing material (manufacturer/brand), I'd find an installer in your area familiar with that material and either ask them to clean it or ask them for a recommendation of who you can call. I think it'd be all too easy for a cleaning company to nod and smile at you while you insist on how you want it cleaned, then use a pressure washer anyway (because it's faster, duh) and walk away leaving you with a damaged roof.
posted by Kadin2048 at 3:07 PM on January 24, 2018 [8 favorites]

Response by poster: Kadin2048, that sounds in line with what I remembered hearing from inspectors. I will try more roofing companies - so far the ones we've tried asking will only install.
posted by joan_holloway at 10:02 PM on January 24, 2018

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