I have a 2nd floor hot-mopped deck with grouted slate tile placed on top of it. During rain, water comes out between the hot mop and tile layers and runs down the side of my home, leaving stains on the siding and windows.

One contractor who took a look at this thinks it's rust and indicative of something below the deck rusting out, ie: a much larger problem. Another contractor (the one who put in the hot mop) believes that the water deposits are from the slate tile and are more mineral deposits from the slate leeching out rather than rust.

Is there a way to determine if this is rust or mineral without finding the source of it?

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Note if it helps that I've scraped off the deposits on the glass with a razor blade a few times, they used to be thicker. It's easy to scrape off and get a powder form.

  • I think it is rust and it looks like it is actually coming from behind the clapboard. It appears to me that you have water running down this exterior wall on the inside. This should be checked. Jul 21, 2015 at 15:22
  • 1
    Mix some of the red dust with some aluminum powder and light it on fire... then you'll know.
    – Mazura
    Jul 31, 2015 at 3:01

2 Answers 2


Well I think that the best way to determine that is that you ask yourself is there any iron or steel elements which could corrode? If you built the house (bought it new etc.) you should know that. Also you can check design blueprints to check this out. Also check if metal is detraining which means rust. From what I see in the pictures this doesn't look like rust. Anyway if none of this is helpful or isn’t helpful enough notify me so we will try to find different approach.

  • Unfortunately I don't know what elements might be under the surface as I bought the home already built. I don't understand what you mean by 'metal is detraining'
    – Gregable
    Apr 22, 2015 at 8:13
  • Sorry bad spelling....I meant metal deterioration. I’m guessing that you don't have any design blueprints either? Only thing which could be rusting is façade substructure....but normally they are made out of materials which are rustproof.....I'm almost sure that those are just water stains....try to clean them with some detergent. I think it will fix your problem. Apr 22, 2015 at 13:20

It turns out that this was rust. The flashings above this window (around the deck above had significant rust. We ended up reconstructing a large section of this to deal with the issue.

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