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?
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.