What's that growing on my concrete tile roof????

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Old 07-07-16, 12:07 PM
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What's that growing on my concrete tile roof????

20 year ago we needed to replace our Los Angeles English Tudor wood shake roof. Fire laws banned wood so we found concrete tile (16" x 8" and 9 pds each) that looked great! They have withstood all our earthquakes and droughts but a few years ago an odd reddish growth appeared. See attached pix. Anyone know what this is? If it's mold, is it harmful? How do I get rid of it? Any and all responses gratefully appreciated.

Joe Bronx

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Last edited by PJmax; 07-07-16 at 01:03 PM. Reason: reoriented 1st pic- enhanced pic 1 and 2
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Old 07-07-16, 01:08 PM
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I assume it's some sort of mold. A thinned down [w/water] bleach or a bleach/TSP solution should get rid of it.
 
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Old 07-07-16, 01:25 PM
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I appreciate your response but "I assume" concerns me. I've been warned not to use bleach until I
know what it us so I'm hoping some one recognizes it and responds. Many thanks!
 
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Old 07-07-16, 01:33 PM
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You could always test it on a small area first. I use a bleach/water solution for all types of exterior cleaning during the prep stage of repaints - never had any issues.
 
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Old 07-07-16, 03:33 PM
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Might be lichen... but can't tell from so far away. I think it looks good. Patina.
 
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Old 07-07-16, 05:04 PM
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Good idea but what ratio bleach to water do you suggest?
 
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Old 07-07-16, 05:26 PM
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How about a close up picture.
 
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Old 07-08-16, 03:09 AM
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I generally mix 40-50% bleach with water and will add TSP to the mix if needed. I'll wet the substrate first, then apply the bleach solution, let it set but not dry and then rinse. It may be necessary to retreat or scrub stubborn areas. An old broom works well for scrubbing.

Not sure why you've been warned against bleach, while I don't recall ever washing cement shingles I've used a bleach solution on all types of masonry with good results [brick, block, concrete, stucco, stone]
 
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Old 07-08-16, 12:53 PM
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Many thanks for the detailed response. I'll try it. Bleach warning came from my brother in law who studied masonry in prison. He's not out yet or I'd have him on the broom. Again, Thanks for your advice.
 
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Old 07-08-16, 01:19 PM
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I'm not aware of bleach harming concrete but I suppose it could as it can harm wood, vinyl and other substrates BUT as long as you use common sense, don't use the bleach straight out of the jug and rinse it well - shouldn't be a problem.
 
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