I was wondering if anyone might have an idea what happened here:
This house had a flat roof with hardy siding, I treated the roof with SH & down-streamed a house wash.
It was a very hot day so I’m wondering if this is caused by the bleach drying, or maybe it somehow leaked through from the roof? It only happened on one section of the house.
All input is greatly appreciated.
I’m going answer this with a question. Was this on the side with gutter or down slop of the roof? And there is no such thing has a flat roof they all sloop one way or the other, sorry law of physics. Second, would what is the roof covering?
That’s not true, but it sounds real cool doesn’t it 
Read this site regarding washing painted hardy board. Some paints are adversely affected by SH. I will let you read which one.
It might be that you just need to re-wash it, but I don’t know. I am not there and I don’t know what paint is on it.
Have you ever seen a roof that was totally flat that did not allow for water to drain off in some form or fashion?
Maybe hot roof mix got on the siding and dried?
That’s a strange question. Yes, I have seen a flat roof. No, I haven’t seen a roof that didn’t drain. Lot of today roofs have several inches of rock with drains every 6 ft or so. Point is, a flat roof doesn’t defy physics. Got good advice, give it. Don’t have good advice, remain silent, or talk about fishing or something cool. The thread is about bleach damage on Hardie plank.
I’ve washed probably 300 new-construction houses with Hardi-back and haven’t “bleached” one yet. However, the jerkwad who owned the house I just bought managed to do it. I have no idea what he used or at what strength, but I’ll have to repaint the house at some point. 
Hopefully you saw it before you bought it and could factor that into your offer.
The market is crazy atm so I did see it but couldn’t get much out of it concerning price.
Roof mix will damage any painted surface if it sits on it for more then a few sec. House wash mix will not 95% of the time. You should have had someone standing on the ground constantly spraying water on the hardi board sections to less the chances of this happening. It is what it is now.
Agree with the others, the roof mix found its way down and wasn’t rinsed in time, or it found its way down and that paint has the dreaded organic paint.
All so, flat roofs indeed are a thing and do not defy physics. While they are more prevalent in the commercial setting they are used in residential in some areas. They usually end up being used as a patio off the second floor. If they have a parapet then they usually drain via scuttle holes.
