Im hoping to not get killed for this question ive read tons and think i know my answer but was looking for some thoughts from the pros as im just getting into this sh mixing. I batch mixed for my brothers roof with pretty bad black stains. Mix ratio was around 5% with a cheap 6.5% bleach. Went back the next day to check it out and couldnt really tell i did anything? And am also having trouble gettting black mold spots off of siding at a 2% mix? Do i need stronger mix? New to the using sh and am trying to be careful of plant life i pre wet then during and after but wanted some thoughts? Thanks
1- how did you mix it
2- how did you apply it
3- how much did you use
4- what were you cleaning
5- what did you have for breakfast
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Cheap and OLD SH probably your problem. I buy my 12.5 from the wholesaler that sells to the pool places. I order the 53 gallon drum as their stock 5, 15 and 30 gal drums are anywhere from 1-4 months old. They have to special order the 53 gallon drums and when they come they sell them directly to me tested that day from the manufacture date stamp on it. 6.5% usually bought from places like target or Walmart who knows how long has been sitting on the shelf before it was sitting in stock before then wherever, that could be six months old and dropped to 4% or less. Try it full strength see how it works.
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i have a 4.5 gpm 12v with a 40 gallon tank and 50ft of hose. ive got it on a wagon that i can pull around the house with me like i said just stepping into this game. i mixed 20 gal of water with roughly 2.5 oz of 6.5% bleach and about 5 oz of apple wash. Stirred the tank a bit then drove to the house. spraying it on siding with dirt and black mold. Soaked the the one side of the 2 story house bottom to top. let sit a about 5 minutes rinsed with my 3gpm pressure washer. And that may be the problem i didnt eat breakfast that day. And the roof was mixed same way with 5 oz per gallon in a 10 gallon batch used it all on a 20x20 area? Thanks for any feedback
2.5 per gallon 
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For roofs you need at least a 4% mix. The cheap 6.5% stuff you got is probably pretty old and already down to like 4 or 5%. If you want it to work on the roof you just need to use that straight. But you need to be careful with roof mixes because it will kill plants and change colors on fences and paint.
Don’t get household bleach from Walmart, go to a pool store or Lowe’s/Home Depot and get pool shock. Better bang for the buck.
Ah, there’s your problem.
Say you want to achieve 3% SH potency and the bleach bottle says 6%, you’d have to do 10 gallons of bleach and ten gallons of water.
As for store bleach losing potency, I seem to remember our resident chemist pointing out the chlorine in SH will stabilize at 6.5% and is why you see that percentage on the bottles in the store.
One more thing, not sure about your location but in my area you need special add-on insurance to clean roofs separate from the normal pressure washing insurance.
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This makes my head hurt
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Lol I was waiting for that.
ok makes sense! Thank you! And i knew id make someones head hurt! 
But was better than i thought id get away with. Ive read alot but just couldnt quite figure it out. So was prepared to be whipped for the dumb question and just read remarks. And on the roof washing i wanted to test it out before i decided i could get into it efficiently. Thanks for heads up on insurance though.
This is a SH Mix Ratio Calculator I made to make it easy to
calculate the strength and weight of your Batch Tank.
Input into Calculator:
- Total Batch gallons needed
- Starting SH % strength
- Final SH % strength needed
Open file with Google Sheets, or MS Excel.
Edit values in green cells only, all others cells should be protected from edits
Moderator’s note: spam link removed
20 Gallon tank, You want a 4% mix using 6.5% bleach from the store.
You would need 12.30769231 gallons of bleach to hit that ratio.
Desired Strength / SH% your using * Tank Size in gallons = 12.3
This seems right, right?
yep, that checks out
