Cleaning a Metal Roof
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Identification
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Before starting a roof cleaning project, it's important to understand exactly what you're dealing with. The three most common aesthetic problems facing owners of metal roofing are zinc oxide (causing a white, powdery stain, often streaked along the seams), algae or mold (dark green to black staining, usually developing in wet, shaded areas), and rust.
Each of these culprits requires a different approach to effectively cure the problem and leave you with a beautiful metal roof when you're through.
Preparation
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Zinc Oxide: Remove the worst of the stain with a piece of steel wool or similar mildly abrasive cleaning pad. Zinc oxide stains can be an eyesore, but they're not dangerous to you or your roof, and will not prevent any paint or coatings from adhering to the roof.
Algae or Mold: Mix a mild chlorine bleach solution in preparation for pressure washing. While washing the roof with a pressure washer may remove the stain initially, if the algae or mold spores are not killed bleach, the stain will likely return.
Rust: Similar to zinc oxide stains, the best method for removing these stains is steel wool or other mild abrasive. If the rust is particularly stubborn, a stiff metal brush may be used, but be careful not to overdo it, as a stiff brush could cause damage to the underlying metal, threatening the integrity of the roof.
Washing
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In all three cases, after the above preparatory work is completed, prepare a solution of trisodium phosphate (a mild detergent available in most paint or hardware stores) in a dilution of one-half cup per gallon of water. As noted, if you are cleaning a roof stained by algae or mold, include chlorine bleach in this solution as well. The quickest and easiest method of applying the solution is by using a pressure washer, but it is strongly recommended that you follow the manufacturer's instructions very carefully as a pressure washer used carelessly can cause unexpected damage to the roof.
After the entire roof has been thoroughly washed using the TSP/bleach solution, be sure to spend an adequate amount of time thoroughly rinsing the roof as well. Pay special attention to areas where moisture can pool so the chemical is not allowed to dry on the roof.
Final Steps
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To make sure your finished product is as clean as can be, especially if you plan to paint or coat the roof, apply a short strip of wide masking tape to the roof, pressing it down firmly, then peel it off. If it comes right off, laden with bits of dirt or debris, you've got more work to do. If it adheres tight and peels off clean, you've got a roof ready for painting or coating.
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