This is a commonly asked question that is very important because if the roof is not thoroughly cleaned, the coating will adhere to the dirt and degraded roofing material instead of the actual roof surface. This will cause poor adhesion, blisters, delamination and dirt and other particles to be adhered to the underside of the applied coating - preventing proper adhesion of the coating to your roofing surface.
How to Properly Clean Your Roof:
- Pressure Wash the Roof - Pressure washing is the most common technique for cleaning the roof, as it is effective and can incorporate detergent to improve cleaning efficiency. It is important to note that you must be careful not to damage seams, penetrations, flashings and side and end laps when cleaning with a pressure washer.
- Brush the Roof with a Heavy Bristle Broom & Water Hose - If you don't have a pressure washer accessible, you can use a heavy bristle broom and water hose to clean the roof. Trisodium Phosphate cleaner (TSP) or other hard surface cleaner can be used to remove stubbornly adhered particles.
- Rinse ALL Debris Off Roof - After the roof is sufficiently cleaned, it must be thoroughly rinsed and all debris flushed down the drains or into the gutters. Residual dried soap film prevents adequate coating adhesion. If the coating is adhered to dried soap film, it would be similar to coating a bar of soap - you won't get acceptable adhesion!
How to Test the Roof for Adequate Coating Adhesion (after the roof is cleaned, rinsed and dry):
- Apply 2" masking tape to an area of the roof that had previously been dirty, such as a ponded water section or a drain sump. Press the tape firmly into the roofing substrate.
- Peel the tape back:
The photo above shows the back side of masking tape after pressing on un-cleaned modified bitumen (above) and after thorough cleaning (below).
- If it is difficult to peel off and there is no dirt or degraded roofing material on the backside of the tape, proceed with coating.
- If it peels easily with dirt or degraded roofing material embedded on the underside of the tape, you MUST clean again and retest!
This simple technique should be done on numerous areas of the roof, especially in sections that were the dirtiest and most difficult to clean. Testing your roof like this will insure a suitable substrate for coating and a successful project!