By William A. Kirn, RRC Technical Director National Coatings Corporation
While asphalt is considered very hydrophobic (read: it makes a great waterproofing material) when it is formulated as an emulsion, it contains additives to keep the asphalt stable in the water carrier. However, the additives that enable the asphalt to remain dispersed and stable in the emulsion can have a negative effect on the adhesion of any material applied on top. This can be especially true of a coating.
There are some simple tests that can be conducted to predict how well a coating will adhere to the dried asphalt emulsion.
- First be sure the asphalt emulsion is fully dried through completely down to the substrate to which it was applied. Remember the first place the coating dries is at the air interface (read: skins over) and the last place it dries is on the existing roof. If the asphalt emulsion has not fully dried, there will be adhesion problems going forward. If the asphalt were applied in one thick coat at high spread rates, (>4 gallons per Sq in one coat) then it may take weeks for the asphalt emulsion to dry. If the weather conditions are marginal, high dew point, fog, rain, then it will take even longer for complete dry-through.
- Some asphalt emulsions are so heavily laden with emulsifiers and surfactants (“soapy stuff”) that they can even reemulsify. This can be checked by applying a spot of water to the dried asphalt and rubbing with one’s finger. If the water turns black and feels slippery, then the asphalt emulsion contains too much residual “soaps” to allow for adequate adhesion, especially in areas of ponded water. Thorough rinsing with water using a pressure washer (being careful not to damage the roof with high pressure) should dissolve the soaps. Remember to flush all the water off the roof and not allow it to collect (and dry) in ponded areas.
- If water rinsing was required, the simple spot test described above in #3 should be repeated to insure the washing has thoroughly removed the soapy materials. If the surface still feels soapy, then the rinsing process must be repeated.
- A final field test is conducted on the dry asphalt covered surface. A piece of 1” masking tape about 5-6” long is pressed onto the surface. It is then pulled back at 180o direction. If the tape has satisfactory adhesion, then it will not peel away easily nor will its adhesive surface be contaminated with dirt, degraded asphalt or other materials that will interfere with adhesion.
- These two tests will aid in determining and predicting the adhesion of the coating to the asphalt.