Being smart about applying a roof coating falls mainly on the coating applicator in the field, which is perhaps a blunt way to start an article, but in truth, the applicator is the last line of defense in making sure the surface is ready before the coating is set upon it. In nearly 20 years of formulating coatings the author has had or read of instances where an angry contractor has called wondering why the coating is not adhering to the surface. The response that comes to mind: the coating is sticking exactly how it is supposed to on the surface to which it was applied. After all, the coating itself lacks intelligence and cannot decide whether it sticks here or there. It goes wherever it is applied and adheres to what is beneath it. Unfortunately, often times, exactly "what is beneath" the coating may not be what the contractor intends for the coating to adhere to, resulting in unwanted headaches, angry phone calls, and problems in the future. A properly adhered coating begins with a properly prepared surface. All of the formulating, testing, field trials, etc., are of little use if the substrate is not treated with the same care and attention paid to applying the new coating. This article will focus on techniques available to contractors to improve the adhesion of a roof coating so that the coating can spend more time on the roof and not delaminated or damaged, making more problems for everyone involved.
It’s All about Contact Points
At its simplest level, the key to coating adhesion can be summed up in two words: contact points. If a mineral surfaced modified bitumen sheet were magnified and viewed along its edge, one would observe countless ridges and valleys. One would see the same profile in a sanded piece of wood prior to painting. Wood workers know that these peaks and valleys in the sanded wood provide the coating with more "tooth," or contact points, for the paint or varnish to adhere to than the same piece of wood sanded smooth. The same is true with a roof surface.
When possible, a rough surface is desired because of the increased surface area and greater number of contact points. If a contaminate is present on this rough surface, the coating may adhere to the contaminate and not the intended surface at that specific point. The contaminate can be anything: dust, dirt, bird droppings, water, fungus, algae (see problem below), or even an existing coating. Any interruption of the continuity of the coating/substrate interface, however small, can be a detriment to the bond integrity over its life. The goal of surface preparation is to give the roof coating as many contact points as possible to give it the best opportunity to perform as the contractor and manufacturer intend. In the next section, a list of common bond breaker problems is shown along with helpful ways to improve contact points and enhance adhesion. This is not the total list; many contractors will encounter problems not listed here. But this list starts the process of thinking about those problems not mentioned. The common theme is that each of these problems break the continuity of the coating/surface interface, reducing contact points, and ultimately resulting in adhesion issues and reduced service life. Additionally, it is recommended that the applicator take advantage of the knowledge a trained roof coatings professional can offer because they know the coating best and will suggest the best way to get the coating to adhere right the first time.
Common Bond Breaking Problems and Solutions to Increase Contact Points
PROBLEM: End Lap or Seams Opened - Fish-mouthed open seams are ways for water to enter underneath the membrane and between the coating/surface interface by creating an unprotected edge. Even if the gaps are small, a water molecule is much smaller and these openings cannot be fixed by putting a coating over it no matter how thick it is applied.
SOLUTION: Open seams should be repaired using similar materials, using recommended roofing practices.
PROBLEM: Wet Insulation or Sub layer - In some situations, the surface feels dry but may be holding pockets of wet insulation beneath. Unfortunately, there is no way of knowing if - through the day - that water flows through some point of ingress on the roof surface. Upon coating, the coating acts as a barrier and the water pushes the coating off of the roof surface from behind.
SOLUTION: An infrared scan of the roof before coating will identify wet areas that can be removed and corrected ahead of coating. This could also be an opportunity to address poor drainage in that area and build up the surface to direct water to the drain.
PROBLEM: The Weather - Part of preparing a roof for a coating is knowing the environmental condition of the roof not only before the coating is applied, but a few days after the coating is applied. Remember that unless a fast curing coating such as a two-component polyaspartic or polyurea is being applied, where cure times are measured in minutes or less, the typical roof coating will achieve full strength in timespans of hours or days. During that process, even though the physical work of the contractor may be done for the day, the coating is busy curing or drying and is susceptible to all of the problems that would have plagued it during application. Even the best surface preparation and application done on an ideal sunny day could go wrong if something as unassuming as the dew point come close to the coating's surface temperature during its setting time. The dew point is the temperature at which no more water can be put into the air. When the surface temperature and the dew point approach each other, the water in the air must condense on a surface. Dew usually happens on cooler nights, especially when the day has been humid. High dew points (>50°F) usually forewarn dew events the next day because evening temperatures can come close to this. This is where careful study of the weather can be a great benefit.
SOLUTION: Roofing contractors should be amateur meteorologists. Take some time to learn about basic weather terminology. Find a trusted accurate weather source such as noaa.com or Wunderground to provide detailed 48-hour forecasts. Aside from the obvious forecast of rain, pay close attention to numbers like dew point, and falling atmospheric pressure, which indicate a coming rain event. Morning dew should be completely removed before applying a coating. Like dirt and grime, water is a contaminate on a roof surface that will interrupt the continuity of the coating/surface interface, reducing the contact points.
PROBLEM: Ponded Water - This problem could also be filed under "Same Roof/Different Surface," but because it is so prevalent, it was given its own section. Roofs are designed to move water off of them as quickly as possible. As ideal as this concept is, the reality is that ponded water is an unavoidable fact of life. Whether caused by poor building design, improper drainage caused by elevation differences, lack of maintenance on HVAC units dripping rusty condensate in an area, or external variables such as wind or varying snow loads, identifying ponded water areas before coating is critical to the success of the coating. It should be noted that the author is not referring to short duration ponded water, such that occur after a rain storm. These instances are considered acceptable by roof manufacturing associations. However ponded water that remains for more than 48 hours can cause problems for roof coatings.
SOLUTION: Ideally, the drainage should be addressed before the coating is applied. However, this cannot always happen - for example, in the case of a restoration coating applied after the fact. In situations where the drainage cannot be addressed, the first thing to realize is that the surface under the ponded water is going to have a lot more dirt, grime and other microbes, spores, etc. (see next problem) than drier areas. So the approach to cleaning these surfaces should be more aggressive than what would be done to a drier area. If the drier areas are going to get a pressure washer treatment, the ponded water areas should get more than a pressure washer treatment such as bleach solution, Simple Green®, or ZymeAway® followed by a vigorous scrubbing, thorough rinse, and dry.
The key here, beyond the equally important choice of coating, is to do more in the way of preparation in these low areas than the drier portions of the roof because it is a certainty that the coating will be subjected to the same ponded conditions. Unfortunately, standing water areas are a bane to roofers because - in spite of the best preparation, and by its very nature - water wreaks havoc on roof coatings, period, although some degrade faster than others. There is no "Holy Grail" coating, not that it stops formulators from trying. However, the roof coating stands a fighting chance at a longer service life if the coating is allowed to fully envelop the intended surface and not the "stuff" left from the ponded water. Be sure to follow federal, state, and local laws governing gathering and disposing rinse water. Drying the cleaned surface is crucial also - remember that water is a bond breaker also (see first problem).
Photo 2: Power washing a roof is a great way to remove dirt and grime, but if the water does not go to a drain and sits in a low spot, the coating will be subjected to the same ponded conditions once installed (Click image to enlarge)
PROBLEM: Fungus and Algae - Fungus and algae are present anywhere where there is water, sun (warmth), a food source, and something to sit on and ponded water can provide all of this. Fungus and algae take full advantage of the sun-soaked water and propagate on any surface where the spores can attach, this includes the roof substrate. The spores eat anything they can, including the calcium carbonate filler present in many roof coatings. This is why simply spraying the surface with a power washer to remove surface dirt is not enough, especially where ponded water exists. Sometimes, though, there are tree branches extending above the roof or nearby, dropping their spores onto the surface and discoloring the roof.
SOLUTION: As in ponded water situations, it is best to do more than power wash the surface, such as incorporating a solution of fungus or algae remover like ZymeAway® or similar product designed to remove the microbes from the surface and increase contact points. As with any chemical that is to be rinsed, be sure to follow federal, state, and local laws governing gathering and disposing rinse water. Drying the cleaned surface is crucial also - remember that water is a bond breaker also (see first problem).
Photo 3: When power washing is not enough, scrub with a bleach solution (1 part chlorine bleach, 3 parts water) or roof cleaner followed by a thorough rinse and dry the surface (Click image to enlarge)
PROBLEM: Coating Over an Existing Coating - Due to aging, aesthetics, degradation, or as part of a warranty, there may be a time when a coated roof needs a new coating applied. There is a risk of the new coating, especially a solvent-based one, softening the existing coating and causing delamination or localized blistering later on.
SOLUTION: It is important to realize that the adhesion of the new coating is only as good as the existing coating's adhesion to the metal, mineral surface, etc. Secondly, and equally important, the new coating will inherit the issues of the previous coating if the new coating is applied directly to the old coating. With that in mind, it is recommended that preparation be given proper consideration. If power washing does not remove all of the existing coating, is the risk of not removing the remainder worth the problems later when the solvents in the new coating swell the resin in the existing coating causing delamination? Sanding the residual existing coating on a metal roof helps by creating a rougher surface (more contact points) and also exposes more of the intended surface for the new coating. If time and budget permit, it is recommended to include additional means to removing the existing coating as much as possible whether through sanding on metal, or through cleaning agents and vigorous scrubbing on a mineral surface, single ply, or metal roof.
Another solution would be to consider using a primer. Primers aid in the adhesion of the coating by providing a bond between the roof surface and the coating. It should be noted that a primer is never a substitute for proper roof surface preparation, but priming provides a uniform surface for the topcoat with infinite contact points to adhere to while at the same time, biting into the substrate and existing coating. It is recommended that the coating manufacturer be consulted before a decision is made to use primers in any situation.
PROBLEM: Fasteners on Metal Roof are Missing - Throughout the life of a metal roof, the thousands of screws penetrating the panel, each containing their own grommets or washers, dry out and crack leaving holes for water ingress and the opportunity for corrosion to occur.
SOLUTION: It will be too late once the new coating is applied to address this, so it is important to check fasteners before coating. Replace missing fasteners and tighten loose ones. Fix fish-mouthed seams and, if on a horizontal face, install extra fasteners as an insurance policy.
PROBLEM: Vent Pipes, Protrusions, or Old Lines No Longer Used - During the building's life, repairs to the A/C or interior remodeling may close off and render old vent pipes obsolete. However the building owner does not remove the pipe or take out the drain lines.
SOLUTION: Consider finding out if old protruding pipes or unused drain lines could be removed and the surface sealed over, removing one less vertical surface.
In roof coatings, the surface with the most contact points over the intended surface has an increased service life when compared to the same coating applied to a poorly prepared surface. This article listed some of the common bond breakers that interfere with the continuity of the coating/surface interface, thus reducing the contact points. Some solutions were common sense ones, but others may not have been. A trained roof coating professional is an invaluable source of solutions to these and other questions before the coating is applied. The point to take home is this: when the manufacturer recommends that the surface be free of all dirt, dust, grease, and water prior to coating, do the smart thing and prepare the surface well.
Leonard, J. (2008, January/February). Maximizing Roof-Coating Performance in the Emerging Era of the Cool Roof. Retrieved January 2016, from Journal of Architectural Coatings: www.jac.com
MS Editorial Staff. (2007, March). Roof Coatings Considerations. Retrieved January 2016, from www.Facilitiesnet.com
Opel, J. J. (2008, December). Proper Surface Preparation is Essential Before Applying Roof Coatings. Retrieved January 2016, from www.Facilitiesnet.com
Wilen, J. P. (2012, July). Still Water Runs Deep - Proper Roof Slope and Drainage are Important to Prevent Excessive Water Accumulation. Retrieved January 19, 2016, from www.professionalroofing.net
Jason Smith is the Sr. Research & Development Chemist for The Garland Company, Inc. He has multiple US and foreign patents directly related to roofing and has written several articles related to coatings applications and solvent regulations. Jason is a member of the Roof Coatings Manufacturers Association and serves as the co-chair for its Technical Committee.
Smith received his undergraduate degree in Chemistry from The University of Pittsburgh and his Masters Degree in Polymer Chemistry and Coatings from DePaul University in Chicago.