A properly adhered coating begins with a properly prepared surface. Coating formulation, testing and field trials are of little value if the substrate has not been treated with the necessary care and attention. This responsibility falls mainly on the applicator — the last line of defense when making sure the surface is ready for coating.
This article focuses on techniques available to contractors to improve the adhesion of a roof coating and avoid delamination and damage. Watch a video on the subject at the end of the article.
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, many ridges would be visible. That same peak-and-valley profile would appear on a piece of sanded wood, if properly prepared for painting. Unlike smooth-sanded wood, a surface with these ridges has more "tooth" — or contact points —- to which paint or varnish will adhere.
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 so it has 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 included here, but being aware of these issues will help them anticipate problems not mentioned. The common theme is that each of these problems breaks the continuity of the coating/surface interface, reducing contact points and ultimately resulting in adhesion issues and reduced service life. To ensure dependable performance, applicators should consult trained roof coatings professionals who know the coating best and will recommend the best way to get the coating to adhere right the first time.
End Lap or Seams Opened
Fish-mouthed open seams create an unprotected edge, allowing water to enter underneath the membrane and between the coating/surface interface. Even if the gaps are small, a water molecule is much smaller, and putting a coating - no matter how thick it is applied - over the breaks will not fix the problem.
Solution: Open seams should be repaired using similar materials and recommended roofing practices.
Fish mouths like this one need addressed before coating. A coating will never be a suitable substitute for proper repairs.
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 that water flows through some point of ingress on the roof surface. If it does, the coating can act as a barrier and water builds up behind it, pushing the coating away from the roof surface.
Solution: An infrared scan of the roof will identify wet areas that can be removed and corrected before 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.
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. Unless the contractor is applying a fast-curing coating, such as a two-component polyaspartic or polyurea - where cure times are measured in minutes or seconds — the typical roof coating will achieve full strength in time spans of hours or days. Although the physical work of the contractor may be finished for the day, the coating continues to cure or dry 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 comes 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 previous day has been humid. High dew points (over 50 degrees Fahrenheit) usually forewarn about dew events the next day. 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 such as 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.
Although roofs are designed to move water off of their surface as quickly as possible, ponded water is an unavoidable fact of life. Whether caused by poor building design, improper drainage due to elevation differences, lack of maintenance on HVAC units dripping rusty condensate in an area, or external variables such as wind or varying snow loads, any ponded water areas must be identified before coating if the job is to be a success. Note that instances of short-duration ponded water, such as those occurring after a rain storm, 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, when a restoration coating is applied after the fact. In situations where the drainage cannot be addressed in advance, 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 and spores (see next problem) than the drier surfaces. The approach to cleaning these wet surfaces should be more aggressive than that planned for a drier surface. If the drier areas are going to get a pressure-washer treatment, for example, the ponded water areas should get more than a pressure-washer treatment — such as bleach solution or an enzyme mold and mildew cleaner — followed by a vigorous scrubbing, thorough rinse and dry.
The key here is to do more preparation in these low areas than in 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, despite the best surface preparation, 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 to create it, but the roof coating stands a fighting chance at longer service life if it is allowed to envelop the intended surface and not the stuff left by the ponded water.
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.
Fungus and Algae
Fungus and algae are present anywhere there is water, sun (warmth), a food source and a surface where they can rest; 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, including the roof substrate. The spores eat anything they can, including the calcium carbonate filler present in many roof coatings. Sometimes there are tree branches — extending over 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 incorporate a solution of fungus or algae remover 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 of rinse water. Drying the cleaned surface is crucial also - remember that water is a bond breaker (see first problem).
Coating Over an Existing Coating
Due to aging, aesthetics, degradation or warranty requirements, there may be a time when a coated roof needs a new coating. If the new coating is applied over the old, there is a risk that the new coating — especially a solvent-based one — will soften the existing coating and cause 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 or other surface. The new coating, if applied directly to the existing coating, will inherit its issues, so preparation must be given proper consideration. If power washing to prepare the surface does not remove all of the existing coating, is it worth leaving the residual coating in place at the risk of future problems? For example, the solvents in the new coating could swell the resin in the old coating, causing delamination.
Sanding the residual 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, additional means should be employed to remove more of the existing coating, including sanding on metal or using cleaning agents and vigorous scrubbing on mineral surfaces, single ply or metal roof.
Another solution is using a primer. Primers aid in the adhesion of the coating by providing a bond between the roof surface and the coating. A primer is never a substitute for proper roof surface preparation, but after this stage, priming bites into the residual coating and substrate and provides a uniform surface with a vast number of contact points for the topcoat to adhere to. The coating manufacturer should be consulted before deciding to use primers in any situation.
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 insurance.
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 old vent pipes and render them obsolete. The building owner may not have removed the pipes or taken out the drain lines.
Solution: Find out if old protruding pipes or unused drain lines can be removed and the surface sealed, eliminating a vertical surface.
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. Consult with a trained roof coating professional — an invaluable source of solutions — and prepare the surface well.