Painting a Metal Roof
This article is meant to clarify the answers to some of the most common questions our customers have about painting a metal roof. This information is aimed at the residential customer, but can easily pertain to a commercial application as well. However, commercial facilities will have other more industrial product selections such as epoxy and aliphatic urethane coatings to choose from that are less common in a residential environment. These types of coatings will not be covered in this article. Product selection will be limited to more readily available products with less involved application procedures.
This article is broken down into four basic sections. The first step is to determine what substrate you are painting actually is. This includes both the type of metal and any existing coatings that are in place. The second step is to determine the best method and products to prepare the existing surface for paint. The third step is choosing the correct coating system for the job. The final section will cover some of the dos and don’ts of applying the coating.
Please read through the entire article before beginning your project to familiarize yourself with the process as a whole. It’s all too easy to jump right into a project and find yourself suddenly overwhelmed.
Determining the Substrate
The first step in painting any metal roof is determining what substrate and existing coatings you are dealing with. This is a huge factor in what coating system you’ll want to use and is critical to determining how to prepare the existing surface.
There are several different types of metals used in roofing applications. Some are more common than others. Historically steel and copper have been widely used. Stainless steel and aluminum are newer and more expensive products used in roofing applications and as a result are less common. There are also various Terne products available. Terne is an alloy of a few different metals that is used to coat a steel core and provide anti-corrosion properties.
Let’s start by ruling out the most easily identifiable metal, copper. Copper is pretty easy to spot. It is shiny and copper colored when new, just like a new penny. It dulls and darkens to a brown color as it ages. Eventually it progresses to a green or greenish blue patina as it continues to weather. Regardless of exactly how old it is, copper tends to look nothing like the other “silver” metals used in roofing applications. Copper is also generally left uncoated so the color difference is easy to spot. Most people that opt for a copper roof are taking the patina into consideration as part of their choice and wouldn’t dream of coating it.
Now we can separate the ferrous metals from the non ferrous metals. The term ferrous refers to any metal with a primarily iron base. For our purposes that generally means steel. Most of your older roofs will be galvanized steel or Terne tin, which is also steel, despite the name. While the zinc coating used in the galvanizing process will not attract a magnet, the iron core material will. That means that a magnet will be the quickest and easiest way to determine whether the roof is steel. If a magnet sticks to the material then it is most likely galvanized steel or Terne tin. Stainless steel on the other hand is designed not to rust and the alloy metals added to the mixture generally prevent a magnet from sticking to it. Please note that some alloys of stainless steel will hold a magnet, but these types are not generally used in roofing panels.
Terne tin is a little trickier to determine. The term “tin” is a misnomer in this situation. It is very rare to find an actual tin roof. Generally, a tin roof is referring to one of two things. The first incarnation of the tin roof was a sheet of steel with a tin coating on the surface to prevent oxidization. The second incarnation is what we call Terne tin. This material still consists of a steel base, but it is coated with a Terne alloy in lieu of actual tin. Originally, tern was an alloy made from a blend of lead and tin. When lead was banned from use in construction Terne II was born. Terne II is an alloy made from a blend of zinc and tin.
As you can see, it can be difficult to determine whether a roof is galvanized steel or Terne tin without resorting to chemical tests. Fortunately, it doesn’t really matter. Both galvanized and Terne surfaces are prepared for paint the same way.
Let’s cover stainless steel now. Stainless is generally only used in commercial applications and even then it isn’t common due to the added expense. Stainless steel is naturally corrosion resistant, but it is still sometimes coated with an additional Terne II finish. The finish on stainless steel can be bright and shiny or matte depending on the product chosen. Generally, stainless steel is left uncoated and still retains a good appearance. There is really no need to paint stainless steel so it is one of the few roofing materials that will have a bright silver appearance. The Terne version will be harder to distinguish from normal Terne tin so a magnet will be needed.
Lastly, we’ll cover aluminum. Aluminum is more expense than steel and thus less common. It’s growing in popularity though because it is light weight, corrosion resistant, and dissipates heat quickly. A magnet will not stick to aluminum roofing material. Aluminum does not really corrode if left exposed, but it does discolor badly from the layer of protective oxidization that forms on its surface, making it an ugly natural finish. Most aluminum roofing will be coated, often coming from the factory in an assortment of colors. Sometimes it will also be anodized like the old paraplanes. Aluminum is very thin a flexible material and most often has a factory finish so it’s easy to spot.
Once the type of metal has been determined, you need to figure out what kind of coating if any is currently on it.
Traditionally, oil-based roof paints have been the most common product chosen. Most residential applications of any age will have at least one coat of oil on them and often dozens of coats will have built up over the years. Oil paints get brittle as they age and often start peeling as the number of coats increases over time. The paint film gets so hard and so thick that it eventually can’t keep up with the movement the metal experiences with temperature changes. When that happens the paint film starts to sheer from the metal and peeling results. These flakes of peeling paint will be very brittle and break easily, which makes spotting oil-based roof coatings fairly easy. If you can’t get peeling chips for some reason there is another way to tell if the roof has oil on it. Simply wet a rag with denatured alcohol and rub a spot with it. If little or no color comes off on the rag then you have oil paint. If the color does come off you have an acrylic paint. Alcohol is strong enough to melt acrylic products, but not strong enough to affect oil.
In more recent years acrylic DTM (direct to metal) products have become more common as a roof coating. Certain roofing materials, such as Terne tin and galvanized steel, accept waterborne coatings more readily than oil-based coatings. Acrylics are also more flexible than oil and thus handle movement in the metal better. They are also faster to dry, allowing a painter to turn a job around quicker. The speed of drying can pose its own problems though. DTMs are generally the product of choice for many factory finished metals. Peeling DTM will be flexible and rubbery. The color will also come off when applying the alcohol test.
Another less common roof coating is elastomeric paint. Elastomerics are also waterborne acrylics, but they have elastomeric resins built in as well. These products leave a much thicker and more rubbery film and generally only come in white. Even when a colored version is present they are easy enough to differentiate from a DTM. These coatings are mainly used in commercial or hospitality settings and are mainly used for their heat reflecting properties. This makes them more common in warmer areas such as Florida. Test for this product just like DTM, but pay attention to how thick and rubbery the film is and what color it is.
There is one other type of coating that is commonly found on buildings like barns or storage sheds. Asphalt roof coatings are sometimes used on these roofs because they are extremely cheap and are thick enough to fill pinholes in the metal. You have plain asphalt, fibered asphalt and fibered aluminum asphalt. Asphalt will be black and tarry. Fibered asphalt will look like normal asphalt but have more texture from the material fibers in it. The aluminum variety will look like fibered asphalt, but will have a dull aluminum color to it. All of these coatings are a real pain to deal with as far as recoating with other products.
Bare metal is the best substrate to work with in most cases. You don’t have the existing coating to deal with. Surface condition can be a factor though.
First let’s cover rust and corrosion on steel surfaces. If there is any rust or other corrosion that is loose and scaly it must absolutely be removed via mechanical methods. This can involve wire brushing, scraping, grinding, sanding, and other procedures such as sand or soda blasting. Remove as much rust and scale as possible. When you are left with just a residue you can choose to further treat the issue or to depend on the rust inhibitive properties of your coating system to put a “band-aid” on the problem.
It is recommended to further address the residual rust with a solution of phosphoric acid. Products such as Ospho are recommended for this. Ospho is a light phosphoric acid formulation. You simply pour it on the surface and brush it into place. Let it dry and repeat if rust is still apparent. Ospho will etch the surface of the metal and convert the iron oxide (rust) into iron phosphate, an inert substance that turns the metal black. Sweep away any powder left by the process and you are ready for primer and paint.
If no rust is present on the steel a simple pressure washing may be adequate to clean the surface of the roof. Be aware that a lack of rust indicates the presence of a coating such as zinc galvanizing or Terne. Galvanized or Terne surfaces must be etched with a product like Ospho to avoid adhesion problems with the coating system. When galvanizing or Terne is believed to be present, clean the roof via pressure washing and let dry. Then apply a coat of Ospho and let it dwell for 30 minutes. Rinse clean and allow the surface to dry completely before applying a coating system. You may allow the Ospho to dry and sweep it away in lieu of rinsing it if appearance is not as important. However, rinsing the surface will result in a smoother finish.
Let’s move on to aluminum. Aluminum should not form any major corrosion. A simple pressure washing is generally adequate preparation for an aluminum roof. If desired, the metal can be etched with Ospho just like steel. This will result in a better bond with the coating system. If scaling has formed for some reason handle the issue in much the same way as you would rust on a steel surface.
Next, we’ll cover stainless steel. You really shouldn’t paint stainless steel. It defeats the purpose of using it in the first place. When you absolutely must paint it, follow the preparation instructions for aluminum.
Finally, we’ll cover copper. Copper really should not be painted. It defeats the purpose of using it since copper can last centuries. It also takes away from the natural beauty of the metal. There is also the problem of keeping a coating on it. The chemical reaction that occurs when copper oxidizes burns off just about anything you put on it. So just leave copper alone.
Unfortunately, a painter is seldom blessed with the gift of a bare metal roof to work with. The great majority of the time the roof will have a previous coating on it. It could even have multiple coats present. Some old roofs have been found to have 30-40 layers of paint on them. When that much paint is present it may be time to replace the metal instead of painting it. Make sure you examine that option alongside the option of a proper repaint to determine which one is better in the long run.
If you have an oil coating on the surface in question, which is the most common situation, then you’ll want to remove any loose and peeling areas of paint via mechanical methods. This means scraping, sanding, wire brushing, and so on. Pressure washing will seldom remove all loose debris. Treat any rusting or corroding areas the same as you would if they were present on bare metal. If the majority of the coating is still sound you can get away with spot priming the bare areas you’ve exposed before applying paint. Otherwise, you’ll be best served by removing as much coating as possible before a full coating system of primer and paint is applied. Once the loose material is removed, pressure wash the surface to remove any residual contamination.
Surfaces painted with an acrylic product can be prepared much the same way as a surface coated with an oil product. Just be very careful when removing the loose material. Acrylics tend to peel in strips and keep on peeling if they are not securely adhered, whereas oils break up more readily as they peel.
Surfaces painted with an asphalt product are often a lost cause. Asphalts are extremely difficult to coat over effectively and even more difficult to remove. Generally, anything coated with asphalt probably isn’t material of any quality and you’d be better off replacing it. If you must paint over it, remove anything loose via mechanical methods and give it a good pressure washing.
There are a few final notes concerning preparation. It is recommended to use a detergent such as Jomax or TSP when cleaning and pressure washing a roof. Water alone will not always remove all of the contaminants. A detergent is essential to achieving an optimal surface. Once your surface is clean and ready for coating do not leave it for more than a day or two before applying the primer. The more time you allow the surface to remain exposed, the greater the chance it will become contaminated again.
A Note on Lead
If you have an older roof you should familiarize yourself with the hazards of lead. Many old roof coatings and Terne formulations used lead in the mix. When grinding or scraping where one of these coatings is present you are releasing lead dust into the environment. Breathing this dust can result in lead poisoning.
The EPA has spent the last few years cracking down on lead as well. It’s treated much like asbestos now and heavy fines can result if you make a mistake handling it. Consult the EPA website for more information.
Now that your roof is prepared for paint you need to choose the correct coating system for the existing surface. There are three main types of roof coatings for general residential application. These include traditional oils, newer Acrylic DTMs, and Acrylic Elastomerics.
Types of Coatings
Traditional oil coatings are linseed based long oils designed to bond tightly to the metal and provide as much flexibility as an oil coating can. This is the type of coating that has been used for many decades and remains the most common product in use today. The biggest drawbacks to oil coatings include lack of colorfastness, brittleness with age, and long dry times. The longer dry times make for slower turnaround and require that care is given to the time of day you are painting. In spring and autumn weather, painting too late into the day with an oil product can make it susceptible to spotting from evening dew.
Acrylic DTMs are newer products that have come down from the industrial and railroad divisions. These products have been formulated to take the place of traditional oil coatings in a world that is constantly pushing for GREEN building initiatives. They are very durable, have better colorfastness than oils, better flexibility on metal surfaces, and have a faster turnaround time since they dry rapidly. The drawback to the rapid dry time is that you must be careful not to apply them in direct sunlight or on very hot days to avoid lapping and flash drying, which will result in a less than desirable appearance and possible adhesion issues.
Elastomerics are more of a specialized product. They are more commonly used in commercial and hospitality projects, but can be used quite successfully in a residential application. An elastomeric coating is a waterborne blend of acrylic and elastomeric resins. It is much thicker than a normal acrylic coating and provides a thick rubbery membrane when dry. These types of coatings are generally used in warmer climates such as Florida due to their ability to reflect heat and provide a little insulation from the sun. They often contain ceramic micro-spheres to further promote heat reflection. Elastomerics are most readily available in white, which is also the best color to reflect heat. Other colors are sometimes available, usually on a special order basis, and more common in cooler climates like Canada. Elastomerics behave very much like acrylics with the exception of having a much lower spread rate due to the their thick viscosity. Their heat reflecting properties as well as the ability to fill small pinholes are their strongest advantages. However, most homeowner generally do not like the appearance of elastomeric coatings, especially when they are available only in white.
Choosing a Product
Which type of product to use depends largely on the surface you are coating as well as the intended application. Considering the focus of this article, we’ll assume the intended application is your standard residential roof. Therefore, we can focus on what the substrate calls for.
When to Use Oil
An oil system should be used when you have a very rusty ferrous metal surface that you do not intend to etch and convert with a product like Ospho. Oil roof primers and paints are naturally better at handling rust bleed through and reducing future oxidation of the metal.
Oil products should also be used when the existing coating is already an oil product, especially when multiple coats are present. If you have an old roof with existing oil paint on it you are better off recoating it with more oil unless you are willing to strip the roof to bare metal. Acrylic DTMs and Elastomerics have an aggressive bite and will often bond to old oil without an issue. However, the acrylic and oil products expand and contract and a much different rate. This usually results in the acrylic coating pulling the old oil coating loose, which will result in peeling. The top coat is only as good as what lies beneath it.
When to Use Acrylic DTM
Acrylic DTMs should be used on any surface that is not friendly to oil products. These include aluminum, galvanized, and Terne products. You will get a much better bond to the properly prepared surface of one of these metals from an acrylic than you will from an oil.
Acrylic DTMs should also be used when colorfastness and flexibility are a major concern. They will greatly outperform oil products on both accounts.
Acrylic DTMs are also the product of choice when trying to coat over old asphalt products. There is no guarantee in this situation that anything will really stay bonded to the old asphalt surface, but acrylics have a better chance of adhering than oils.
In general, any new surface should be properly prepared and coated with an Acrylic DTM unless the specific properties of an elastomeric are called for. Oil will eventually disappear under pressure from the EPA. There is no reason to coat a new surface with a dying technology and have to deal with conversion issues in the future.
When to Use Elastomerics
Elastomerics need little explanation. They are such specialized products that very few situations will call for them. They are primarily called for if you are specifically looking for their heat reflecting properties.
Application Dos & Don’ts
Generally, application of any of these systems will consist of applying one coat of rust inhibitive primer and two coats of finish with proper dry time between coats. It is important that the primer be rust inhibitive on ferrous surfaces. In the case of oil products you will most likely be looking at a red oxide metal primer. Acrylic DTMs are also available in a red oxide variety. Both types also commonly come in gray and sometimes white, but red oxide is preferred for the greatest rust inhibitive capability. Elastomeric coatings should be applied over a coat of red oxide acrylic DTM primer.
If you have a sound existing coating and are applying a coating of the same type of product over it, i.e. oil over oil, then fully priming the roof is not necessary. Spot prime any bare areas and then apply two coats of finish product over the entire roof.
Application via brush, roller, spray or a combination thereof is acceptable provided proper film build of each coat is achieved. Use a film gauge to check this if necessary.
Avoid applying a coating in direct sunlight, especially waterborne coatings. Also be sure to honor the approved range of temperatures for the product in question. Generally, surface temperatures should fall between 50 and 90 degrees Fahrenheit. Surface temperature refers to the temperature of the surface you are painting, not the ambient air temperature. A metal roof that has been in direct sunlight for a while can be much hotter than the air around it. Too hot of a surface can cause the solvents in a product to flash off too fast and prevent proper bonding. This can also result in visual defects such as dull and shiny spots and a crinkled film. Too cold of a surface can retard drying and curing of the product and also result in adhesion issues.
Avoid painting too early or too late into the day, especially in spring or autumn weather. Dew is often the culprit when visual problems occur on an exterior paint film. Painting a surface too early in the morning when the weather has been very cool does not give dew time to dissipate and can result in adhesion and blushing issues. Painting too late in the day allows dew to form before the paint film is dry and can also cause blushing.
Be careful to get good coverage on the seams of the roof. These are the most difficult areas to coat well when applying product with a roller or brush. The flat surfaces are much easier to achieve full coverage.
Refer to the documentation or sales representative of the specific product you choose to use for full recommended application procedures.
The proceeding information should make it clear that application method is only one small part of the equation when dealing with a metal roof. Properly identifying the existing surface and correctly preparing it for a coating are essential steps to achieving a good looking paint job that will last for many years.
The length of this article may make painting your roof seem like a formidable task, but be assured that it is not. There are simply numerous variables that need to be covered. Once you have identified the substrate and chosen a product to apply, you’ll always know the best system for future use. Then it all comes down to common sense and knowing how to use your painting equipment.
This article was written with an emphasis on identifying the surface to be painted, properly preparing it, and choosing which type of coating is best for your application. Application tips have been provided but application procedures have been kept to a minimum. Once you’ve chosen a brand and specific product for your job, it is best to adhere to the manufacturer’s recommended application procedures first and foremost.
I hope this information has been helpful to those of you about to tackle a roof painting project. Please feel free to comment with any questions or concerns. I’ll try to keep this article up to date with any new information that seems relevant.
I have worked in retail and commercial sales of residential and industrial coatings for over a decade. I am not a chemist or a metallurgist, though I do frequently have input from the chemists and field representatives of the paint manufacturers with which I deal. My knowledge is based solely on my real life experience with the products and situations discussed above, not on any official training. Please use this information as a starting point for your project, but discuss the details with your local paint representatives before proceeding.
By Chad Simmons – Morris Paint & Floor Covering, Inc.