Will a Roof Coating Extend the Useful Life of Your Roof?
By Jack Gray, Roof Online Editor • Updated April 8, 2023
Table of Contents
- Is a Roof Coating a Good Idea?
- How Roof Coatings Work (or Don’t)
- Roof Coatings on Shingled Roofs
- The Roof Coating Industry
- Roof Coating Warranties
- A Recent Roof Coating Experience
- Related Pages
Is a Roof Coating a Good Idea?
Roof coatings can be a good idea, sometimes.
Roof coatings can extend the life and improve the performance of your roof, sometimes.
But sometimes a roof coating can be a terrible idea.
A roof coating could easily be one of the biggest wastes of money you’ll ever end up kicking yourself over.
The thing is, a roof has to be in good condition when the coating is applied in order for you to see a benefit.
How Roof Coatings Work (or Don’t)
Roof coatings don’t somehow rejuvenate the roof by infusing it with new strength and vigor.
A roof coating simply protects what is already there. If there’s almost nothing there, a roof coating will provide almost no value for the money.
A roof coating is not a roof. It should never be counted on as the main barrier between the weather and your building. Roof coatings depend on the underlying roof in order to function.
This is why building codes have almost nothing to say about roof coatings except to repeat the minimum slope requirement for flat roofs and list the ASTM material standards that apply.
A common feature of flat roof membranes is that as they age, they become weaker and less flexible.
Asphalt chains break down and asphalt roofing becomes brittle. The plasticizers added to PVC roof membranes migrate out of the material. Every roofing material weakens with age.
As the roofing material becomes more fragile over time, it becomes more and more likely to split, tear, and develop holes due to the expansion and contraction of the membrane while it undergoes normal daily thermal cycling.
The roof membrane expands when it heats up during the day and shrinks when it cools down at night. A roof coating won’t help a roof that no longer has the strength to resist these cycles.
If the coating is applied, say, halfway through the life of the roof, it can slow down the processes which weaken the roof. By doing this, a roof coating will extend the life of a roof.
That is the main benefit of a roof coating.
But if the coating is applied after the roof is already worn out, it does no good.
Roof Coatings on Shingled Roofs
This is simply a bad idea and goes against the recommendations of every reputable shingle manufacturer.
In almost no case will a roof coating be suitable for a pitched roof. According to the Asphalt Roofing Manufacturers Association:
“coatings may negatively impact the performance characteristics (including the fire classification, algae resistance, impact resistance, etc.) of the roof assembly”
“the problems reported after asphalt shingle roofs have been field coated include shrinking of the coating, which may result in unsightly curling and/or cupping of the shingles or loosening of the granule surfacing of the asphalt shingles. In addition, non-permeable roof coatings may create a vapor-retarding layer by sealing the voids around and between the shingles. If this occurs, it may contribute to moisture accumulation within the roofing system.”
The moisture that accumulates in the roof sheathing generally comes from interior humidity which condenses on the underside of the roof sheathing.
Heating and air conditioning lead to different levels of humidity and temperature between the inside of the house and the outside air. These differences lead to condensation.
If you coat the roof, that moisture will have nowhere to go. It will remain trapped in the sheathing, laying the groundwork for dry-rot and mold.
Another good source for impartial information about the roof coating products marketed for use on asphalt shingles is Myth Busting: The risks and unverified benefits of field coating asphalt shingles in Professional Roofing magazine (published by the National Roofing Contractors Association).
The Roof Coating Industry
Roof coatings are barely regulated by building codes and there are no official standards governing roof coating performance. Because of this, the roof coating industry is saturated with shady coating salesmen, suppliers, and manufacturers.
Many roof coating companies are simply groups of marketers who buy their coating material on the cheap from some industrial chemical manufacturer with poor quality control, and then outsource the coating application work to poorly-trained applicators.
This doesn’t mean that there aren’t any reputable roof coating manufacturers who produce quality materials and will be honest with you about what a roof coating can and cannot do.
It simply means that you need to be as careful when buying a roof coating as you would be if you were buying a used car.
Roof Coating Warranties
All roof coatings come with a warranty. You can buy roof coatings with 5-year, 10-year, 15-year, and, in some cases, even 20-year warranties. The catch is that these warranties are material warranties.
A material warranty simply guarantees that when the coating material left the factory, it was free of defects.
A material warranty does not cover the application of the coating. If the applicators put the coating on too thin, if they forget the reinforcing fabric at flashing locations, if they apply the coating while the roof is wet, the warranty doesn’t cover it.
A roof coating warranty will never cover problems due to the performance of the existing roof, and that’s the real issue. If you have a 10-year warranty on your roof coating but the roof has serious problems and starts to fail in 3 years, your coating warranty won’t help you in the least.
If your roof has blisters or areas with wet insulation, and the roof coating delaminates and falls off in those areas, the coating warranty won’t cover it.
Roof coating warranties are, for the most part, something that roof coating salesmen can point to in order to convince you to buy their product.
Roof coating warranties are practically worthless. I want to emphasize that. If you think that your roof coating warranty is going to cover you if your roof starts leaking, you are almost certainly wrong.
A Recent Roof Coating Experience
We were hired by a Fortune 100 company to oversee a major re-roofing project at one of their largest manufacturing facilities. The facility has dozens of buildings and roof areas that total in the millions of square feet.
Some of those buildings had spray polyurethane foam roofs that had been installed on top of two layers of old built-up roofing. The job consisted of grinding off (scarifying) a few inches of the existing foam on a few of those roofs, cutting out and replacing wet insulation, and applying a new layer of foam with a new silicone coating.
This job went very well, the new roofing was a high quality installation, and the client was set for another 20 years of good roof performance on those buildings.
The following year they contacted us again about doing the same thing on another, larger, roof area.
We had infrared and nuclear capacitance surveys of the roof performed to identify areas of wet insulation. This would normally be overkill, but there were three existing roof systems in place on the building, which made identifying wet areas within the roof particularly challenging. We started putting specifications together. We lined up contractors to bid on the work.
Then a roof coating salesman got hold of the facilities director.
Right before the job was supposed to go out to bid, we got an email from the facilities director suggesting that he was now planning to simply re-coat the roof and asking us for our opinion on a certain brand of acrylic roof coating.
He pointed out that they were giving him a 10-year warranty and that the price would only be about 20% of what our proposed project would cost.
This was our response:
Our Roof Coating Advice to the Facilities Director
Dear Xxxxxx –
Regarding the spray polyurethane foam roofs:
We do not consider these roofs to be good candidates for a roof coating application, especially an acrylic roof coating. Here are our concerns:
Roof Coating Compatibility Issues
These roofs have an existing silicone coating. Silicone coatings are known for being incompatible with other coating chemistries.
The following is a quote from the Spray Polyurethane Foam Alliance (the industry organization which certifies SPF contractors and establishes the standards that govern SPF roofing):
(From SPFA-102: A Guide for Selection of Elastomeric Protective Coatings Over Exterior Spray Foam Applications)
“Compatibility with other Elastomeric Coatings: Acrylics have been used with other elastomeric coatings; however, manufacturers should be consulted to ensure compatibility. Acrylics are not recommended to be used over existing silicone coating.”
Coating Over Wet Polyurethane Foam
These roofs have more than 800 locations where the foam is water-saturated. Roof coatings cannot be applied over a wet substrate. If a coating is applied over areas of wet foam, the coating will rapidly delaminate in those areas and fail.
Those wet areas are the exact areas that are currently most in need of remediation. Even with the graphic from the infrared survey as a guide, the coating contractor will not be able to identify areas of wet foam that have formed since that survey was performed.
If the roof is scarified and foamed, all wet areas will be identified and removed during the scarifying process.
There are also many locations where the existing coating has worn away to a considerable extent, so pressure washing the entire roof surface (which would be necessary) will likely lead to new areas of wet foam forming right before they apply the new coating.
Coating Over Roof Blisters
These roofs have many, many areas where blisters have formed in the foam.
Blisters expand and contract as the air inside them warms and cools. They also get bigger over time as the thermal cycling process repeats.
Acrylic coatings are relatively brittle and would likely fail in many blistered locations. The blisters would have to be identified and repaired during the coating process.
We are not confident that this could be done effectively. This would not be an issue if the roofs were scarified and re-foamed.
New Spray Foam Application Before Coating
The existing foam in more than 800 locations (wet foam and blisters) would have to be cut out and replaced with new spray foam.
We do not know whether the coating contractor is qualified to perform this work. As these areas would not be properly prepared (scarified), we doubt that the new foam in all of these areas would be successfully installed without new blisters forming soon after the application, which would damage the new coating.
Acrylic Roof Coatings and Ponding Water
These roofs currently have large areas where ponding water is an issue.
Acrylic coatings are possibly the worst-performing coatings under ponding water conditions.
Acrylic coatings have high permeability; when water sits on top of them for prolonged periods, the water eventually penetrates the coating and gets into the substrate, causing the coating to delaminate and fail.
Roof Coating Warranty
Roof coating warranties almost always only cover the coating material itself as being free from manufacturing defects.
It would not cover any problems caused by existing issues with the roofs. It would not cover coating failure due to ponding water, wet foam, blisters, etc.
Yours Truly, etc.
The Rest of the Story
The facilities director thanked us for our input. Two days later he told us he was going to go ahead with the roof coating idea.
We billed them for our work up to that point and had nothing more to do with the project.
We did find out how everything turned out, though.
One of the roofing contractors on our standard bid list did a lot of leak repair work at that facility, and they had also been amazed that the facilities director would go ahead with the roof coating application despite their own recommendations against it.
We were on the phone with the roofing contractor about six months later, talking about a totally unrelated roofing project, when they brought it up.
The new roof coating started to fail within three months (three months!).
The areas where the coating delaminated and began falling off made up about 40% of the total roof area. The roof was shedding pieces of roof coating into the parking lot constantly.
The roof coating manufacturer, who had convinced the facilities director to use their product in the first place, refused to do anything about it.
They pointed out that the warranty was just a material warranty and that the coating failure was solely due to the poor condition of the existing roof.
And the facilities director was no longer the facilities director.
Think twice about that roof coating!
About the Author
Jack Gray is a principal roof consultant and vice president at the Moriarty Corporation, an award-winning building enclosure consultant firm founded in 1967. He is also the editor of the Roof Online website.
Mr. Gray has been involved in the roofing industry for over 25 years, with training and practical experience in roof safety, roof inspection, roof installation, roof condition assessment, estimating, roof design & specification, quality assurance, roof maintenance & repair, and roof asset management.
He was awarded the Registered Roof Observer (RRO) professional credential in 2009.
He also served as an infantry paratrooper in the 82nd Airborne Division and attended Cornell University. Read full bio.