Concerning roof coatings and flat roofs in Miami, it is shocking see so many roofs turned black with algae stains. People are more aware of their home’s energy efficiency but not realizing the potential savings.
Residential flat roofs require maintenance, primarily roof coatings, and the sooner the better. In Miami, almost every home either has a shingle roof, tile roof or metal roof. The majority of those have a flat roof attached. Usually they are out back over a patio, addition or Florida Room and sometimes over a front porch or carport. There are even some that are covered entirely by low slope roofing.
What many don’t realize is how important roof maintenance is to their flat roof. Out of sight, out of mind? Sloped roofs such as tile, metal and shingles require little maintenance other than keeping them clean and free of debris. Flat roofs, as a consequence of being flat, require a more rigorous maintenance regimen or they are doomed to a short life cycle.
Blame Roof Salesmen
Many homeowners perceive flat roofs as maintenance free systems. This idea has been perpetuated by roof salesmen and manufacturers who like to point out the “maintenance free” white granular surface of the products which cover the vast majority of residential flat roofs today. Nothing in South Florida is maintenance-free.
Gloeocapsa Magma is a cyanobacteria which grows on everything down here, eventually. Also known as black algae, it is the black film we see on sidewalks and pavers as well as our roofs. Flat roofs also develop this film and though there is debate over whether it directly damages the material, there is one indisputable fact – it is black! When a roof turns black it absorbs the sun’s rays rather than reflect and, as a consequence, the roof membrane’s aging accelerates. Specifically, the aging process is thermal shock, the deterioration of a material caused by daily expansion and contraction due to temperature changes.
Elastomeric Roof Coatings Are Proven
Elastomeric roof coatings are a proven remedy for the effects of thermal shock. Their elastic characteristics and durability have been documented by such authoritative entities as ASTM, CRRC and Energy Star®. A roof that is stained black can reach surface temperatures of 150º or more. The temperature range a roof experiences is much narrower with the application of a quality roof coating, from around 75º at night to about 100º on a sunny 90º day. Without coatings that range goes from 75º to 150º – that’s a lot of expansion and contraction going on.
Some folks turn to roof coatings after several years when a roof begins to leak and develop problems. By then it is usually too late. The primary use of roof coatings is to protect the roof from the effects of solar radiation and thermal shock, not to fix leaks. You will not see anything on a manufacturer’s warranty about stopping leaks, despite the claims you may see in their marketing. Coatings can help prevent leaks but, when used without re-enforcement, are strictly a maintenance application.
Roof Coatings Can Double the Life Cycle
My mantra for roof coatings is, “You can do it, we can do it or you can have someone else do it – but DO IT – and the sooner the better”. Coatings are an investment. If 25% of the roof’s cost is invested in a coating application and applied within the first three years of the installation, the life cycle can easily double. That is good math.
Every year that goes by renders the roof coating that much less effective. Indeed, a roof can be too old and worn for coatings to be practical and may even cause the roof to fail faster than if it were left alone. Occasionally I must give a client the “zero option” when their roof is too far gone with not much to do other than replace. Sometimes roof materials crack under the stress created by slight shrinking that occurs while a coating cures. A roof must first be inspected to determine if it is strong enough to support a roof coating.
About Roof Coatings & Ponding
Ponding is an important issue when considering roof coatings. Ponding water is any water left standing after it has stopped raining. Standing water is not good for the roof’s membrane. Unfortunately, most flat roofs in Miami are designed perfectly flat. They’ve settled a bit and developed ponding areas, some so badly designed they completely fill with water before they start draining. That is a 100% ponding situation – not good. Some flat roofs are sloped the wrong way, negative drainage built into them by the GC!
Ponding can be corrected with a well-designed system at the time of replacement through the use of tapered insulation. Sadly, many contractors will not even offer this option and when they do, the owner often declines due to its considerable cost. Small ponding areas which dry soon after the rain stops are inconsequential.
Miami Dade Excessive Ponding Rule
Miami-Dade County considers any water still on the roof after 48 hours to be excessive ponding. You’ll see this standard mentioned in manufacturers’ literature. That is the point at which they recommend correcting ponding areas. 
Their water-based coatings are not made to withstand extreme ponding conditions. Ideally, ponding conditions due to structural defect are corrected during a re-roof through the use of tapered insulation.
There are three ways to address severe ponding on an existing roof: 1) Build up the area with additional material. This usually does not completely eliminate the ponding water completely but “pushes” most of it over the edge while also displacing it into smaller ponds. The smaller, shallower ponds evaporate quickly.
2) Rebuild the structure. In extreme situations ponding can be so widespread that filling the area is impractical and against Code. It is common for Miami-Dade’s 25% rule to come into play. It states that if a repair covers more than 25% of the roof it must be replaced. That rule applies to many trades in the Code. The problem with rebuilding structure is it’s outside the scope of a roofing contractor’s license. Rebuilding structure requires a GC – very expensive.
3) Solvent-based roof coatings. Generally, water-based roof coatings can be used on roofs with little or no ponding. Solvent based coatings should be used on roofs with moderate to extreme ponding.
Water-based vs Solvent-based Roof Coatings
Be aware that solvent-based roof coatings are much more expensive. It would not be wrong to use solvent-based coatings on the ponding areas and water-based on the rest of the roof – in that order! Acrylic coatings account for a vast majority of the residential roof coating market. They’re available in water-based or solvent based formulas and ideal for the systems found on most homes.
Other coatings available are polyurethane, butyl rubber, hypalon, and silicone. Without going into great detail on each we can say three things about them. They are all generally commercial applications, solvent-based and expensive . Silicone coatings are becoming more popular and cost effective. There are 100% silicone coatings that are effective under permanent ponding conditions which exceed the 48 hour rule.
Roof Coatings are The Future
There are literally hundreds of different roof coatings on the market in the US with local or regional manufacturers in every major city and several national brands. To become familiar with the coatings we favor and use regularly, go to the Maintenance Page on this site.
The successful use of a coating is dependent on a thorough examination of the roof’s condition, followed by a well-executed cleaning, surface prep and application. The future is bright as homeowners, roofing contractors, maintenance managers and architects are recognizing the economic, environmental and performance advantages of elastomeric roof coatings.
