Before we discuss metal roof coatings, let’s start with some basics on Metal Roofs. Metal roofs come in various styles, materials and types. In the residential market particularly, you have metal roofs that can look like shingle/slate, tile, shake, and vertical panels (which is common in the commercial roofing market). The materials used can be steel, aluminum, copper & zinc, metallic coatings and painted. As far as types you can have vertical seam, pre-formed panels and granular coated panels. For this article we are going to focus on commercial applications which are usually vertical seam or pre-formed panels, usually made of steel or steel that has been coated with aluminum and zinc called galvalume, or steel that has been coated with zinc called galvanized. Lastly, metal roofing can be classified according to its connector system. There are two main types, concealed fastener systems (for example the standing seam roof) and an exposed fastener system which is typically on a pre-formed panel (which are generally cheaper). These exposed fastener roofs include the old corrugated panels that you would see on sheds and barns. Exposed fastener systems are most common in warehouses and industrial buildings.
The primary benefit of a metal roof is it’s longevity. Manufacturers offer 50 your warranties and some offer a lifetime warranty. They claim their products last two to four times longer than asphalt shingles. Not having to do several re-roofs during the life of the roof, investing in a metal roof makes financial sense. Metal roofs are also lightweight, sometimes allowing them to be installed directly over old roofs. Another benefit of metal roofs is their fire rating. Some insurance companies will give you a discount if you have a metal roof.
However even with the superior benefits of a metal roof, with the passing of time and wear, metal roofs can develop some problems, especially exposed fastener roofs. The exposed fasteners on a metal roof have a neoprene grommet that will degenerate with time. Also, the thermal shock (expansion and contraction) effect on all roofs will make water intrusion possible, especially at the vertical and horizontal seams and all penetrations. The roof can be in excellent shape overall, but can develop leaks at these areas, the fasteners and seams.
There are several solutions available when water intrusion on a metal roof becomes too problematic. Obviously you can tear off the existing metal roof and replace it with a new metal roof. Unless the roof has so much structural damage (like severe rust through), this is a very costly option. There are also layover systems that can be used. You can use your current metal roof as the substrate and put a single ply system (such as TPO) with insulation over it or you can use a product like Roof Huggers and put another metal roof on top of the current roof. Both are cheaper than a tear off and re-roof. But the most popular option today is to use a fluid applied system. On a sloped metal roof, the best option is an acrylic coating system. This system can be rolled on or sprayed on. It is a monolithic application and can come with 10, 15 and 20 year warranties.
On this application, you will first do a thorough inspection of the roof. Examine all fasteners (screws with the neoprene washers) to see if any have backed out or are missing. Replace the bad fasteners with an oversized screw with a new neoprene washer. Then take a power washer and clean the entire roof deck. If you have a metal roof with grease trap units, like on a restaurant, special degreasers may have to be used in those spots. Once the surface has been cleaned, you now began the prep work prior to the first coating. Different manufactures use various names for this product, but you want to coat every screw, all horizontal and vertical seams and all penetrations. This product can be called Flashing Grade, Butter Grade or Brush Grade. Some manufacturers require you to do what is called a “three course”. What this means is if it is required for the horizontal seams, you would brush down a layer of Flashing Grade, then insert a polyester fabric (usually 6 inches wide) into the wet Flashing Grade, work it in so there are no creases or fish mouths, and then come back on top of that fabric with a second layer of Flashing Grade. Thus it is three coursed. This is often done on all horizontal seams, all rooftop penetrations and on some vertical seams. This is usually the most time consuming aspect of coating a metal roof. This should be allowed to cure for approximately 12-24 hours. Again, this varies by manufacturer. The next step is to apply the acrylic base coat. This coat may or may not be the same color as the finish coat. Whether it is applied by roller or a sprayer, you will usually apply 18-24 wet mils on this coat. Again, allow to dry/cure for 12-24 hours and then come back the same way with the finish coat. Usually the finish coat is going to be a bright reflective white. This too will usually be applied to 18-24 wet mils.
A final advantage to the acrylic coating, is every so often, say every 10 years, the building owner can have the roofer come back out, clean the roof and apply another coating. Again depending on the manufacturer, that can give you another 5-10 year coating warranty and at a reduced cost because the timely prep work was done the first time you coated it.
Coatings truly are your best option to make your metal roof watertight, so long as the roof is structurally sound.