Standing Seam Guide part 2: Coatings
March 14, 2019
Choosing the right coatings for a metal roof is one of the most important decisions you can make in a roofing decision. And most homeowners don’t question or understand what they are being sold by a metal roofing contractor.
THE BIG RULE!:“Don’t buy any metal roof until you know what the metal is coated with, and how that coating will perform.”
Why have a base coating? Bare steel needs some form of rust inhibitor, and metal manufacturers apply a layer of rust resistant materials to keep the panel from breaking down long term. There are two basic types of these base coating processes: Galvanized and Galvalume.
- Galvanized is thin layer of metallic element called Zinc that is applied to both sides of base steel.
- Galvalume is a mixture of Aluminum and Zinc applied in a thin layer to both sides of base steel.
I get asked which coating is better and my answer is always “Both”, and after their confused look, I explain the following. Both Galvalume and Galvanized are great rust inhibitors, what really matters is how much coating is applied and what paint is applied to the top of it. These coatings are measured in the number of tenths of an ounce, applied to both sides of one square foot of a steel sheet. Now, that may sound like a mouthful, but it breaks down fairly simply.
You will see coating amounts that range from .30 ounces (noted as G30) all the way to 1.85 ounces (noted as G185), of Zinc coating on steel. Lower coatings amounts are used on things like steel tube for interior electrical wiring. It isn’t exposed to the elements and is more cost effective to keep the coating amount down. At the very high end of the range, is typically reserved for screws and nails that won’t be painted and used in treated lumber, so they won’t react with high levels of copper in the wood. All you really need to remember is for metal roofing purposes, a coating level of .90 ounces, called G90, is the high standard and will work extremely well.
G90 Galvanized ( note the large Zinc crystal, this pattern is refereed to as “spangle”)
You will see coating amounts range for .40 ounces (AZ40) to .60 ounces (AZ60). This narrower range is because Galvalume is designed to be coated with a top layer. It is coated either with paint or stone aggregate. A good level for metal roofing is usually anything AZ55 or higher.
There are three major top coating categories: Non painted, Polyester Painted, and PDVF aka Kynar Painted.
None painted coating may sound like an oxymoron, but it does exist. Bare Galvalume roofs are very popular and we put many of them on each year. They cost effective because you aren’t paying for the application of paint. And they are a good choice for anyone seeking a more ranch or country style look. They may start off a shiny silver but will soon oxidize to a dull gray, and have a long life span. They usually come with a top coating of clear acrylic paint. This helps them run through a roll forming process, and also aids in slowing down oxidization. The biggest downside here is, it is only one color.
If you desire your standing seam roof to have color, Polyester based paints are the most economical. One of the up sides, they come in a wide range of colors and cost less to manufacture. On the down side, they are water based paints and do not have the life expectancy of other more expensive paints. The biggest concern of them being “fade resistance”. Because metal roofing has such a high sun exposure, fade resistance can be huge factor of paint performance. Polyester based paints can begin to show signs of fade as early at 7-10 years.
Fade is major enough concern, that “Super Polyesters” were created to extend fade resistance. This was achieved by adding ceramic pigment to the paint. The issue of it being a water and polyester based paint was still an issue. A few years ago, some long term testing by the paint manufacturers themselves, was published. It showed Super Polyesters had strong fade resistance for the first 15 years, but a sharp fall off after that point.
PVDF or Kynar Painted:
PVDF stands for Polyvinylidene fluoride, and also goes by the brand names Kynar or Hylar. PVDF’s are desired for their resistance to solvents, acids, and hydrocarbons, making them far less likely to break down. As well as, their extreme resistance to fade. In the real world, that means the paint won’t break down, standing up to a myriad of conditions in the environment. Kynars are truly lifetime paints, and carry a warranty to back that up. No peeling, delamination or cracking, for as long as you live in your home. They are the best paints available for fade resistance, with no fade warranties ranging 30 to 35 years. The only real downside here is a small increased cost for this high performance paint.
Fluorite Crystal PVDF structure (balance and symmetry making it more impervious)
Metal Roofs are energy efficient in general, and you will see a BLOG dedicated to it in the future. For the purpose of this BLOG, here are some facts about energy pertaining to coatings alone. It is true that lighter colors are more reflective than darker colors, but, but there are some ways around that rule. Bare Galvalume is a naturally reflective surface, and it does fit the ides of a metal roof being energy efficient.
Painted metal roofs get a part of their energy efficiency from their coatings, as long as they use reflective pigment technology. This works because the ceramic pigment utilizes a crystal called Fluorite, that is super heated to expand its particulate size. The pigment now reflects infrared light, which is the portion of light that carries the most energy. This allows the paint pigment to be any color, including dark shades, to be reflective. Yes, your roof can be dark brown to match your house and still be energy efficient!
One factor that should not be overlooked, but, often is when considering a metal roof, is the environment it will be installed in. We at AL-CO Metal Roofing, have installed many roofs right on the coast, and even directly on the water. The salt air is a real problem for steel products, and roofs are no exception. The issue comes when you combine salt with water and iron, which is found in steel. The Zinc or Galvalume coatings do help as a barrier against this.
The issue comes wherever there is a cut edge. Without a significant amount of coating, the cut edge will slowly being to rust. The industry term is “edge crawl”, and we see it all the time. Red spots start showing up on the cut edges of panels about the 7-9 year mark, and by year 11-13, they are leaking and look horrible. Look to an Aluminum panel as the solution here.
Standing Seam Metal Roof coatings are just like many things in life, you get what you pay for. I suggest you base your decision on these basic factors.
- If you want your roof to have color to compliment the house, then step up to a Kynar painted panel. They have, by far, the best track record of all exterior paints.
- If a gray metal exterior fits your home, then save the money, and install a Galvalume roof.
- Read manufacturers warranties. Pay attention to what is or is not covered, and for how long. Also, note what they say will be paid for. Will they only cover materials based on original cost or will the value be greater. Also, steer away from any warranty with heavy proration, where the value of your warranty diminishes over time.
- No steel metal panels close to the coast. Some panels can’t be inside a mile to salt water and others may get as close as half a mile. Read the warranty if this gets to be an issue, and if so, move to Aluminum.
If the point of installing a metal roof is for it to be the last one you buy, then why settle for any coating that won’t live up to that standard. It basically becomes wasted money.