Jill asks: What is the best roofing product to use on a carport and pergola? The carport roof covers both, which are part of the same structure. I am aware that there are plastic-coated steel sheets, polyester-coated, zinc, and polycarbonate sheets. It’s a minefield. There are lots of trees overhead, so everything goes green including the roof and patio. I was thinking of maybe going for a dark green-colored roofing product. What do you think? Kindest Regards
Steve Bliss, of BuildingAdvisor.com, responds: You are right that there are a lot of options for roofing a carport and similar structures. Assuming that you want to keep all the trees overhead and that everything will “go green,” then you are wise to go with metal or plastic roofing. These are certainly easier to clean than shingles, tiles, wood, or other materials. Also moss and lichen will shorten the life of asphalt or wood roofing materials and could have a similar effect on lower-cost metal roofs by trapping moisture against the roof surface.
Metal Roofing Options
A good quality metal roof will outlast plastic, but each has its pros and cons. As you noted, there are many options to choose from for both metal and plastic. Not surprisingly, the most expensive options will last the longest.
With metal roofing, the type of metal, its thickness, and the quality of the paint will determine the lifespan of the roof. Typical low-cost steel roofing has a light galvanized layer and, if painted, uses polyester coatings. A step-up would be silicone-modified polyester (SMP) paint.
Getting green-colored roofing to match the debris and mold/moss/algae sounds reasonable if you plan to let things grow on the roof. If you go with a pre-coated steel roof, choose a smooth finish as this will be easier to clean than a textured coating. Also be aware that the finishes used on inexpensive metal roofing will lose their luster in a few years.
Unfinished metal roofing is another option. Copper would be great because it will naturally inhibit the growth of mold and algae, and it will turn green over time from oxidation. But it is one of the most expensive roofing materials you can buy. Galvalume roofing is another option that is much less expensive and will inhibit growth to some extent because of the zinc in the galvanized coating. Aluminum is another long-lasting and durable metal, but does not have the anti-mold properties.
Corrugated sheet metal products are much less expensive than traditional standing-seam metal roofing and easier to install. They were originally developed for agricultural and utility buildings, nut are not marketed to the housing market as well, and would be a good choice for your application.
Corrugated Plastic Panels
Plastic corrugated roofing is another inexpensive option that might be suitable for a carport.
In general, high quality plastic panels – polycarbonate or fiberglass (FRP) — will be more expensive than cheap metal roofing, but less expensive than premium metal products. I would avoid PVC and acrylic panels as they are not very durable.
Plastic panels are stiffer and easier to install than metal, but less durable overall than premium metal. They are more prone to damage from hail or stray baseballs, and from damage during installation. Also, they are not recommended in hot, sunny climates. Thermal expansion can cause damage around screw holes and can warp thin, corrugated plastics. As with any building material, thicker panels will be stronger and more durable than very thin ones.
In general, polycarbonate is stronger than FRP and more resistant to impacts and chipping, but scratches easily and will show dents. Some polycarbonate panels have a UV-resistant film that will prolong their life span. Polycarbonate is available in a single-wall corrugated sheets or more expensive double-layer “twinwall” configuration popular for greenhouses.
Most plastics are translucent, which might be good for a pergola or carport where light is welcome. But you can also see debris, bird droppings, and dirt build-up. Some can be walked on; others can support very little weight. Some have a smooth surface that will be easier to clean than those with a textured surface. Many have anti-scratch or UV coatings that will help with their appearance and longevity, but they are still are prone to scratching. For a pergola or carport, plastic roofing is a good choice, but I wouldn’t recommend them for prime living space.
With any metal or plastic panels, installation details are critical as they need to allow for expansion and contraction of the materials without creating leaks or damage around the screw holes. Also, pay attention to the cleaning recommendations as some types of chemical cleaners or abrasive scrubbing may harm the panels. Also look at warranties. Roofing warranties are usually for materials only and are pro-rated, so they are not worth a whole lot but give you some idea of how much confidence the manufacturer has in the materials. Along those lines, carefully follow the manufacturer’s recommended installation details or any warranty protection will be lost.