Roofing: MATERIALS ENCYcLOPEDIA
Applications for this system
Metal sheets or tiles
Strapping over roof framing on 16- or 24-inch centers
Some codes require a sheet membrane under steel roofs
Ratings Chart for Metal Roofing
The ratings chart shows comparative performance in each criteria category. Click on the tabs below for detailed analysis of each criteria.
- HOW THE SYSTEM WORKS
- ENVIRONMENTAL IMPACTS
- EMBODIED CARBON
- ENERGY EFFICIENCY
- MATERIAL COSTS
- LABOUR INPUT
- SKILL LEVEL REQUIRED
- SOURCING & AVAILABILITY
- CODE COMPLIANCE
- RAINWATER COLLECTION CAPABILITY
- FUTURE DEVELOPMENT
Metal Roof System
Metal roofing is a very broad category of products, with many variations and styles available. The key variations between different products are:
Types of metal
The two most common types of metal roofing are steel and aluminum. Copper, stainless steel and zinc alloys are also available at premium prices. Every metal has a different environmental impact, but all should be fully recyclable. Manufacturers offer varying levels of recycled material in their roofing products, and this can have a large impact on the environmental costs.
Gauge of thickness
Different thickness gauges of metal are widely available, typically ranging from 29 gauge at the thinnest, 26 gauge in the mid-range and 24 gauge at the thickest. In some jurisdictions, codes may indicate a minimum gauge requirement. Cost rises with thickness, as does weight and difficulty of installation with the trade-off being increased durability.
Paints, powder coatings, alloy coatings and stone chip coatings are among the options available. From an environmental point of view, the coatings on metal roofing are often the least environmentally friendly element of the roof. In particular, chemical compounds including Teflon, polyurethanes, acrylics and paints can contain elements that are environmentally detrimental during their production and are spread from the roof to the ground as the coatings wear. It is very important to thoroughly research the coatings used on any metal roofing you are considering purchasing, especially if you intend to collect and use rainwater from the metal roof.
Metal roofs use either exposed fasteners (usually screws with an integral rubber or neoprene washer) or hidden fasteners. Exposed fasteners tend to be less expensive and easier to install, but can have a shorter lifespan.
Closures and Flashings
Each roofing company will tend to offer its own version of the basic ridge, hip and valley flashings, as well as different trim options, drip edges and chimney flashings. Raised rib roofs also come with foam closure strips to prevent water and pests from entering the area under the raised sections of roofing. Look for flashing systems that have generous widths and logical fastening, overlapping and drainage provisions.
Among these variations, there are also three basic types of metal roofing:
Sheet-style metal roofing
Sheets of metal with a pattern of ribs are ordered to match the lengths required for the particular roof. There are many rib styles, and the sheets can vary in width and gauge of thickness. The sheets are installed with the last rib of one sheet overlapping the first rib of the adjacent sheet. The sheets are typically fastened using roofing screws that have a rubber or neoprene washer under the head. The screws are used on the raised ribs or on the flat sections of roof, depending on manufacturer recommendations and installer preference. The ribs in sheet-style metal roofing offer strength to the sheets and allow for the use of simple and cost-effective strapping systems to support the sheathing. In general, sheet-style metal roofs will be the least expensive, fastest to install option.
Sheet-style metal roofs can also come in versions that mimic the appearance of shingles, clay tiles or other types of roofing; these are usually higher in cost and slightly more time-intensive to install.
Standing seam metal roofing
Sheets of metal are ordered to match the lengths required for the particular roof. The sheets will have steep, sharp ribs along both edges and a completely or relatively flat profile elsewhere. Fastening systems vary by manufacturer, but the ribs will overlap and be attached to the roof deck in a manner that does not leave the fasteners visible nor penetrate the surface of the roof with the fastener. For this reason, these roofs can have greater durability but can be slower to install and are often more expensive. Depending on the manufacturer, a solid roof decking must be provided for standing seam metal roofs, raising the costs compared to ribbed sheets. Standing seam roofs are typically installed by professionals.
Metal roofing tiles
Tiles or shingles are fastened to the roof decking (usually a solid deck and not strapping) in a manner common to other shingle products. Fasteners are used along the top edge of the shingle or tile and covered by the next course of shingles or tiles. Some products use an interlocking system as well as fasteners to join the individual shingles or tiles, and these will tend to have a longer lifespan and be slightly slower to install. The size, shape and profile of shingles and tiles can vary widely between manufacturers. This type of roofing will usually take longer to install than either of the sheet-style roofs.
Environmental Impact Rating
The category of metal roofing covers a wide variety of products. There are three main components to consider when weighing the environmental impacts of a metal roofing system:
1) Type of metal and recycled content
Each type of metal will have its own footprint at the mining and processing stages. Be sure to consider the regional source of the ore and the amount and intensity of the processing required to transform it into roofing. Recycled content makes a meaningful difference in the intensity of metal roofing products, making high-percentage recycled roofs an environmental imperative.
2) Decking and underlayment requirements
A large portion of the impact of a metal roofing system will be the supporting elements of the decking and underlayment. Some metal roofs require only thin wooden strapping on 16- or 24-inch centers, while others call for a solid plywood or lumber deck, which significantly raises the amount of material required and the environmental impacts. Systems that require a membrane material to be laid under the whole roof will also raise impacts, and these membranes are often made from plastics that can have serious implications in their creation, use and disposal. Be sure to consider the impacts of the whole system.
The coatings on a particular metal roof may far outweigh the environmental impacts of the metal itself. Coatings can range from metallic alloys like galvanizing to paints, epoxies, polymers and other chemical coatings. Depending on the type of coating, the impacts may be negligible or considerable, making research a very important part of selecting a metal roof. It can be difficult to assess the impacts of paints and other coatings, as most manufacturers do not release the chemical composition of their coatings.
The coatings will have two types of environmental impact. At the time of mixing and application, varying amounts of toxins will be released into the environment, and a certain amount of the coating will also get dispersed into the air and water around the building during the roofing’s lifespan. This second environmental impact is especially important if water is being collected from the roof. The NSF rating agency offers advice on roof coating products that have been tested for use in rainwater systems.
Harvesting — High
While different types of metals have varying impacts at the harvesting stage, they are all degrees of high impact. The extraction of ore is a high-intensity activity that is accompanied by major impacts on the immediate environment. Destruction of flora and fauna, contamination of soil and water from runoff and tailings ponds and the need to build access roads are all part of mining ore. Harvested ore is heavy, and incurs high transportation impacts, especially considering it is often transported long distances from source to production facility.
Manufacturing — High
The process of transforming raw ore into finished metal roofing product is energy intensive, and is responsible for significant greenhouse gas emissions, contaminated water, toxic dust, airborne dioxins and other impacts. It is often completed in distinct processes with transportation of heavy materials between facilities in the process.
The manufacturing and application of the coatings for steel roof products, in particular paints and powder coatings, is responsible for significant quantities of airborne pollutants, VOCs and contaminated water. Petrochemical underlayment materials can carry similar negative impacts.
Metal roofs with recycled content can have significantly lowered impacts. With many common roofing metals, energy input can be reduced by a factor of as much as ten. Some products come with as much 65% post-consumer recycled content, and these choices mitigate harvesting and manufacturing impacts considerably.
Transportation — Moderate to High
Sample building uses 1600 kg of metal roofing:
2.4 MJ per km by 15 ton truck
1.5 MJ per km by 35 ton truck
0.4 MJ per km by rail
Impacts will be based on the distance traveled from manufacturing plant to distributor, retailer and finally the job site. Roofing materials are heavy, and large trucks are used to move even fairly small quantities.
Installation — Low
On-site, the installation of a steel roof is not very energy intensive. Hand tools and small power tools are used. Sheets and tiles are individually quite lightweight and are usually moved into place manually.
Waste: Low to high
Recycleable – Metal offcuts. Quantities can be negligible for simple roofs, but can be significant if there are multiple hips and valleys.
Landfill – Empty caulking tubes, offcuts of foam adhesive strips, membrane or underlayment offcuts. Quantities will be low.
Chart of Embodied energy & carbon
Energy Efficiency: N/A
Steel roofing lies outside the building enclosure, and may seem at first glance to have little impact on energy efficiency. However, the color of the roofing can make quite a difference on cooling loads within the building in warm weather. Light-colored roofs reflect more sunlight than dark roofs, and the resultant heating of the roof surface on dark roofs has an impact on temperatures within the building. The more cooling days in your climate, the more important it is to choose a roof with a light color.
Material costs: moderate to high
There are many kinds of metal roofing, covering a wide range of price points. Sheet versions will tend to be less expensive than shingle or patterned versions, and price will rise with gauge of steel, paints/coatings and trim details.
Labour Input: easy to moderate
Sheet-style metal roofs are among the least labor-intensive roofs to install if the design is fairly simple. The more hips and valleys involved, the more installation times will increase. Sheet roofing usually comes cut to the desired length from the factory, and if the roof is simple there may be no cuts required during installation. If there are many hips and/or valleys, many sheets may need cutting, adding significant time to the installation. Sheet-style roofing is best installed with a crew of at least two and as many as four people, as long sheets can be difficult to handle and cut.
Shingle-style products take longer to install than sheets, but are easier to cut and manage on complicated roofs, so labor inputs will be more favorable on more complex roofs.
Working at heights to install roofing has inherent dangers. Proper setup and safety precautions should always be taken when working on a roof.
Skill level required for homeowners
Decking — Easy
A homeowner capable of the carpentry skills to frame a roof will be able to install the required strapping or decking for a steel roof.
Underlayment (if required) — Easy
Underlayment products are typically sheet products that come in rolls and are straightforward to install.
Steel sheets or tiles — Easy to Moderate
The cutting and installation of metal roofing products takes some practice, and a beginner would do well to try a small roofing job before tackling a whole house project. The basic process of squaring and preparing the roof is important, especially with sheet-style roofing, as it is easy to have sheets begin to run crooked and look sloppy. Each manufacturer will have particular instructions for their products, which should be followed.
Caps and Flashings — Easy to Difficult
Typical ridge and hip caps and perimeter flashing are quite simple to install. Intersections and penetrations through steel roofing can be tricky to flash properly, and the more of these that exist on a roof the harder the job of keeping them watertight. Some experience or the ability to carefully follow written directions is important.
Sourcing & availability: Easy
Metal roofing products are widely available everywhere in North America.
Durability: Very High
Metal roofing is among the most durable roofing choices. If it is installed in a way that prevents leakage from the start, it will typically last forty to eighty years, and often carries long (15–25 year) warranties.
Metal roofing is an acceptable solution in all building codes. Requirements for gauge of metal, roof decking and membranes will vary by region, so be sure to know the local requirements for metal roofs.
RAINwater collections capability: high
Earthbag foundations will have no direct impact on indoor air quality. A well-built foundation can help keep the floors and walls of the building dry and prevent other IAQ issues.
Resources for further research
The basic metals and alloys and the formats for metal roofing are well established and unlikely to change much. What may improve is the environmental friendliness of the coatings, or access to useful information about coatings and their impacts. A wider variety of light-colored roofing is also likely to become available as the value of reflective roof surfaces is better understood. Recycled content may also increase as consumer awareness drives the market toward more environmentally friendly products.
Tips for a successful metal roof
1. Be sure to understand and follow the manufacturer’s installation instructions. Every product will have different procedures, and these should be carried out accurately.
2. Leaks in metal roofs happen predominately at seams, particularly hip, valley and other flashing seams. The cost difference between different metal roofing systems is often based on the quality of the flashing and trim components, and it is worth using high-quality options. Generous positive lapping and clear drainage paths are critical, as are proper use of closure strips and well-applied caulking where required.
3. Sheet-style products with exposed fasteners must have the fasteners secured at the proper torque. If the fastener is driven too deeply, it can dent the metal and cause leaks; if it is not driven deeply enough, water will be able to get under the integral washer.
4. Sheet-style products require careful squaring of the roof surface before installation. It is difficult to make corrections in the sheets once the installation has begun, and angled sheets will create a jagged roof edge that looks untidy. Strapping or roof decking can be used to create a decent, square surface for the roofing.
5. Sheet-style products can be long and awkward to work with. Be sure to have a work crew capable of handling the full-length sheets without excessive bending that might damage the ribs.
6. Metal roofs are slippery to work on. Be sure that workers have adequate training and safety equipment. Avoid walking on the metal roof once it is in place, and be sure that if people are going to walk on it, they do so safely and without damaging the roof.