Residential Metal Roof Systems
Various types of metal roofing systems can be found in residential structures. Two common types are standing seam metal roofs and flat lock metal roofs. Metal is also used in chimney flashings, wall flashings, trim, valleys, and box gutter liners. Metal systems are recommended by roofing professionals because of their longevity, durability, appearance, resistance to wind, reflectivity, and resistance to fire. Many metal roofs currently in service around the world have been in place for hundreds of years.
The successful installation of a metal roofing system begins with several design considerations, including the following:
- Roof Pitch – Standing seam metal roofs are typically recommended on slopes greater than 3/12. Flat lock roofs are recommended on slopes 3/12 or less.
- Type of Metal – Dissimilar metals can corrode when they come in contact with one another or drain on each other. Also, some metals are naturally weathering while others require painting. In most situations, copper and galvanized steel should not be used on the same roof system.
- Thickness of Metal – The gauge or weight of metals must be considered for different types of installations and applications. Copper is commonly used in weights of 16 ounces per sq foot or 20 ounces per square foot. Galvanized steel is measured by gauge. 26 gauge steel is thicker than 28 gauge steel (Lower gauges are thicker than higher ones). Aluminum is measured in inches. Common thicknesses of aluminum (from thinnest to thickest) include .019, .024, .027, and .032.
- Interior Building Conditions – Vapor retarders or corrosion protection for the underside of the roofing panels must be considered if vapor is able to infiltrate the roof panels. Effective vapor retarders include self-sealing ice and water guard or synthetic underlayment.
- Type of Fastener – Rivets, nails, and screws used in metal roofing applications must be compatible with the metal being installed. For example, Galvanized roofing fasteners should be used with galvanized steel panels, and copper or stainless steel fasteners should be used with copper panels. Careful consideration should also occur when fasteners penetrate treated wood.
- Thermal Movement – Metal, like all building materials, expands and contracts when exposed to changes in temperature. Metal roofs and box gutter liners can fail if thermal movement is restricted. (Please see the article below that appeared in Professional Roofing magazine.)
- Underlayment and Slip Sheets – Combinations of Rosin paper, asphalt felt, and (high temperature) ice and water guard are used to allow the metal panels to expand and contract without tearing the underlayment. The low melting point of the asphalt must also be considered as some metal systems can reach temperatures of 200 degrees Fahrenheit.
- Metal Finish – Three types of metal finishes can be installed on residential structures – naturally weathering metals, factory-applied painted systems, and field applied pre-painted systems. Naturally weathering (and corrosion resistant) metals include copper, stainless steel, and zinc. These metals do not require a painted finish as they oxidize to form a protective coating or patina. The seams on these systems are soldered. Pre-painted systems include steel and aluminum. It’s important to note that one drawback to these systems is that seams cannot be soldered and must be sealed using caulk. Field painted finishes include galvanized steel systems. Galvanized steel seams are soldered and then base metal can be painted to eliminate corrosion.
After a proper design is specified, a skilled sheet metal foreman will install the metal system. These individuals must be skilled at soldering to successfully install flat lock metal roofs, box gutters, and flashings. Solder is used to fuse metal seams together to ensure that they are watertight. Deer Park Roofing does not recommend the use of caulk as a substitute for solder.
Standing Seam Metal Roofs
Metal standing seam roofs have been gaining popularity as a residential roofing system. These roofs should be installed with concealed fasteners to eliminate the risk of water infiltration at the screw heads. The panels are installed with seam heights of 1 to 2 inches. The seams enable thermal movement within the metal panels, add structural support to the system, and provide a watertight seal. Pre-painted metal panels come in a wide range of color options and can be fabricated from steel or aluminum. Factory-applied paint systems will perform much better than field-applied paint systems.
Standing seam metal systems can also be designed using copper or galvanized steel. The galvanized steel panels can be designed to replicate the ‘tin roofs’ used in previous centuries. Panels can be fabricated in 10 ft lengths using a break or in custom, lengths using a roll former. These roofs are designed to last upwards of 60 years, approximately 3 times as long as asphalt shingle roofing systems.
Flat Lock Metal Roofs
Flat lock roofing systems are fabricated out of copper, galvanized steel, and sometimes stainless steel. These types of roofs are commonly found on cupolas, domes, spires, porches, and low sloped roof areas. Metal panels are fabricated in a sheet metal shop with four interlocking seams, two bent upward and two bent downward. Panels must be similar in size – 18 inches by 24 inches or smaller and secured to the roof deck with metal cleats to absorb thermal movement and prevent damage to the panels or the seams. Flat locked seams must be soldered on all roofs with a slope less than 8/12.
For information about built-up gutter systems, box gutters, and/or box gutter liners Click Here.
The following types of metals are commonly used in roofing systems:
Steel is the most common metal used in residential roofing because of its strength and cost-effectiveness. Steel is a metal alloy that consists of 98% iron and 2% carbon. Steel that is used for metal roofing must be painted or galvanized to prevent it from corrosion.
Galvanized steel is coated with a thin layer of zinc. When the zinc coating deteriorates, the roofing components will begin to corrode when exposed to water and oxygen. The rate of corrosion on steel roofs depends on the pitch of the roof, the level of humidity, and the presence of a protective coating. Steep sloped steel roofs will last longer than lower slopes because the water drains faster and the metal dries quicker. Galvanized steel is easily soldered and can be used for almost any flashing detail while pre-painted steel cannot be soldered.
Most building codes require a minimum thickness of 26 gauge steel. The National Roofing Contractors Association recommends a minimum thickness of 26 gauge steel for all types of gutters.
‘Tin Roofs’ are actually made from steel that contains a tin/lead alloy coating. Roofing professionals referred to this type of metal as Terne. Terne metal was easily soldered, durable, and cost-effective in comparison to other metal roofing systems. These roofs are commonly found on residential structures in 28 or 30 gauges. Periodic maintenance is required in the form of painting.
Copper Roof systems are known for their durability, longevity, and beauty. Copper sheets used for roofing applications are measured in ounces per square foot instead of thickness. The most common copper weights are 16 and 20 ounces. Copper is easily soldered and requires very little maintenance. Copper is a very soft (malleable) metal and lacks the strength that other metals possess.
Aluminum is a corrosion-resistant material that can be used to install standing seam roof systems, gutters, and downspouts. Aluminum has a very high rate of thermal expansion (and contraction) relative to other metals. For this reason, emphasis must be placed on proper design when aluminum is specified on roofing projects. Aluminum used in roofing is pre-painted in a variety of colors. Common aluminum thicknesses (in inches) are .019 or .024 inches for trim or flashing. The National Roofing Contractors Association recommends a minimum thickness of .032 aluminum for gutters.
Aluminum is very difficult to solder and is not recommended for many flashing details.
Understanding thermal movement in residential metal roof systems is key to long-term performance
Roofing contractors often are amazed at the time-tested craftsmanship of sheet-metal systems that are removed from historical structures. It’s not uncommon for some of these systems to date to World War II. The one thing all long-lasting systems such as standing-seam, flat-lock, and built-in gutters have in common is their ability to accommodate thermal movement. Unfortunately, when thermal movement is restricted, contractors may witness failures in as little as five years.
Contractors that install metal roof systems on steep-slope structures, must understand the direction and effect of thermal movement on various metal types.
The principles behind thermal movement are easily misunderstood. Anything that is not visible to the naked eye can be overlooked. It’s not likely, for example, for a roofing contractor to notice a ¼-inch expansion in an aluminum panel. In addition, failures caused by thermal movement usually occur over dozens of thermal cycles or seasons, and the original installers or designers sometimes are not around to witness the failures. For that matter, a homeowner or building owner could have sold the property, as well. Failures associated with thermal movement also often are misdiagnosed and blame is often placed upon poor workmanship or improper soldering.
When thinking about thermal movement, it helps to visualize the movement of liquid mercury in an old-fashioned thermometer. As temperature increases, the atomic particles in the mercury move faster, causing it to expand. Conversely, as the temperature decreases, the atomic particles slow down and the liquid metal contracts.
The rate of thermal movement varies by the type of metal installed and the temperature of the base metal. It is important to note ambient air temperature will differ from the temperature of the metal, and direct solar radiation plays a major role in this process. Also, the surface temperature of metal roofing materials can range from 25 degrees Fahrenheit cooler than ambient air temperatures in the winter and more than 100 degrees Fahrenheit warmer than the ambient air temperature in direct summer sunlight. Based on these temperatures, a base metal could have a temperature change of over 200 F.
The formula Delta L = L•Delta T•Ce where Delta L is the change in length, Delta T is the change in temperature and Ce is the coefficient of expansion for each metal, which can provide the amount of expansion and contraction in common roofing materials.