Maintaining Metal Roofs

This article covers some of these maintenance alternatives and is based in part on the author’s presentation at the 9th Conference on Roofing Technology held earlier this year sponsored by the National Institute of Standards and Technology ONIST) and the National Roofing Contractors Association (NRCA).

Before discussing corrective and maintenance procedures, an overview of some of the potential difficulties that can occur in metal roofing is appropriate. Many or all of these problems also can be associated with any and all types of low-slope roofing.

Slope Considerations

Slope is a major design factor that determines the success or failure of many roofs. In an attempt to economically cover more interior space, slope is minimized and a single slope typically is used for the entire roof surface.

This initial cost savings is in fact a false economy because of the longer term problems that eventually are created. Lack of suitable slope is a major shortcoming of many existing metal roofs.

Improper slope allows snow and ice to accumulate, which eventually results in deflection of the deck. This deflection will in turn allow water to back up at end laps as well as at gutters.

Deflection also can lead to the opening of side-lap seams, thereby allowing moisture to enter. Cycling action on an improperly sloped roof will draw water into lap areas and cause rusting in the overlap with resultant leaks.

Another area of slope design problems involves the placement of heavy equipment on the roof. This also can result in deck deflection with the same consequences as described above.

An additional difficulty that rooftop equipment presents involves flashing details. The combination of an irregular deck configuration along with water build-up presents a severe test to the flashings on these roofing systems.

Material and Design

The industry’s understanding of the relationship between design and materials in metal roofing has grown dramatically. From an historical point of view, metal roofing can be traced back as far as 400 years ago.

Many of the great cathedrals of Europe were roofed with ⅛”-thick lead shingles. The deck was solid English oak fastened to huge timbers. These roofs had slopes in the range of 4 inches to 6 inches per foot.

Obviously, the materials and lengthy construction times of medieval Europe are out of the question today. The 24-26 gauge steel used now does not even approximate ⅛”-thick lead shingles. The use of steel results in considerable movement at fasteners and seams.

Such a metal roof can be subject to severe deflection due to its light gauge if there is insufficient substrate support from the purlins and beams. The addition of rooftop units (not a problem on medieval cathedrals) and the necessary traffic to maintain them certainly complicates today’s roof performance requirements.

The lead tiles used on the cathedrals were uncoated. With their thickness and the designed slope, they have performed well for centuries. Today’s metal panels must be protected with a variety of coatings and treatments that are subject to wear and must be maintained.

Fasteners were completely covered on medieval cathedrals. On many of the metal roofs now in need of maintenance, the fasteners have been left exposed.

It is important to remember that a great deal has been learned from the problems of these older configurations of metal roofs. The systems in present use have incorporated a number of changes that overcome the shortcomings of previous designs.

For example, fasteners no longer are exposed, proper consideration is given to slope and drainage, acceptable levels of support are designed into installations and allowances for movement are built in.


One other factor that has contributed to the need for metal roofing maintenance systems is improper workmanship during installation. This subject usually brings a strong reaction because of pride of workmanship and the legal ramifications involved.

Nevertheless, missed or improperly installed fasteners, poor flashing work, ill-fitting panels as well as a number of other mechanic-related factors are the cause of many problems on jobs.

Metal roofing is comparable to single-ply materials because only one layer of waterproofing is used. If any error is made in the installation, it will translate directly into a leak in the building.

Summary of Problems

The metal roofs addressed in this article were applied as inexpensive alternatives to built-up roofing systems. They represent a substantial inventory of square footage and have reached appoint at which maintenance is critical to extend their service lives.

It is apparent that consideration must be given to the following concerns in any maintenance recommendation:

  • Very low slopes or deflection of panels.
  • Exposed fasteners.
  • Excessive movement of components and failure to accommodate for this in the system’s design.
  • System components that are less substantial than required for the application.
  • Penetrations and rooftop equipment.
  • Heavy traffic on the roof because of equipment maintenance or other needs.
  • Surface coating that are not performing adequately.

Maintenance Needed

Coating manufacturers are offering an increasing number of metal roof maintenance systems. They have recognized that this remedial area represents another outlet for their products.

This is especially important because the success of single-ply and modified bitumen roof systems has reduced significantly the amount of build-up roofing being installed.

Built-up, or course, has been a primary area of coating usage for maintenance applications. Coating also have served as surfacings for new, hot-applied installations.

Metal roofs obviously have been protected with coating applications for decades. Basically, paints and coatings that were used for general metal protection also were employed for metal roofs.

Standard coatings that often have been recommended are asphalt-based aluminums in either fibrated or non-fibrated forms. But manufacturers recognized that in many situations, a simple recoating is not sufficient.

At least five years ago, systems began to appear that consist of a number of different products. These various materials were used in combinations depending on the severity of the problem’s condition or the length of service required from the repair of the roof.

Understand the Need

It is of considerable importance that a thorough evaluation be made to determine the requirements or degree of maintenance necessary for the metal roof. Metal roof construction varies significantly, but the areas that almost universally require remedial treatment are:

  • Fasteners
  • Seams
  • Projection or penetrations
  • Rooftop equipment
  • Rust
  • Failed coatings

A thorough evaluation should be made of the degree of maintenance necessary.

It is important to decide what problems exist and select the proper degree of maintenance that is necessary. The options that are available include:

  • Paints or rust-preventative materials.
  • Coatings that also offer a degree of waterproofing.
  • Systems containing several products to address a variety of existing problems.

For example, if seams are a problem, a single coating system should not be selected that does not use tapes or reinforcing membranes with mastics. If rust is the major problem, a single coating system should not be selected that does not use tapes or reinforcing membranes with mastics. If rust is the major problem, the expense of a full system employing sealants, mastics and tapes would be unnecessary.

In actuality, maintenance needs seldom will be as clear-cut as indicated above. A combination of treatments most likely will be necessary, especially if a large roof area is involved.

As with all successful roofing projects, several criteria must be followed:

  • Professional and knowledgeable assessment of the problem.
  • Comprehensive preparation of the specification.
  • Selection of experienced and responsible applicators.
  • On-site instruction and inspection during the application.


For the purposes of this article, paints are considered to have little or no waterproofing capabilities. Their three primary purposes are to:

  • Treat existing rust and inhibit the formation of new rust.
  • Improve the aesthetics of the roof.
  • Produce a more reflective surface and reduce temperatures in the building.

Painting must not be considered a means of correcting leaks.

The varieties of products used are as numerous as the types of paints and treatment available. Alkyd rust paints, zinc oxide primers, one and two-part epoxies, aluminum-pigmented materials and rust conversion products all are employed for this application.

Sometimes primers are used in conjunction with a top-coat product. But some manufacturers are incorporating rust-inhibiting chemicals in their surfacing products and are suggesting or requiring two coats. This eliminates the need for having two separate products on the job.

As in all applications of paints, it is critical that the surface be prepared properly. All loose rust must be removed by some means of abrasion and the substrate must be clean to allow adhesion of paint.

It is important that the previous coating be well-adhered and unaffected by the application of the paint. Test applications should be performed to determine compatibility as well as to check for bleed-through.

To repeat an earlier point, painting must not be considered a means of correcting leaks. Paints can improve reflectivity and aesthetics, but leaks must be remedied by other means.

Coating Options

Coatings are materials that are more heavily-bodied than paints. Because of this, they can be applied in greater thicknesses or build.

Coatings generally have somewhat better mechanical properties than paints. Specifically, they can be formulated to provide good elongation and recovery characteristics, as well as good fatigue resistance.

Due to these attributes, coating supply a greater degree of waterproofing properties than paints and will seal pinhole leaks. But they should not be viewed as a long-term solution to leaks at bolt heads, joints or other areas of the metal roof that are prone to problems.

In the category of coatings are included asphalt-based aluminums, rubberized asphalt cutbacks, modified asphalt emulsions and latex-based products.

These materials serve the functions of paints (aesthetics, reflectivity and rust treatment), but they also provide an additional degree of waterproofing not offered by lower viscosity materials.

Maintenance systems offer a comprehensive approach to remedial treatment.

Maintenance Systems

Metal roof maintenance systems offer the most comprehensive approach to remedial treatment. They consist of a number of different products.

Primers:  These materials may be solvent or water-based and are low in viscosity to ensure penetration and sealing of rust. They typically have rust-inhibiting additives that act to seal the rust and prevent access to oxygen in the air.

Other additives also can be formulated into the primers that chemically react with the rust to inhibit its spread.

Paints:  This class of products was discussed previously and is more highly pigmented than primers. Paints offer more weatherability, reflectivity and hiding than primers because of their higher pigment contact. Paints generally are higher in viscosity than primers and therefore do not have the same ability to penetrate rust.

Coatings:  Once again coatings are one step up in viscosity compared to paints. They offer the advantages of paints but provide a greater build or film thickness.

Sealants and Mastics:  These products are much heavier in viscosity than coatings and can be used at much heavier rates or application. They can be packaged in cartridges or cans and are supplied in trowel, brush or even spray grades.

Sealants and mastics are used to stop leaks. Their heavy body allows the placement of enough volume of material to create a substantial water barrier. They are used primarily in seam areas, over bolt heads or other types of fasteners and in general where different components of a system come together.

These products many times are used in combination with some type of reinforcement with some type of reinforcement mat or scrim. The reinforcement is embedded between two layers of the sealant or mastic to give greater integrity and strength to the application.

Many manufacturers are using polyesters as the reinforcing mat because of their ability to elongate and recover through many cycles, called fatigue resistance.

Sealant and mastic materials can be made from a large number of different polymer and resin materials. Acrylics, urethanes, butyl and silicone rubbers are just a few of the chemical types that can be used.

Many times, the sealant simply is a higher viscosity version of the coating. This ensures compatibility between these two materials on the roof.


To avoid the necessity of the mastic-scrim application, tapes now are being offered. These materials are self-sticking and have the scrim already incorporated into the tape.

Polyester and polyethylene are two of the more common reinforcements being used in tapes. The advantage of tapes, besides easier installation and less dependence on the installation skill of the mechanic, is that there is no wasting of time while curing is taking place. This allows the rest of the application to proceed without delay.

Surfacings:  A number of optional surface finishes are available. The dark-colored or black coatings can be left as is or finished with aluminum or white paints.

Colored granules can be embedded in the surface coating if desired. Specially-matched colors of coatings are being offered by many supplier as well.

Criteria should be established now to ensure quality materials, specification and applications.

Remedial Technique

The decision on the type of treatment necessary is best made by someone experienced in metal roof maintenance. Obviously, a simple paint job is far less costly than a full systems approach. Depending on an owner’s needs and projected tenure in a building, a less extensive maintenance approach may work well.

In the past, applicators tended to provide a lesser product and service than was required. A simple paint job greatly improved a roof’s aesthetics, and the occupants noticed the lower heat build-up in the building if a light color was used. But this unfortunately did not stop leaks, and in the long run it did not resolve the owner’s needs.

The metal-roofed building was viewed as low-cost and not deserving of expensive maintenance treatment. But the surprise came when the owner found out the price of replacing the “low-cost” building.

Regarding warranties, manufacturers offer a five-year warranty against leaks if a roof is treated by the complete system application.

The warranty generally is renewable for an additional five years after an inspection, correction of defects and a reapplication of coating. Warranties are much more limited or not offered if a simple painting or coating application is performed.


Maintenance of metal roofs is a market that has been relatively ignored until the last five or six years. It traditionally has been considered a low-end market and not worth pursuing by the major suppliers and contractors.

But with the growth and acceptance of metal roofs over the last 10 to 20 years, this maintenance area is being perceived as a potentially significant market by coating manufacturers.

Development work now is being directed toward identifying products with specific performance advantages for metal roofs. In the past, existing products were borrowed from built-up applications or general building maintenance areas. It is expected that a number of suppliers will enter this market in the near future as its potential is recognized fully.

On a final note, it would appear that a rather unusual opportunity exists to develop meaningful performance standards for these products and systems.

Rather than attempting to reach agreement long after manufacturers have established positions, criteria should be established now to ensure quality materials, specifications and applications.

Organizations such as ASTM, the Roof Coating Manufacturers Association (RCMA) or even the MBMA would be likely candidates to develop these standards. Perhaps we can gain a head start on control of a product area instead of trying to catch up as we have done so frequently in the past.