Inspecting and Repairing Metal Roofs

Inspecting and Repairing Metal Roofs By Richard L. Fricklas (click here for bio) In the October 2002 issue of this newsletter, the many facets of metal roofing were introduced. The list of types was more diverse that bituminous, rubber and plastic roofing put together. Many building owners have only had to deal with the membrane roofing systems so far, but metal is growing in applications and has lost the stigma of being only for 'pre-engineered' buildings.

About those types Metal Construction News lists the most widely specified types of metal roofing as:  Standing Seam-with trapezoidal and vertical seams. These are also frequently 'hydrostatic' and structural in nature. The 2002 article pointed out that these systems solved the most frequent problems with previous metal roof systems: thermal movement, lack of metal durability and lack of watertight connections.  Through-fastened, ribbed-Because of the through-fasteners, these systems are suitable only on roofs of considerable slope so that water will run downhill (hydrokinetic). At these higher slopes, color and appearance are important, so usually painted metal is used. Many of these systems require a subdeck and waterproof underlay, as contrasted to the structural standing seam roofs that require neither.  Through-fastened, corrugated-These may bring to mind the Quonset huts of WW II, although such buildings are still available today. The main advantage to these structures was simplicity. Sheet steel was corrugated, and sheets were bolted to structural members with little elegance. The buildings were generally uninsulated, and were intended to be 'temporary', even though many are still around 60 years later.  Batten seam-Raised battens provide an architectural feature with strong shadow lines. "Real" battens of wood may be placed between metal pans while a batten cover protects the wood and the raised sides of the adjacent pans. More recently, all- metal batten systems have also be used with good effect. These systems are also generally hydrokinetic, with the water flow parallel to the battens.  Foam-cored sandwich-These are generally quite different. (Not all are metal skin systems). The seal at panel joints is critical and usually proprietary to a single manufacturer. The panels serve as structural roof deck, thermal insulation and roof all in one. Penetrations and flashing details are equally critical. A recent innovation is to use polymer-clad metal and to seal the panels to one another with a heat-welded single-ply batten.  Tile, shake and shingle-profiled panels, individually formed metal shingles and stone- coated metal are residential products that compete with clay and cement tile, as well as wood shakes and shingles. These will not be given our attention today, as excellent references are available elsewhere under "steep roofing'. Where do we begin? Since corrugated, through-fastened metal is the granddaddy, let's begin our maintenance discussion here. Since steel is strong and versatile, it was the natural choice for the Quonset hut. Corrugations provided the structural strength. Applying a thin zinc coating to both the top and bottom surfaces of the steel sheet provided corrosion resistance. This is called galvanizing, and G-60 denotes 0.60 ounces of zinc, total weight, per square foot of steel. G- 90 is a thicker coating, and since erosion of the zinc is the primary weathering mechanism, is more durable than G-60.

Zinc is a more active metal than iron, hence under corrosive conditions will preferentially corrode (to zinc oxide), protecting the steel against rust (iron oxide). Rusting is more of a problem in humid environments and those where acid rain or fumes are present.

Corrosion protection consists of removing loose rust by abrasion, applying primers and durable top-coatings. However, coatings by themselves do not restore corrugated roofs that are having other problems such as holes. The holes occur primarily from the through-fastened screws and bolts. Since materials expand and contract with temperature change, the nice round holes that were drilled when the bolts were installed will eventually elongate. In addition, the elastomeric washers on the screws were natural rubber or SBR, and have embrittled, no longer providing a waterproof seal. Current washers use EPDM, even more durable than the neoprene washers that replaced the earlier materials. Other holes are due to penetrations made for vent stacks, etc. While these most likely had some sealant when installed, the sealants also embrittle and fail. If fasteners have backed out, replacement with a screw with an increased thread diameter (and new washer) usually works.

Galvanic incompatibility is a concern with zinc alloy coatings. It is necessary to avoid contact with copper salts, such as from copper or brass flashings higher up on the roof or wall. Condensate from copper A/C coils will also erode galvanized and zinc-aluminum alloy (Galvalume®) surfaces. Compatible screws are also needed. Most combine polymer coatings with zinc dip. Brass and copper must be avoided. Stainless steel fasteners are used with all-aluminum roof systems.

Metal roof maintenance must address all these issues. One approach is to first bridge all openings and failed washers with a flexible membrane such as polyester fabric embedded in elastomeric sealant. These will accommodate some thermal movement, whereas the coatings by themselves cannot. Examine each equipment and pipe penetration on the roof, determining if it is still needed. In many cases, the equipment that the penetration supported is long gone. The more penetrations we can eliminate the simpler roof repairs will be. Butyl sealants, when sandwiched between metal panels and at flashings are extremely durable and are available as either pumpable materials or preformed tape. These are used to reset flanges of rooftop equipment that is still in use.

Since many of these early structures lack thermal insulation, applying sprayed-in-place polyurethane foam not only bridges the small holes in the system (and provides stiffness and stability), but also tremendously upgrades the thermal performance of the building. Thermal movement of the metal panels is dramatically reduced now that the panels are insulated. Durable coatings of urethane elastomer, acrylic, or silicone complete the installation. The surface will still have the form of corrugated, with a texture ranging from orange-peel to treebark. However, if the foam comes down to ground level as a sidewall, it is vulnerable to physical abuse by passing traffic. Use of double-thick coatings will provide extra protection.

The big leap-Structural Systems The 2002 article cited a number of dramatic improvements in design and durability of metal roofing. These included the use of zinc-aluminum alloys or pure aluminum coatings to protect the steel panels. These have vastly improved corrosion resistance as compared to the earlier galvanized finishes, and are commonly warranted for at least 20 years. Colored finishes of polyvinylidene difluoride (PVDF, Kynar 500® or Hylar 5000®) also are remarkably durable. However, if abraded or when the time comes to recoat, PVDF is a difficult surface to recoat without taking special precautions.

The problems of thermal movement at flashings and fasteners were solved by the use of 'floating clips and curbs'. In many cases, a two-piece clip is used, with the base secured to the structural supporting members, while the upper part is embedded in the seam of the panels and 'floats' with the panel. These systems limit through-fastening into the structural support to a single point in the panel run, generally the eave. Movement accumulates towards the ridge, which flexes to accommodate the accumulated movement, while remaining watertight.

The use of butyl tapes, pumpable sealants and factory-injected sealant in the side-seams greatly improves waterproofness. Flexible boots of EPDM or silicone rubber are used at penetrations, allowing the panels to move relative to the beam or pipe.

Seam design, using portable seaming machines, eliminates leakage at side seams. Some machines make a complete 360° 'Pittsburgh Lock". From a maintenance perspective, such secure seams make replacement of damaged panels quite complex.

Inspection and maintenance techniques-Because of the raised seams, water flow must always be downhill, parallel to the rib. At curbs and other downstream penetrations, diverters must be used to redirect the water around the obstruction. The best-designed curbs are premanufactured with side-rib integrated to mesh with the roof panels. The upstream horizontal flange sits beneath the upstream panels, so that water flow does not buck the metal laps. Compatibility between the roof panels and curbs is important. Don't allow 15-year galvanized to be used with 30 year roof panels.

While many structural panels have factory-installed side-lap sealant, field application of pumpable or tape sealant is necessary at end laps. The sealant needs to be placed on the 'wet-side' of the seam fasteners. The fastener has a washer to keep the upper panel watertight, but water could back-up into the seam and find the fastener holes through the lower panel if the sealant is installed too high up in the joint. A feeler-gage can detect the location of the sealant, if this is the suspected cause of a leak. Applying sealant on top of a leaking lap is never a satisfactory solution to misplaced sealant.

Flashing Problems Side-wall and end wall terminations can be the source of frustrating leakage. On the sidewall, remember that the wall is stationary but the roof panels are designed to move. This requires a two-piece flashing system, with the base piece consisting of an angle that is bent vertically and sealed to the panel. This vertical leg extends up the wall but is not fastened to the wall. A counter-flashing that is attached to the wall covers the top edge of the base flashing; shedding water out onto the body of the roof panel, and differential movement takes place between the base flashing piece and the wall counterflashing.

At endwalls, the panels must be cut short of the endwall to allow for thermal expansion. Usually a flexible membrane is attached to the upper end of the roof panels and serves as an air and water seal. It is covered with a metal cover to match the roof appearance, but the cover only provides a physical shield for the membrane, while the flexible membrane provides the waterproofing.

Exterior drainage is preferred with metal roof systems. If the structure is so wide that internal gutters are needed, they must be very corrosion resistant. Stainless steel gutters are recommended, as it will be very hard to remove a failed gutter with all the panels attached to it. Rehab of a failed internal gutter may include application of a sheet single-ply membrane.

Retrofitting Metal Roofing In addition to the sprayed-in-place foam systems mentioned previously, metal retrofit systems are readily available. Many use a sub-purlin system that bridges the existing vertical seams, and may be designed to be high enough to permit the addition of more thermal insulation as well as providing a base for the new metal roof system.

Naturally the single-ply industry recommends a single-ply solution, using rigid insulation to match the height of the seam, with an additional insulation layer to span the first layer. This is followed by membrane roofing and flashing.

The recoat industry also targets metal roofing as a maintenance opportunity. Elastomeric coatings are widely accepted for rehabilitation of basically sound structural metal roof systems, especially when combined with flexible polyester membrane for bridging laps, flashings and holes.

It is only fair to note that metal roofing is used as a recommended retrofit for built-up, single ply and even shingle roofing!