Pre-painted Galvalume® Sheet:
A Guide to Best Practices
Because GALVALUME sheet roofing and siding are long-lasting products, it is important that fasteners have a service life equivalent to that of GALVALUME panels and accessories. Fastener costs are minimal relative to the overall cost of a building project, so no benefit is gained by using inferior fasteners. The correct selection of a fastener is important for long-term structural performance and aesthetics.
A wide range of fasteners is available for use with prepainted GALVALUME sheet. These fasteners are also supplied in self-drilling and self-tapping types with various metal and graphite-free polymer washer combinations to suit the fastening requirements of panel manufacturers.
Fastener materials include carbon steel, 400- and 300- series stainless steel and aluminum. Carbon steel fasteners are available with special heads that include cast zinc-aluminum, nylon and 300- series stainless capped heads. Fastener manufacturers can also top coat metal fasteners with factory-applied polymer coatings that color-match prepainted GALVALUME roof and wall panels. (Note: Carbon steel fasteners are available with electroplated and mechanically-plated zinc and cadmium coatings; however, these coatings typically are too thin (0.5 mil. or less) and not recommended for prepainted GALVALUME sheet.) For all fastening applications, users should be guided by recommendations of fastener and panel manufacturers.
Steel Roofing During installation fasteners should be properly tightened and not overdriven or driven at an angle (Figure 10). Leaks can potentially occur at improperly-seated fasteners, as well as at underdriven fasteners or fasteners that "back out" over time. Water leaking at fasteners can saturate insulation and cause inside-out corrosion. Overdriving a fastener may also cause a depression on some panel profiles, which can then collect water and accelerate corrosion. Driving tools equipped with depth-sensing nose pieces and suitable RPM speeds can assist in avoiding these problems. Impact-type tools should not be used.
Double-bead and triple-bead tape sealants are commonly used at overlapping panel ends on roofs and at laps on roof ancillaries such as curbs. Sealants are also available in tubes for flashing and ancillary sealing. Sealants should be sandwiched completely within the lap and not applied on the panel surface, where they can be degraded by UV radiation. Roof panel manufacturers can provide design details about placement of sealants at overlapping panels, ancillaries and flashings.
Butyl, polyurethane, acrylic and silicone sealants can be used. Neutral cure silicones are recommended. Acid cure silicones liberate corrosive by-products that smell of vinegar. Only aliphatic urethanes should be used. Butyl sealants should be used where there is no exterior exposure, such as between laps. Only acrylics containing very high solids (>80%) are recommended. Sealant manufacturers can make specific recommendations about flexibility, resiliency, UV tolerance and weathering.
Cut or sheared edges expose the steel, metallic coating and paint coating to the environment, thereby creating a galvanically-active area at which corrosion can begin. The degree to which such corrosion occurs at a cut edge depends on the thickness of the sheet, the paint system, the environment and the angle of exposure, i.e. vertical (wall panels) or sloped (roof panels).
Roof Drip Edges
Roof drip edges on prepainted metallic coated sheets are subjected to the most severe cut edge corrosion because rain and dew collect and are retained at such edges. The initial sign of corrosion at roof drip edges is usually visible as microblistering on the surface of the panel back from the cut edge. As time goes on, paint microblistering continues to creep back from the edge.
On some metallic coated products, paint microblistering continues at a steady rate and eventually red rusting occurs on the surface of the sheet near the edge. Prepainted GALVALUME sheet shows microblistering, but its rate decreases with time. Thus, over the long term, prepainted GALVALUME sheet provides better performance than conventional metallic coated sheets at roof drip edges.
Sidewall Drip Edges
For prepainted GALVALUME sidewall drip edges, no significant corrosion is noticeable to the naked eye when proper installation practices have been followed. These practices are described in Table 3.
Good Practices Guidelines To Avoid or Minimize Paint Microblistering At Cut And Drip Edges
- When roof panels are sheared or cut in the shop or field, the burr produced by the shear cut should be placed down to limit water retention at the edge.
- Drip or drain ends on roof panels at eaves should be bent down slightly to avoid collecting water on both the top and bottom sides of the panel.
- If possible, particularly in severe marine and industrial environments, field-cut edges should be painted or otherwise protected, such as with shielding trim parts.
- Sidewall panels at the ground level should not contact the concrete footing. Water can collect at the crevice contact point between the panel and the footing, as well as infiltrate into insulation on the inside wall, which can cause inside-out corrosion. To isolate the sidewall panel from the footing, sill plates that provide adequate drainage should be installed on the concrete footing (Figure 9).
Vertical Edges on Siding
Slitting of the vertical or long edges of prepainted GALVALUME siding is not recommended for applications where the slit edge is boldly exposed to the environment. Building panel manufacturers should order prepainted GALVALUME sheet for roll forming that will give the required finished siding width without slitting to size and thus avoiding exposed cut edges. The long edges of such full width GALVALUME sheet are protected by the GALVALUME coating and paint "wrap around" during hot dip metal coating and paint coil coating.
Overlapping Roof and Sidewall Edges
Corrosion is less severe at overlapping edges along the sides and ends of prepainted metallic coated panels than at drip edges, because rain and dew drain freely and are not retained at overlapping edges. Where possible, full-length panels should be used on roofs and walls to avoid short, overlapping panels with sheared ends and exposed cut edges.
Trim and Sidewall Fitted Edges
Cut edges are also present on trim and sidewall fitted edges around doors and windows. The cutting practices described under "Field Cutting" should be used for clean, burr-free edges when fitting sidewalls around wall openings. Such cuts should be protected and hidden from view by trim parts. Where possible, trim parts should also be bent and hemmed so that the cut edge is folded out of view.