Galvalume® Steel Roofing: Good Practices Guide
Installation Is Key To Performance For Contractors
The most popular of all GALVALUME roof systems is the standing seam roof system. It's made up of metallic-coated steel sheet panels joined together at their sides with standing seams and fastened to the roof substructure with concealed fasteners and clips.
On new roofs, structural GALVALUME steel panels are typically installed directly on purlins. Concealed clips are attached to the purlins, and the panels and clips are joined together by forming, crimping or snapping at the seams.
For most retrofit applications, panels are installed on a sub-framing system which, in turn, is attached to the existing roof structure.
To provide positive water drainage and maximize performance, the recommended minimum slope for GALVALUME roofs is 1/4:12 — otherwise, water collects to form ponds.
Some panels on this 20-year-old standing seam roof were installed with a slope less than the 1/4:12 minimum. Water collected at low areas and caused corrosion of the Al-Zn coating.
Compatible accessories are important to ensure that a GALVALUME roof achieves maximum life. Fasteners and flashings should always be made from material designed to last as long as the GALVALUME roof. Incompatible or substandard accessories can lead to failure of one or more components.
Neutral cure sealants should be applied at end laps to prevent moisture from entering panel lap joints. Crevice corrosion has occurred on this 20-year-old "screw down" GALVALUME roof, near the Gulf of Mexico in Florida, due to poor end lap sealing. The light zinc-electroplated fasteners were also rusted.
When installing fiberglass blanket insulation under GALVALUME roofs, be careful to seal all vapor barrier seams and repair punctures, penetrations or holes in the vapor barrier. Condensation of water vapor on the underside of the roof, along with saturation of the insulation, can cause inside-out corrosion. Leaking rainwater at loose fasteners can also saturate insulation and cause corrosion from inside.
Fasteners are used extensively on "screw down" roofs and, to a lesser degree, on standing seam roofs. Quality fasteners with a life matching that of the Al-Zn coating should be used.
The heavy zinc-electroplated steel fasteners on this 17-year-old roof in Florida are in good condition and free of rust.
Insulation should not be exposed where it will be continually wet and in contact with the underside of roof panels. Here wet, exosed insulation at the top wall panels caused corrosion from the underside of a 20-year-old roof on Florida's Atlantic Coast.
Immediately after installation, make sure panels are swept clean to remove construction debris such as unused fasteners, metal filings, pop-rivet stems and pieces of flashing. The rusting of debris is often mistaken for roof corrosion.
Architectural GALVALUME steel roofing is usually installed on a metal or wood deck fastened to the roof structural system. Concealed clips or screws fix the panels to the deck. It's recommended that a 30-lb. felt paper be used between the roof and deck.
In all installations, a minimum 1/4:12 slope should be provided for maximum performance over time.
Care must be exercised during unloading, storing and installation of panels to avoid damage by bending, warping, twisting, scoring and corrosion. To avoid storage corrosion, stacks should be stored off the ground on skids-at an angle for drainage-and protected with a loose-fitting waterproof cover.
Roof panels are delivered to the job site in packaged, blocked and banded stacks. The stacks must be handled properly with cranes and fork trucks to avoid kinking.
The 0.022-inch thick "screw down" roof panels on this 16-year-old building were kinked from careless handling causing water to collect and form ponds.