Galvalume general questions

1. Why is galvalume sheet called by other names? Are all of these products the same? 

The product is a 55% aluminum-45% zinc alloy coated sheet steel developed by Bethlehem Steel and sold commercially under the trademark galvalume, starting in June 1972. Bethlehem Steel later licensed other major steel companies to produce and sell the product using its patents and technology. In North America, the galvalume trademark is used by ArcelorMittal Dofasco (Canada), U.S. Steel, Wheeling-Nisshin, Steel Dynamics and AM/NS Calvert. Steelscape, LLC uses the trademark ZINCALUME廬, Ternium USA and Ternium Mexico use Galval, Cincalum or Zintro-Alum.

2. How is it made?

Both galvalume and galvanized sheet steels are made by a "continuous hot dip" process. Coils of cold rolled steel are welded end-to-end and processed continuously on the coating line at speeds up to 600 feet per minute. The uncoiled sheet is first cleaned to remove rolling oils and mill dirt, and to reduce surface oxides so that the surface will accept the coating. These continuous sheets are first fed into a molten coating bath contained in an open top, brick-lined heated pot. The sheet then passes around a roller submerged in the coating bath and exits the bath vertically, pulling out with it an envelope of the coating material. As it exits, the sheet proceeds through a pair of opposing air knives, which are positioned above the bath and equidistant from the surface of the sheet. Most modern production lines have a coating thickness gauge feed-back control, which automatically adjusts knife air pressure and position to ensure that a uniform coating is applied. Various finishing steps are carried out to complete the process, after which the sheet is wrapped around a reel into a coated steel coil.

3. Why is it a good material for roofing?

galvalume sheet is an ideal material for roofing because of its extraordinary outdoor corrosion resistance and resulting long life. It can be readily rollformed into a variety of panels, as well as formed and stamped into tile and shingle facsimiles. It can also be factory painted to impart color and extend durability. Most important, it can be used in modern day steel roof designs which are strong, yet light weight. And unlike conventional nonmetallic roof systems, galvalume sheet roofs won't crack and peel when subjected to the sun and weather.

4. Galvalume sheet is sold with AZ50 and AZ55 designations. What do they mean?

galvalume sheet is made to meet ASTM Specification A792. This specification covers a number of requirements, including coating weight. AZ50 and AZ55 are the English unit coating weight designations indicating that, respectively, 0.50 ounces per square foot and 0.55 ounces per square foot of the aluminum-zinc alloy coating have been applied to both sides of the galvalume sheet. Converting these coating weights to coating thickness, AZ50 is equivalent to about 0.8 mil (0.0008") on each side of the sheet, and AZ55 to about 1.0 mil (0.001") on each side. In metric units, AZ150 is equivalent to about 20 microns and AZ155 to about 25 microns on each side.

5. What is the reflectivity of galvalume sheet?

galvalume sheet has good heat and sunlight reflectivity. Its bright, white reflective surface makes it an ideal material for roofing. During the summer it makes buildings cooler by efficiently reflecting away sunlight and reducing the amount of heat transmitted into the building. Likewise, it makes buildings warmer in the winter by reflecting inside heat from the underside of the roof back into the building. This good thermal reflectivity translates into energy savings by reducing cooling costs in the summer and heating costs in the winter. Tests have shown that only half as much heat from solar radiation is transmitted into a building fitted with a galvalume roof compared to a galvanized roof, and almost one-third less heat compared to a terra-cotta roof.

6. What are the advantages of a galvalume standing seam roof (SSR) compared to a built-up roof?

Leaking conventional, flat nonmetallic roofs are the biggest problem on nonresidential buildings for architects and building owners. Conventional roofs made with organic materials deteriorate even under normal environmental conditions. They embrittle from the heat and ultraviolet radiation from the sun. Then, as result of temperature fluctuations, they develop cracks, splits and tears. Finally, because they are flat, ponding water inevitably penetrates these brittle, cracked systems and leaks into the building, causing damage to goods and disrupting activities inside the structure. Leaks can be patched early in the life of the roof, but as time passes, leaks become more frequent and an expensive replacement of the roof is required. Such replacements often require tearoff, extra labor and even a temporary building shut-down. galvalume standing seam roofs (SSR) offer a weathertight, maintenance-free roof system that will last for decades on commercial, office and factory buildings. galvalume SSRs are economical to install, leak proof, maintenance-free, energy efficient, noncombustible and long-lived.

7. Can a galvalume SSR be installed over an old flat built-up roof?

A galvalume SSR is an ideal system for retrofit roofing. Its light weight makes it suitable for retrofit applications over any old, leaking, conventional, nonmetallic roof system. Retrofitting is accomplished by installing to the existing roof structure a light weight sub-framing system, generally made with light structural steel sections, that provides a minimum 錄:12 slope for the new galvalume SSR. By installing the roof directly over the existing nonmetallic roof, costly and time-consuming tear-offs are eliminated, and activities inside the building can continue without interruption.

8. How does the corrosion resistance of galvalume sheet compare with that of galvanized sheet, and how long can a galvalume sheet roof be expected to last?

galvalume sheet has been successfully used in roofing applications for more than 40 years; it has been evaluated in outdoor R&D tests for well over 50 years. Based on these tests, in which corrosion weight losses were measured and compared with galvanized, galvalume sheet is projected to outlast galvanized sheet (with an equivalent coating thickness) in various atmospheres by up to nine times. Likewise, recent inspections of 82 low slope galvalume roofs up to 35 years old in the eastern U.S. confirms the R&D results. These roofs are in excellent condition and are projected to last 50 to 60 years before requiring major maintenance.

9. Should architectural galvalume sheet roofs be installed on decking or over shingles on residential applications?

Wood or steel decking is normally used for prepainted galvalume architectural roof systems on both residential and nonresidential applications, but it is not recommended to install the panels in direct contact with the decking. Instead, 30 pound felt, or an equivalent, should first be installed on the decking and the roof panels then installed over the felt. For residential applications, the roof panels would be installed over roofing felt on wood decking for new construction applications, or in applications in which the old shingles would be removed from the deck.

Roofing manufacturers should be contacted for design and installation details both in warmer regions where condensation can occur under roof panels, and also in cold regions where snow, ice and water can collect at eaves and valleys and leak into the building.

10. What types of insulation are recommended for galvalume roofs? Are there any to be avoided?

By far, fiberglass roll insulation is the most commonly used insulation for galvalume steel roofs on nonresidential buildings. It can be used in any of the available thicknesses to give the required insulation value. One of the main benefits of retrofitted galvalume roofs installed over leaking nonmetallic flat roofs is the ability to add insulation under the new galvalume roof and realize significant energy savings from reduced heating and cooling requirements. In such instances, fiberglass roll insulation can either be placed on top of the old nonmetallic roof or under the new retrofit roof.

Wet insulation, which may be rain soaked at the site or wet from condensation or a leak in the roof, should not be in contact with galvalume roof panels. Insulation should be dry when installed and kept dry after installation. Fiberglass insulation retains water and can cause rapid inside-out corrosion on galvalume sheet panels. Also, spray-on insulation is sometimes used inside buildings under roofs and on siding. This insulation may contain fire retardant chemicals that can be corrosive to galvalume sheet. Manufacturers should be consulted about such fire retardant additives if spray-on insulation is used.