Material

Focus on Insulation

Metal-deck corrosion: Three case studies

A combination of factors was found to have contributed to major, unpredictable patterns of deck corrosion

by Richard P. Canon, RRC, PE

The roofing industry has exhibited a growing concern in recent months regarding corrosion of metal decks. Several recent articles in Professional Roofing ¹ and NRCA Bulletin 15-91 ² have discussed a growing concern in the roofing community regarding corrosion of metal decks. During the last two years the author's roof consulting firm has investigated three projects that have experienced severe, premature corrosion of metal roof decking. Two of the decks were primed but not painted, and one was galvanized (ASTM A 446 steel with hot dipped galvanized coating conforming to ASTM A 525, G-60). The roof assembly for each project is shown in Figures 1, 2, and 3. Table A summarizes the history and data regarding each project. All three used phenolic insulation as part of the system. This article will discuss the evidence that a combination of factors involving the insulation, moisture in the system and inadequate protection by the deck treatments may have contributed to deck corrosion in these three projects. Project No. 1 The discovery of corrosion of the metal deck was initially made on the roof of Project No. 1 in 1985, about three years after construction, when several new penetrations were cut in the asphalt built-up roof (BUR). This exposed moderate-to-severe corrosion of the deck. This corrosion was initially thought to be dormant and isolated to active leaks. Two years later a small area of corroded deck was replaced after rust was observed on the bottom of the deck over an area used to charge batteries for lift trucks. The rusting was explained as being related to battery acid fumes, a theory later invalidated. Originally, both occurrences were rationalized as localized anomalies and not a source of major concern. In 1989, six years after construction, pieces of...