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Our firm did some forensic investigations on such a building at the request of the owner when warehouse personell discovered rust falling from the ceiling. When we opened the foil covering, we found that a good portion of the joist hangers and the nails through the roof plywood were rusted into non existance. They were totally rusted away in many places and additionally the sub purlin and purlin connecting hardwar had begun to show some damage. Also the framing and plywood of the roof showed significant areas of dry rot.
This was a major problem, of course, since all that was holding the roof together was the roofing material.

At 05:44 PM 09/09/1999 -0700, you wrote:
>I am currently involved in condition assessments of some tilt-up warehouse
>buildings in Los Angeles that are all about 20 years old. We have noticed
>in many of the buildings that there is corrosion staining on some of the
>joist hangers and other roof framing hardware. In some cases the staining
>is more severe than others; from ground level it does not appear that there
>is a significant amount of cross section loss due to corrosion. All of the
>buildings have insulating foil.
>Our initial thought is that the corrosion is due to the foil trapping
>moisture from either condensation or roof leaks. We are thinking that if
>the foil is replaced with a breathable material and the hardware is
>cleaned, then future corrosion will be slowed and it is thus not a serious
>problem (I do think that a few localized repairs and replacements will be
>necessary though). However, we have conflicting reports indicating that it
>may also be necessary to repaint all of the hardware in order to stop
>future corrosion.
>We have seen similar corrosion problems in past projects, but we are still
>trying to assess the root causes and the effectiveness of remediation
>measures. It seems like this might be a fairly common issue. Does anybody
>have experience assessing a similar problem? How serious of a problem is
>it? If we opt not to paint all of the hardware (just clean it and remove
>the surrounding foil) will the corrosion (and staining) reappear? Will
>painting (with a coating such as Rust-Oleum) stop all future corrosion?
>All input would be greatly appreciated. Further, does anyone know of a
>"corrosion engineer" that may have experience in this area, and be able to
>give some specific technical data?
>Thank you.
>Brennan Johnson
>EQE International
>Irvine, CA

R. Sandy Pringle
Special Inspection and Materials Testing - Fully Accredited
Hermosa Beach, Pasadena & Carson, CA
(800)598-1970 Fax(310)353-2248

Ignorance is more expensive than education....