Historic sheet metal restoration · Lesson 4 of 4

Historic sheet metal restoration: defects, verification and case study

Defects in tinplate, terneplate, galvanized iron and zinc and their causes, the parts of a pressed-metal cornice, a National Park Service case study from findings through repair, attachment and paint, and the inspection routine and evidence packet.

You will be able to: Diagnose common defects in old tinplate, terneplate, galvanized and zinc work from symptom to cause, follow a published cornice repair from findings to paint, and set the inspection routine and evidence packet a reviewer needs.

10 min · 12 slides · 7 questions

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Slide 1 of 12

Tinplate and terneplate defects

Read each defect as symptom, cause, evidence, then options.

Tinplate roofing can corrode on the underside from water vapor condensation if the tin is not protected by paint or a nonacidic vapor barrier.

At Hyde Hall, New York, a flat-seam terneplate roof corroded from the underside: unpainted there, with condensation unable to evaporate.

Sources

Metals in America's Historic Buildings, Part 3 (Waite, deterioration and preservation methods; second edition, 1992) - U.S. National Park Service, Technical Preservation Services
Preservation Brief 4: Roofing for Historic Buildings (Sweetser, 1978; a dated brief) - U.S. National Park Service, Technical Preservation Services

Glossary

Modillion: In the case study, a stamped zinc ornament under the cornice, 12 in wide and projecting 24 in, soldered together from six stamped pieces.
Dentil band: One of the named parts of the case-study cornice; there the dentil band was brake-formed galvanized steel in sound condition.
Armature: In the case study, the steel ribs to which the cornice facing was bolted, tied to the steel trusses by twisted wires.