Historic restoration and conservation · Lesson 4 of 4

Historic restoration and conservation: defects, verification and case study

How sheet metal deteriorates, flashing and drainage failures, temporary protection and harmful repairs, cleaning metals gently, inspection schedules, what a fabricator can verify, a worked thickness check, and a published repair case.

You will be able to: Diagnose how historic sheet metal fails, choose gentle corrective options, set an inspection schedule, verify a delivered part against the approved drawing, and document a repair from a published case.

13 min · 11 slides · 11 questions

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Slide 1 of 11

How historic sheet metal deteriorates

Brief 4 says sheet metals deteriorate by pitting or streaking from chemical action.

Causes include pollutants, acid rain, lichen and moss acids, alkalis washed from mortar or cement, and tannic acids from cedar or oak.

Iron rusts unless well painted or plated.

Lead tears because of creep, the gravitational stress that moves it down the slope.

Sources

Preservation Brief 4: Roofing for Historic Buildings (Sarah M. Sweetser, February 1978) - U.S. National Park Service, Technical Preservation Services

Glossary

Creep (lead): The gravitational stress that makes lead move down a roof slope until it tears.
Patina: A surface layer that forms on some metals, such as copper and bronze, that may be both protective and a distinctive finish, so it is not removed.
Nominal thickness: The thickness a metal weight is rated at, such as 0.0270 in for 20 oz copper in AFS's material register; real sheet is checked against it within the specified tolerance.
Cyclic inspection: Inspection at a set interval against a checklist, with the findings recorded and kept with the maintenance plan.