New York & Baltimore old roofs with imported slates
Demand for slate had been high on the eastern seaboard in the United States for roofing material since the early part of the century.
In the 1930s it had been reported that as much as one-half of the roofs in New York City and one-third of those in Baltimore were covered with slate.
Almost all of this slate was supplied by imports from Great Britain.
The prices were competitive, in part, because transportation costs were offset by the use of slate as ballast in ships crossing the Atlantic.