Because the pattern, detailing, and craftsmanship of slate
roofs are important design elements of historic buildings, they
should be repaired rather than replaced whenever possible. The
purpose of this Preservation Brief is to assist property
owners, architects, preservationists, and building managers in
understanding the causes of slate roof failures and undertaking
the repair and replacement of slate roofs. Details contributing
to the character of historic slate roofs are described and guidance
is offered on maintenance and the degree of intervention required
at various levels of deterioration.
The relatively large percentage of historic buildings roofed
with slate during the late-nineteenth and early-twentieth centuries
means that many slate roofs, and the 60 to 125 year life span
of the slates most commonly used, may be nearing the end of their
serviceable lives at the end of the twentieth century. Too often,
these roofs are being improperly repaired or replaced with alternative
roofing materials, to the detriment of the historic integrity
and appearance of the structure.
Increased knowledge of the characteristics of slate and its detailing
and installation on the roof can lead to more sensitive interventions
in which original material is preserved and the building's historic
character maintained. Every effort should be made to replace deteriorated
slate roofs with new slate and to develop an effective maintenance
and repair program for slate roofs that can be retained.