When trouble occurs, it is important to contact a professional,
either an architect, a reputable roofing contractor, or a craftsman
familiar with the inherent characteristics of the particular historic
roofing system involved. These professionals may be able to advise
on immediate patching procedures and help plan more permanent
repairs. A thorough examination of the roof should start with
an appraisal of the existing condition and quality of the roofing
material itself. Particular attention should be given to any southern
slope because year-round exposure to direct sun may cause it to
break down first.
Some historic roofing materials have limited life expectancies
because of normal organic decay and "wear." For example,
the flat surfaces of wood shingles erode from exposure to
rain and ultraviolet rays. Some species are more hardy than
others, and heartwood, for example, is stronger and more durable
Ideally, shingles are split with the grain perpendicular
to the surface. This is because if shingles are sawn across
the grain, moisture may enter the grain and cause the wood to
deteriorate. Prolonged moisture on or in the wood allows moss
or fungi to grow, which will further hold the moisture and cause
Of the inorganic roofing materials used on historic buildings,
the most common are perhaps the sheet metals: lead,copper, zinc,
tin plate, terne plate, and galvanized iron. In varying degrees
each of these sheet metals are likely to deteriorate from chemical
action by pitting or streaking. This can be caused by:
- airborne pollutants;
- acid rainwater;
- acids from lichen or moss;
- alkalis found in lime mortars or portland cement, which
might be on adjoining features and washes down on the roof
- tannic acids from adjacent wood sheathings or shingles made
of red cedar or oak.
Corrosion from "galvanic action" occurs when
dissimilar metals, such as copper and iron, are used in direct
contact. Corrosion may also occur even though the metals are
physically separated; one of the metals will react chemically
against the other in the presence of an electrolyte such as
rainwater. In roofing, this situation might occur when either
a copper roof is decorated with iron cresting, or when steel
nails are used in copper sheets. In some instances the corrosion
can be prevented by inserting a plastic insulator between
the dissimilar materials. Ideally, the fasteners should be a
metal sympathetic to those involved.
Iron rusts unless it is wel lpainted or plated. Historically
this problem was avoided by use of tin plating or galvanizing.
But this method is durable only as long as the coating remains
intact. Once the plating is worn or damaged, the exposed iron
will rust. Therefore, any ironbased roofing material needs to
be undercoated, and its surface needs to be kept well
painted to prevent corrosion.
One cause of sheet metal deterioration is fatigue. Depending
upon the size and the gauge of the metal sheets, wear and metal
failure can occur at the joints or at any protrusions in the
sheathing as a result from the metal's alternating movement
to thermal changes. Lead will tear because of "creep,"
or the gravitational stress that causes the material to move
down the roof slope.
Perhaps the most durable roofing materials are slate and tile.
Seemingly indestructible, both vary in quality. Some slates
are hard and tough without being brittle. Soft slates are more
subject to erosion and to attack by airborne and rainwater chemicals,
which cause the slates to wear at nail holes, to delaminate,
or to break. In winter, slate is very susceptible to breakage
by ice, or ice dams.
Tiles will weather well, but tend to crack or break if hit,
as by tree branches, or if they are walked on improperly. Like
slates, tiles cannot support much weight. Low quality tiles
that have been insufficiently fired during manufacture, will
craze and spall under the effects of freeze and
thaw cycles on their porous surfaces.