The natural weathering of roofing slate manifests itself as a
slow process of chipping and scaling along the cleavage planes.
Paper-thin laminations flake off the surface of the slate and
the slate becomes soft and spongy as the inner layers begin to
come apart, or delaminate. The nature of the sound given off by
a slate when tapped with one's knuckles or slating hammer is a
fair indication of its condition. High-grade slate, when poised
upon the fingertips and struck, will emit a clear, solid sound.
Severely weathered slates are much less sonorous, and give off
a dull thud when tapped.
The weathering of slate is chiefly due to mineral impurities
(primarily calcite and iron sulfides) in the slate which, in concert
with alternating wet/dry and hot/cold cycles, react to form gypsum.
Because gypsum molecules take up about twice as much volume as
calcite molecules, internal stresses result from the reaction,
causing the slate to delaminate. This type of deterioration is
as prominent on the underside of the roof as on the exposed
surface due to the leaching and subsequent concentration of gypsum
in this area. Consequently, deteriorated roofing slates typically
cannot be flipped over and reused.
The chemical and physical changes which accompany slate weathering
cause an increase in absorption and a decrease in both strength
and toughness. The tendency of old, weathered slates to absorb
and hold moisture can lead to rot in underlying areas of wood
sheathing. Such rot can go undetected for long periods of time
since, often, there is no accompanying leak. Due to their loss
of strength, weathered slates are more prone to breakage, loss
of corners, and cracking.
Slates with low calcite content tend to weather slowly. Dense
slates, with low porosity, likewise decay slower than slates with
equal calcite, but with a greater porosity. The pitch of a roof
can also affect its longevity. The steeper the pitch, the
longer the slate can be expected to last as water will run off
faster and will be less likely to be drawn under the slates by
capillary action or driven under by wind forces. Spires and the
steep slopes of Mansard roofs often retain their original slate
long after other portions of the roof have been replaced. Areas
of a roof subject to concentrated water flows and ice damming,
such as along eaves and valleys, also tend to deteriorate more
rapidly than other areas of the roof.
Mechanical agents, such as thermal expansion and contraction
and the action of frost, are subordinate in the weathering of
slate, coming into play only after the slate has been materially
altered from its original state by the chemical transformation
of calcite to gypsum. The more rapid deterioration of slates found
on roof slopes with the most severe exposure to the sun, wind,
and rain (typically, but not always, a southern exposure) may
be attributable to the combined result of the deleterious effects
of impurities in the slate and mechanical agents. Atmospheric
acids produce only negligible deterioration in roofing slate.
It is difficult to assess the procedures by which a piece of
slate has been fabricated without visiting the quarry and observing
the process first hand. The location and size of nail holes, grain
orientation, the condition of corners, and the number of broken
pieces are all things which may be observed in a shipment of slate
to judge the quality of its fabrication. Nail holes should
be clean and with a shallow countersink on the face of
the slate for the nail head; grain oriented along the length of
the slate; and, corners left whole. An allowance for 10% breakage
in shipment is typically provided for by the quarry.
Installation problems often involve the improper nailing and
lapping of slates. The nailing of slates differs from that
of other roofing materials. Slate nails should not be driven tight
as is the case with asphalt and wood shingles. Rather, they should
be set such that the slate is permitted to hang freely on the
nail shank. Nails driven too far will crack the slate and those
left projecting will puncture the overlying slate. Nail heads
left exposed accelerate roof deterioration by providing a point
for water entry. Nonferrous slater's nails, such as solid copper
or stainless steel, should always be used since plain steel
and galvanized nails will usually rust out long before the slate
itself begins to deteriorate. The rusting of nineteenth-century
cut nails is a common cause of slate loss on historic roofs.
When joints are improperly broken (i.e., when slates lap the
joints in the course below by less than 3" [7.5 cm]), it
is possible for water to pass between the joints, through the
nail holes and ultimately to the underlying felt, where
it will cause deterioration and leaks to develop. Insufficient
headlap can also result in leaks as water entering the joints
between slates may have a greater tendency to be wind blown beyond
the heads of the slates in the course below.
Occasionally, individual slates are damaged. This may be caused
by falling tree limbs, ice dams in gutters, valleys, and chimney
crickets, the weight of a workman walking on the roof, or a naturally
occurring fault in the slate unit. Whatever the form of damage,
if it is caught soon enough, the roof can usually be repaired
or selectively replaced and deterioration mitigated.
The ability to lay slate properly so as to produce a watertight
and aesthetically pleasing roof requires training, much practice,
and the right tools. The installation and repair of slate
roofs should be entrusted only to experienced slaters.