If repair is not possible and a new slate roof must be installed,
it is important to remember that more than just the replacement
of the slate is involved. The old slate should be removed to prevent
overloading of the roof timbers. Stripping should be done in sections,
with felt installed, to avoid exposing the entire subroof
to the weather. ln the process, rotted wood sheathing should
be replaced and the roof timbers checked for signs of stress
including deflection, cracking, and twisting. If such conditions
are found, a structural engineer experienced in working with older
buildings should be consulted. Other repairs, such as chimney
repointing, which may require access to the roof should be
completed before the new roof is put on.
Drawings and specifications for a new slate roof should
be prepared by a restoration architect, especially if the project
is going to be competitively bid or if the roof is particularly
complex. Standard specifications, like those published in 1926
by the National Slate Association may be used as a basis
for developing specifications appropriate for a particular project.
The specifications and drawings should contain all the information
necessary to replicate the original appearance of the roof as
closely as possible. Certain changes may have to be accepted,
however, since several types of slate once prominent in this country,
such as ribbon slate, are no longer quarried. It is wise to anticipate
the replacement of older roofs so that proper planning can be
undertaken and financial resources set aside, thereby, reducing
the likelihood of rash last minute decisions.
Roofing slate is sold by the square in the United States. One
square is enough to cover 100 square feet (13.3 square meters)
of plain roof surface when laid with a standard headlap of 3"
(7.5cm). When ordering slate, considerable lead time should
be allowed as delivery may take anywhere from 4 to 12 weeks and
even as long as 1 year for special orders. Orders for random
widths of a particular slate can generally be filled more
quickly than orders for fixed widths. Once on site, slates
should be stored on edge, under cover on pallets.
A roof and its associated flashings, gutters, and downspouts
function as a system to shed water. Material choices should
be made with this in mind. For example, use a single type of metal
for all flashings and the rainwater conductor system to avoid
galvanic action. Choose materials with life spans comparable
to that of the slate, such as nonferrous nails. Use heavier
gauge flashings or sacrificial flashings in areas that are
difficult to access or subject to concentrated water flows.
Flashings are the weakest point in any roof. Given the permanence
of slate, it is poor economy to use anything but the most durable
of metals and the best workmanship for installing flashings. Copper
is one of the best flashing materials, and along with terne,
is most often associated with historic slate roofs. Copper is
extremely durable, easily worked and soldered, and requires little
maintenance. Sixteenounce copper sheet is the minimum weight recommended
for flashings. Lighter weights will not endure the erosive action
of dust and grit carried over the roof by rain water. Heavier
weight, 20 oz. (565 grams) or 24 oz. (680 grams), copper
should be used in gutters, valleys, and areas with limited accessibility.
Lead-coated copper has properties similar to copper and is
even more durable due to its additional lead coating. Lead coated
copper is often used in restoration work.
Terne is a less desirable flashing material since it must be
painted periodically. Terne coated stainless steel (TCS)
is a modernday substitute for terne. Although more difficult to
work than terne, TCS will not corrode if left unpainted; a great
advantage, especially in areas that are difficult to access.
Once a metal is chosen, it is important to use it throughout
for all flashings, gutters, downspouts, and metal roofs. Mixing
of dissimilar metals can lead to rapid corrosion of the more electronegative
metal by galvanic action. Where flashings turn up a vertical surface,
they should be covered with a cap flashing. Slates which
overlap metal flashings should be nailed in such a manner as to
avoid puncturing the metal. This may be accomplished by punching
a second hole about 2" (5cm) above the existing hole on the
side of the slate not overlapping the metal flashing. It is important
that holes be punched from the back side of the slate.
In this way, a shallow countersink is created on the face of the
slate in which the head of the nail may sit.
The use of artificial, mineral fiber slate is not recommended
for restoration work since its rigid appearance is that of a manmade
material and not one of nature. Artificial slates may also have
a tendency to fade over time. And, although artificial slate costs
less than natural slate, the total initial cost of an artificial
slate roof is only marginally less than a natural slate roof.
This is because all the other costs associated with replacing
a slate roof, such as the cost of labor, flashings, and tearingoff
the old roof, are equal in both cases. Over the long term, natural
slate tends to be a better investment because several artificial
slate roofs will have to be installed during the life span of
one natural slate roof.
Clear roof expanses can be covered by an experienced slater and
one helper at the rate of about two to three squares per day.
More complex roofs and the presence of chimneys, dormers, and
valleys can bring this rate down to below one square per day.
One square per day is a good average rate to use in figuring how
long a job will take to complete. This takes into account the
installation of flashings and gutters and the setup and breakdown
of scaffolding. Tear-off of the existing roof will require additional