by Joseph Jenkins, Jenkins Slate Roofing Services
Beautiful to look at, slate roofs are simple, low-tech roofing systems
made of natural materials: primarily stone (slate) with wood boards
or battens and metal fasteners (nails). When the slates need to be
replaced, they can be discarded as clean fill, unlike the toxic waste
of petrochemical roofs; as such, slate roofs are sought after by the
ecologically minded.
When the entire life of the roof is taken into consideration, a slate
roof is arguably the least expensive roof money can buy. I recently
looked at a beautiful slate roof, 120 years old, on a cathedral in
Arkansas. In 1881 the cost to install this ornate, 11,700-sq.ft. roof
— with a 220-ft. spire — was $765.00 for the labor and $1,166.50
for the materials. Even adjusted for inflation, it should be obvious
that this was money well spent.
Some types of slate wear out sooner than others, and once they wear
out, they can’t be saved. There are many types of roofing slate,
each with its own particular qualities and idiosyncrasies. The 120-year-old
cathedral roof mentioned above was installed in an ornate pattern
of black and green slate. The green slate originated in Vermont, the
black in Pennsylvania. The black slate had a life expectancy of about
120 years, and having reached that age, it was showing a lot of delamination,
softening, and crumbling. The green slates, however, remained hard
and showed no deterioration after 120 years — it’s anyone’s
guess how much longer they would last. If the entire roof had been
installed with the green slate, it would not need to be replaced.
But because half of the slates were of this softer black variety that
had reached the end of its life, the roof was not repairable; it had
to be replaced (with new slate, of course).
It is imperative that people who own or work on slate roofs know
the different types of roofing slate — their origins, longevities,
characteristics, and qualities — and be able to identify the
slate on the roof in question. If sight identification is not possible,
then they must be able to send a slate sample or photo to someone
who knows slate to have it identified. In the U.S., roofing slate
is still being quarried in Virginia, Pennsylvania, New York, and Vermont;
a century ago there were hundreds more American slate quarries, including
such states as Maine and Georgia, and the differences between slates
from the various quarries were sometimes striking. Knowing the history
of slate quarries is important for people who work with traditional
slate roofs.
Even if a slate roof is composed of very long-lasting slate, the
metal flashings can wear out and leak before the slate begins to fail.
These flashings are sheetmetal joints installed to prevent water entry
between the various planes of the roof, such as in the valleys, along
dormer walls, and around roof penetrations such as chimneys. The most
common older flashings were made from terne-coated steel, which is
steel coated with a lead/tin combination, erroneously but commonly
called “tin.” Terne-coated steel must be painted regularly
to avoid corrosion. Copper flashings (either plain copper or lead-coated)
were used primarily on institutions and upper-scale residences; sheet-lead
flashings were often used on older buildings, especially around plumbing
vent pipes. Terne flashings could last 90 years or longer, if they
were kept painted. Copper flashings, because they typically are not
painted, begin to corrode, pit, and leak after about 60 to 70 years,
in areas of high wear such as valleys. For this reason, older copper
flashings should be painted to extend their effective lives.
When flashings begin to fail on a slate roof that is made of sound
slates, only the flashings should be replaced, not the entire roof.
This work is routine for slate-roof-restoration professionals. One
of the extraordinary characteristics of slate roofs is that they are
designed to be taken apart and put back together. Broken slates, worn
flashings, rotted sheathing boards, or any element of the roof can
be removed and replaced without replacing the entire roof. Because
of this unusual maintenance characteristic, slate roofs can be made
to last as long as the slate itself will last, which could be hundreds
of years.
When repairing or restoring a slate roof, individual slates are removed
to expose the existing flashings, which can then be removed and replaced.
After the removed slates are put back into their original positions,
the repaired roof should look much the same as it did before the repair,
except with new flashings. A good repair is invisible to the layperson.
It is not uncommon for a century-old slate roof to have 50 or more
slates fail from simple attrition. Slate is a natural stone with faults
and hairline cracks, and slates will eventually break here and there
on a roof. A typical 20-square roof (2,000 sq.ft.), with a typical
10-in.-x-20-in. slate, will have about 3,400 slates. If 50 fail after
a century, then the roof’s failure rate is less than 1.5% per
100 years — phenomenally small. One missing slate, however, is
all it takes to create a leak, which in turn may cause someone to
shout, “Tear off the roof and replace it!” More often than
not, people unknowingly lose a good, repairable slate roof when they
could have easily repaired or restored it.
Faulty slates should simply be removed and replaced. Replacement
slates that match the originals in size, shape, and color must be
used whenever possible. Replacement slates must never be fastened
in place with visible straps or face (exposed) nails. Instead, there
are two generally accepted methods of fastening replacement slates
into place: the nail-and-bib method and the slate-hook method (see
diagram below).
The nail-and-bib method is perhaps the most widely used. It involves
nailing the replacement slate with a nail in the slot between the
overlying slates (see illustration) and then sliding a “bib”
flashing under the overlying slates and over the nail head. The bib
is often bent slightly to friction-fit it into place. It can be composed
of aluminum, copper, or other non-corrodible metal, but the bib should
not be shiny and reflective like stainless steel, as it may then be
visible from the ground on a sunny day. Instead, copper or brown-painted
aluminum (coilstock) are preferred. A common-size bib is 4 in. x 7
in.
A slate hook is a hard wire hook made of galvanized steel, copper,
or stainless steel, approximately 3 in. long. A small exposed loop
hooks the replacement slate in place (see illustration). In this instance,
an exposed repair device is acceptable because the tiny hook is almost
invisible from the ground. Stainless-steel hooks are stronger than
copper. Slate hooks are preferable to the nail-and- bib on particularly
hard slates and on new slate roofs, especially for repairs.
The tool required to remove slates from a roof is the slate ripper:
a swordlike object that slides up under the slate and pulls out the
two nails which hold it in place. A slate hammer, another important
slate-roofing tool, has a hole punch at one end, used to punch holes
in slates for nailing.
Some slate hammers also have shanks designed to cut slates, which
is done by a chopping motion against a straight edge, typically a
slater’s stake. Salvaged slates readily punch without breaking
and leave a clean hole with a “countersunk” characteristic,
into which the nail head sits. New slates can be hard and brittle,
and require some practice for easy punching with a slate hammer. Standard-thickness
slates (3/16 in.) are readily cut with a simple handheld device, a
slate cutter.
Contractors should work on slate roofs using hook ladders, which
keep their weight off the slate while giving them a safe work platform
to cling to. It is not proper to work on slate roofs by walking on
them using ropes, as walking on slate roofs breaks the slates, which
is the primary reason low-slope slate roofs fail prematurely.
Slate roofs can, in some circumstances, be carefully walked on by
a qualified slate roofer — which means someone who will repair
any slates he or she may break while moving about. It is improper
to tar or coat the surfaces of slate roofs, or to use surface tar
for repairs. Not only is it unsightly, but it won’t stop leaks
permanently and will ruin the slates.
One of the biggest problems facing older slate roofs today, and a
cause of many leaks, is not natural attrition, flashing failures,
broken slates, or global slate failures. It is, put plainly, bad work.
There are many unqualified persons attempting to repair slate roofs,
who don’t know what they’re doing. In my own slate-roof-restoration
business, fully half of the work we do is the removal and replacement
of faulty repairwork. Slate-roof owners pay good money to have their
roofs abused, then they have to pay good money again to have it repaired
correctly. Abused roofs include the ones that are walked on by “Bigfoot”
and the ones that are face-nailed, tarred, repaired with non-matching
slates, coated, or re-flashed incorrectly.
Furthermore, roofing contractors who have little or no expertise
in slate roofs may advise a client to replace a slate roof which could
have many decades of life remaining — and a client will listen
to bad advice if it is the only advice that can be found. All these
factors combined can make a frustrated client want to forever remove
a slate roof, no matter how much longer it could last if repaired
properly.
Low-slope slate roofs will fail prematurely because people will walk
on them over the years and break the slates. The resulting leaks are
often repaired by non-professionals because these roofs are readily
accessible. These repairs tend to be done poorly: The roof will still
leak, resulting in more traffic on the roof. A downward spiral of
deterioration thus begins, ending with the demise of the slate roof.
The lowest slope advisable for a slate roof is 4:12; however, for
the roof to last a long time, the slope should be too steep to walk
on, which would bring it up to about 8:12 or steeper.
Nails are sometimes said to be the cause of slate-roof failure, but
they are often not at all to blame. It is true that nails will corrode
on an older slate roof, but this problem is most likely under two
general conditions: (1) The nails were of poor quality when initially
installed, and/or (2) the slate has reached the end of its life, and
moisture is now penetrating the roof and corroding the nails. Originally,
in Wales, slate roofs were installed with wooden pegs driven through
a hole in the top center of the slate. The slate/peg combination was
then hung over a horizontal lath on the roof — no nails were
used. The weight of the slates overlapping each other held the roofing
in place. In the U.S., slates are nailed in place with two nails situated
about a third of the way down the slate, along the outside edges.
The slates are nailed into boards (roof sheathing, usually 1-in. thick)
or into horizontal wooden strips (slating lath or battens, usually
1x2s or 1x3s), depending on the predilection of the installer. Lath
roofs are common in Wales, England, and Europe, so immigrants from
those countries often copied their traditional styles of slate installation
once they arrived here at the turn of the last century. Traditional
Scottish roofs use solid boarding, as is more common in the U.S. Most
of the older slate roofs in the U.S. are nailed with hot-dipped galvanized
roofing nails, although most institutional and upper-scale residential
roofs are nailed with copper nails. Some older slate roofs are nailed
with square-cut iron nails. I have seen many a hot-dipped or cut-steel
nail that has been on a slate roof for 100 years and still in quite
serviceable condition. The exceptions are as mentioned above: poor
nails to begin with (not hot-dipped), or a roof on its last legs due
to slate deterioration.
The need for felt underlayment on slate roofs is another exaggerated
“urban myth,” so to speak. The most common underlayment
on older slate roofs is 30-lb. felt, used to prevent leaking during
installation. After about 75 years, the felt deteriorates almost to
a powder under the slates, but this is not a cause for concern. Many
slate roofs in the U.S. have been installed with no felt underlayment
whatsoever and they do not leak, even after a century. This is true
for virtually all barn roofs, where leaking during installation was
not a concern and so no felt was used when the roof was installed.
A felt underlayment is essential only during installation on a structure
where rainwater can damage the interior. It is very bad advice to
tell someone that they must replace a slate roof because its felt
has worn out, yet such recommendations are often given by roofing
contractors or consultants who don’t know what they’re talking
about.
Nowadays, the trend is to install slate roofs as if they are simply
asphalt roofs with slate on them — which they are not. Traditional,
tried and proven methods of slate roofing are being abandoned and
replaced by methods that cater to the convenience of the contractor
and/or architect. Consequently, architects are now specifying new
slate roofs with plywood roof decks and ice- and water-shield (to
preserve the plywood), as they do for asphalt roofs. Although slate
can be installed on plywood, you cannot expect a plywood roof deck
to last as long as a natural wood deck, which will easily endure 150
years, maybe much longer. I stayed in a house in Scotland last year
with a 215-year-old, original, 1-in. board roof deck, and, of course,
a slate roof in excellent condition. Natural roof decks do not need
ice- and water-shields, a fact that has been proven by countless century-old
slate roofs with natural wood decks and no shields. There is no acceptable
reason to downgrade proven, simple, natural, and fabulously successful
traditional slate-roofing methods. The trend toward membrane-covered
plywood decks under slate will create a whole new set of problems
for slate roofs in the future. When smart roofers and architects stick
with traditional roofing methods, they create for future generations
one of America’s most overlooked treasures: a beautiful, long-lasting
slate roof.
Joseph Jenkins has worked on slate roofs since 1968. He currently
owns and operates Jenkins Slate Roofing Services, providing contracting
and consulting services as well as slate-roofing tools and materials.
Author and publisher of “The Slate Roof Bible,” Jenkins
maintains a website at www.jenkinsslate.com, which includes a contractor
directory, instructions on repairing both slate and tile roofs, slate-roof
installation instructions, source lists for new and used slates, and
a message board, all free to the public; it is also a source of slate-roofing
tools, “The Slate Roof Bible,” and his other books. He can
be contacted at P.O. Box 607, Grove City, PA 16127; 814-786-8209 (phone
and fax).
