ROOF PROFESSIONALS NEED TO KNOW ABOUT SLATE ROOFS
as a PDF
by Joseph Jenkins
roofs are roofs made of rock. The rock is mined or quarried
from holes in the ground, pits, tunnels, and deep shafts.
Its hand-worked into manageable sizes, then split
into roofing shingles with hammers and chisels. The finished
shingles are punched for nail holes and the thin slabs
of stone are fastened to the roof deck with nails. A
typical slate roof will last at least a century and maybe
two. Commercial roof slate quarrying began in the US
around the mid 1800s. Look at some video clips of slate quarries.
Above: Camara Slate Quarry, Vermont (left); Shawn Camara
hand splitting a slate shingle from a block of rock
(center); Brent Ulisky installing the slate shingles
on a roof (right).
There are two general categories of slate roofs upon which
a roof professional will be called to provide consultation
services. They are 1) older slate roofs, and 2)
new slate roofs. By far the most common slate roof
in the US and in the world is an older slate roof,
as such roofs can easily last a century or more in
good working order. Therefore, the bulk of this article
will focus on older slate roofs. However, roof professionals
are also called upon to provide advice when replacing
or installing a slate roof. This issue is addressed
later in this article.
slate roofing systems are fabulously successful systems
that can easily function as a waterproof covering for
100 years, and, if properly constructed, for 150 years,
or even 200 years. Some slate roofs in Europe are still
in good functioning condition after 400 years. Their
longevity, however, is not their only quality. Slate
roofs are made of natural materials primarily
stone (slate) and wood (boards or lath/battens), with
metal fasteners (nails). They are simple roofing systems,
and theyre beautiful to look at. When they do need
to be replaced, they can be discarded as clean fill,
as opposed to the toxic waste of petrochemical roofs.
As such, they are roofs that are sought after by those
who have ecological concerns. Finally, slate roofs are
arguably the least expensive roof money can buy when
the entire life of the roof is taken into consideration.
I recently looked at a slate roof on a cathedral in Arkansas
that is 120 years old (picture below). The cost to install
this 117 square, ornate roof, which has a 220 foot high
spire, was $765.00 for the labor and $1,166.50 for the
materials in 1881. Even adjusted for inflation, it should
be obvious that this was money well spent.
MAIN REASONS WHY OLDER SLATE ROOFS FAIL
1) TYPE OF SLATE
primary reason older slate roofs fail is because of the
type of slate some wear out sooner than others.
There are many types of roofing slate and they each have
their own particular qualities and idiosyncrasies. On
the 120 year old cathedral roof mentioned above, for
example, the slate was installed with an ornate pattern
of black and green slate. The green slate originated
in Vermont, while the black slate originated in Pennsylvania.
The black slate had a life expectancy of about 120 years
and was showing a lot of delamination and crumbling.
The slates had served their useful life and were now
failing. The green slates, on the other hand, showed
no deterioration after 120 years. Its anyones
guess how much longer they would last. If the entire
roof had been installed with the green slate, the roof
would not have needed replaced at this time.
of St. Andrew, Little Rock, Arkansas, showing two
types of slate: PA black and VT unfading green. The
photo below, incidentally, shows the inside of the
steeple, looking up.
So it is imperative that roof professionals who deal with
slate roofs know the different types
of slate, their origins, longevities, characteristics,
and qualities, and be able to identify them by sight.
If sight identification is not possible, then they
must be able to send a slate sample somewhere to have
it identified. Presently, in the US, roof slate is still
being quarried in Virginia, Pennsylvania, New York,
and Vermont. However, a century ago there were hundreds
more slate quarries than there are today, including
in Maine, and the differences between the slates from
the various quarries were sometimes phenomenal. So
a knowledge of the history of slate quarries is also
important for roof professionals who work with older
In addition, it must be pointed out that roof slate has
been quarried and/or mined in Europe for centuries.
Wales has perhaps some of the oldest slate quarries
in the world. Spain is now the largest producer of
roofing slate in the world, and China has entered the
market in a big way. Slate is also produced in South
Africa, India, central Europe, Italy, and elsewhere
throughout the world. These foreign slates are now
entering the American market and being used on new
PA black slate, 120 years old, on cathedral in Little
black and VT unfading green, 120 years old, on cathedral.
Black is worn out, green is still good (above)
unfading green slate on 120 year old house. Slate
is quite good (above)
black slate on house in NJ. Slate is nearly worn
out at 80 years old (above)
if a slate roof is composed of very long lasting slate
material, the metal flashings will wear out and leak
before the slate wears out. These flashings are metal
joints that are installed between the various planes
of the roof in order to prevent water entry. They are
also installed around roof penetrations such as chimneys.
The older flashings were usually made from terne coated
steel (steel coated with a lead/tin combination, also
know colloquially as "tin"), which had to be
painted regularly to avoid corrosion. Copper
flashings were also used, primarily in association
with institutions and upper-scale residences. The terne
flashings could last 90 years or longer if they were
kept painted. Copper flashings, ironically, because they
are typically not painted, will begin to corrode, pit,
and leak in about 60 to 70 years in areas of high wear,
such as valleys. For this reason, older copper flashings
should be painted in order to extend their effective
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 is routine work for slate
roof restoration professionals. One of the extraordinary
characteristics of slate roofs is that they can 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 the
need to replace 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 from the roof in order to expose the existing
flashings, which can then be removed and replaced.
The removed slates can then be put back into their
original positions, and the repaired roof will look
much the same as it did before the repair. Faulty slates
are also removed in order to replace them, and matching
slates must be used whenever possible. The tool required
for removing slates from a roof is the slate
ripper a sword-like object that slides up
under the slate to be removed and pulls out the two
nails that hold it in place. A slate
hammer has a hole punch at one end, used to punch
holes in slates for nailing. Slate hammers also often
have shanks designed to cut slates, which is done by
a chopping motion against a straight edge such as a slaters
stake. Slates readily punch without breaking, leaving
a clean hole with a "countersunk" characteristic
into which the nail head sits. Slates are also readily
cut with a simple hand-held device, a slate
cutter. Slate roofing contractors 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 improper
to work on slate roofs by walking on them using ropes.
It is also improper to tar the surfaces of slate roofs.
Slate is an excellent material to work with, having
extraordinary physical characteristics. It does not
shatter like glass, as many people believe.
flashing repair before, during, and after:
replacement on badly abused residential slate roof
the valley looks completely shot and is heavily tarred
(most inexperienced roofers would tear this roof
off even though it is still highly restorable).
the old metal is removed, as are the tarred slates.
New copper is installed.
The finished valley job leaves the roof looking as
good as new - a one day job for one man.
Some of the tools and equipment that are commonly
used during slate roof restoration.
are additional nuances involved with the assessment of
older slate roofs, such as type of nails,
underlayment, roof sheathing or battens, slope, and previous
maintenance. These factors will influence the longevity
of a slate roof, more or less.
Low slope slate roofs will fail prematurely because people
will walk on them over the years and break the slates.
The resultant leaks are often repaired by non-professionals
because the roofs are low in slope and therefore accessible.
These repairs tend to be done poorly; the roof will
still leak, resulting in more traffic on the roof,
and a downward spiral of deterioration begins, ending
with the demise of the slate roof. The lowest slope
advisable for a slate roof is 4:12. However, the slope
should be too steep to walk on in order for the roof
to last a long time. That would bring the slope up
to about 8:12 or steeper.
Nails are often said
to be the cause of slate roof failure, however, this
is often not the case at all. It is true that nails
will corrode on an older slate roof, but this 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, thereby 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 US, 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
one inch thick) or into horizontal wooden strips (slating
lath or battens, usually one by twos or threes), depending
on the predilection of the installer. Lath roofs are
common in Wales, England and Europe, so immigrants
from those countries may have chosen to copy their
traditional styles of slate installation once they
arrived here in the US at the turn of the last century.
Traditional Scottish roofs use solid boarding, as is
more common in the US. Most of the older slate roofs
in the US are nailed with hot-dipped galvanized roofing
nails, although most institutional and upper-scale
residential roofs are nailed with copper
roofing nails. Some older slate roofs are nailed with square-cut
iron nails. I have examined many a hot-dipped galvanized roofing nail that has been on a slate roof for 100 years and many
have been 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.
roof (above) showing lath roof decking. 100 year
old white oak lath is still in good condition despite
the totally deteriorated valley flashing. Lath roof
(below) being sheathed over with 1x10 green hemlock
lumber prior to reslating.
The need for felt underlayment on slate roofs is another "urban
myth," so to speak. The most common underlayment on
older slate roofs is 30 pound felt. It is used in order
to prevent leaking during installation. After about 75
years, the felt deteriorates almost to a powder under the
slates. This is not a cause for concern. Many slate roofs
in the US 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. The felt underlayment
is only essential during installation on a structure where
rain water can damage the interior. It is very bad advice
to tell someone that they must replace their slate roof
because the felt has worn out, although this sort of advice
is often given by roofing contractors or professionals
who dont know what theyre talking about.
Finally, one of the most common causes of leaks in older
slate roofs is, unfortunately, improper
repairs. Bad repairs will still leak, they look
ugly, and they can be very costly because the roof
owner usually has to pay for them. For example, don't use strap hangers. These factors combined
can make a roof owner, in frustration, want to forever
remove his slate roof no matter how much longer it
will last if properly repaired. In addition, roofing
contractors, as well as professionals, who have little
or no expertise in slate roofs, will advise a roof
owner to replace a slate roof which may have many decades
of life remaining. A client will listen to bad advice
when it is the only advice that can be found. Watch slate roof repair videos.
bad repairs (above) wrong type and shape of
replacement slate, face nailed through overlying
slates ugly, and a sure leak through face
Exposed nail in slot is used to fasten replacement
slate, then caulked another sure leak (should
have used a bib flashing to cover the nail head).
An exposed nail in the slot without caulk or bib
flashing becomes a hidden leak because
the nail head rusts away and the remaining hole is
almost invisible. This is not uncommon on old slate
roofs, but few people can spot this leak. Always
use either a bib flashing or
replacing a slate (see below).
professionals will be called upon to advise on the installation
of new slate roofs. This is where the tar will hit the
fan in some modern roofing circles. There exists today
such a wide gap between natural, traditional roofing
techniques and modern, chemically based roofing techniques,
that most roofing contractors and architects today are
experienced only in the latter. They therefore specify
slate roofs as if they are just a variation of an asphalt
shingle roof, which they are not. Slate roofs are expected
to last a century or two, and the materials used in conjunction
with the slate should have that degree of longevity,
at least. Asphalt and chemically derived roofing has
an average life expectancy of 10-20 years. Therefore,
asphalt roofing systems should never be used as a pattern
for slate roof installation.
The common steep-slope asphalt roof system of today employs
a plywood or other laminated or glued wood product
deck, often covered with a self-adhesive
underlayment. A properly installed slate roof,
however, will avoid any laminated or glued wood product
for decking, as these materials have no record of the
longevity required for a slate roofing system, and
in fact, do have a record of delamination and premature
failure, especially under adverse climate conditions.
Furthermore, ice and water barrier is a product designed
to prolong the life of laminated wood decking, and
therefore has no real purpose on a properly constructed
slate roof. Read about cancer and ice shield. Read how to install ice dam protected eaves.
A properly constructed slate roof, built according to traditional
American techniques, will have a board roof deck either
one inch thick rough sawn lumber, or ¾" planed,
kiln-dried lumber, or 1 ½" planed, kiln
dried lumber (more common on institutions). Tongue-in-groove
lumber is also used. The nails used on new slate roofs
should be either copper or stainless
steel, although hot dipped galvanized roofing nails will
last a century or much longer. Aluminum nails and electro-galvanized
nails should be avoided. It should go without saying
that the design of the roof itself adds much to its
effectiveness and longevity. Low slope slate roofs
(less than 4:12), for example, should be avoided. For
best results, slate roofs should be designed so theyre
too steep to walk on. Also, non-corrosive flashings
should be used, such as terne-coated stainless steel,
or copper. The flashing
can be eliminated altogether, in some cases, by designing
the roof with rounded (slated) valleys and slate hips
and ridges, as is more common in Europe.
fully sheathed board roof deck with a temporary 30
lb. felt underlayment is a very successful tried-and-proven
roofing system used with slate roofs, and should
be used with most new slate roofs for best results.
The felt is optional no underlayment is needed
for the proper functioning of the roof. The valley replacement
on a 104 year old church shown above reveals the
1 rough-sawn hemlock board decking and the
complete absence of underlayment. The slate is PA
black, still in good condition. No water can penetrate
the slate/copper roof system, which is why the underlayment
is optional and is used primarily as a temporary roof covering
until the slate is installed..
Roof ventilation is a big issue today in the roofing industry,
and it should be. Asphalt roofing shingles nailed or
stapled onto plywood decks are roofs systems that will
not breathe they suffocate the roof, and proper
ventilation is imperative. Otherwise the plywood will
delaminate and the roof will fall apart. Slate roofs
attached to board decking, on the other hand, are breathable
roof systems. They are not air-tight theyre
water tight. Ventilation may be necessary to prevent
condensation occurring under the roof sheathing from
warm inside air leaking into the roof space. This is
easily achieved with gable vents or roof vents, air
spaces between the insulation and the sheathing, and
ventilated soffits. However, most older residential
slate roofs had no particular ventilation systems associated
with them. After a century, most are still in good
working order. Such is the advantage of a roof that
can breathe on its own. When these older slate roofs
are retrofitted with roof insulation, care must be
made to ensure that warm air will not come in contact
with cold roof sheathing. Aluminum ridge vents should
be avoided on slate roofs. Such vents are designed
to be used with asphalt shingle roofs as such,
theyre cheaply made, they do not fit the ridge
of a slate roof well, and they interfere with the normal
maintenance of a slate roof.
New slate roofs are only as good as the installation. Contractors
who are skilled at asphalt roofing are not necessarily
qualified to install slate. For example, many new slate
roofs today are having serious
problems such as scores of slates falling out within
the first ten years after installation. This is caused
by the installers walking on the slates during installation a
common practice among asphalt shinglers on asphalt
roofs, but a mistake on a slate roof. Furthermore,
roofing slates are overlapped in such a manner that
each course overlaps two courses below. This particular
overlap is called the headlap, and three inches is
standard. If a proper headlap is not used, the roof
can have serious problems later. Over-nailing
and under-nailing of the slates can also be a problem.
Over-nailed slates are nailed too tightly and the nail
can break through the slate, or the slate can crack
under the stress. Under-nailed slates are not nailed
tightly enough and the nail-head will rub against the
overlying slate, eventually wearing a hole through
it. Poorly installed slate roofs are unfortunately
and unfairly beginning to give slate a bad name. Asphalt
shingle roofers make their money in roof replacement.
Perhaps they have a vested interest in installing a
roof that will fail prematurely. The installation of
slate roofs is an art in itself, and, when mastered,
it creates a roof to be proud of a work of practical
beauty that can benefit many generations.
One major concern today is the inadvertent selection of
bad slates for roofing material, often from foreign
sources. There are bad slates available on the market slates
that will fail within as little as ten years. The best
way to determine the quality of a certain type of slate
is to do a historical analysis. Purple Vermont slates,
for example have been historically proven to easily
last 150 years. PA black slates may last 100 or more
years. Chinese or Spanish slates of an unknown origin
are a total gamble, some failing in as little as one
year. Any roof professional who specifies or encourages
the use of a slate material of unknown origin is asking
for a nightmare. Sources of
new roofing slate.
Much more information about all aspects of slate roofs
is available in the Slate
Roof Bible. Slateroofcentral.com,
and Traditionalroofing.com also
provide an abundance of information about slate roofs.
SLATE ROOF CONSULTING HIGHLIGHTS
was called upon to inspect the roof of the Ford Theater
in downtown Washington DC (below) in 1998 the
site where Lincoln had been assassinated. The roof was
leaking badly in one spot and not so badly in another.
The National Park Service, responsible for the maintenance
of the building, was considering replacing the roof.
There was good access to the roof from an adjacent building
which had a flat roof. You could stand immediately
beside the slate roof and have the drip edge of the
slate right at waist height. The slates looked old,
and, in fact, they were the original 1865 Buckingham,
Virginia slates, installed just prior to Lincolns
death (he visited the theater shortly after it had
been constructed). Surely this 133 year old roof was
a candidate for a re-roofing project.
To make a long story short, after inspecting the roof it
was obvious to me that the roof had already been replaced.
The original slates had been removed about 25 years
earlier, the good slates salvaged, and then re-installed
with copper roofing nails on the side that could be viewed
from the neighboring flat roof. The old slates now
covered an area which comprised about 1/5 of the total
roof. The entire remainder of the roof had been re-slated
with new Buckingham, Virginia slates. Essentially,
the Ford Theater already had a new roof, a fact not
visible from the flat roof below.
A soldered seam on a copper coping of one parapet wall had
split right where water pooled on the parapet. This
was the cause of the primary leak. A metal flashing
between the flat roof next door and the theater building
had pulled away from the building. This was the cause
of the secondary leak. The slate roof did not leak
at all! No one at the National Park Service had been
employed there for more than about 20 years, so no
one remembered when the roof had been replaced, and
there were no records of it either. My visit saved
the federal government a few hundred thousand dollars.
In 2000, I was called upon to look at an up-scale estate
in the New York City suburbs (see below). Some slates
had been found on the ground alongside the mansion
after the winter ice had thawed, and the owners were
concerned. A slate roofing "expert," a "third
generation slater," was called in to inspect the
roof. He subsequently advised that the entire slate
roof needed replaced to the tune of $450,000.
This seemed odd to me, especially considering that
the slate roof was only 12 years old. When I went to
look at the roof, I discovered that the snow guards the
metal fixtures on the eaves that hook under the slates
and protrude up off the roof surface to prevent snow
and ice avalanches had pulled out under the
weight of an excessive snow load. When the snow guards let loose, they pulled some slates off with them and
they all ended up on the ground. The snow guards failed
because there were not enough of them for the size
and steepness of the roof. This had been a particularly
bad winter for ice and snow, and the sparse snow guards couldnt hold the load.
The entire problem was repairable by replacing the missing
slates and doubling the number of snow guards, or leaving
them off completely. And it was all covered by homeowners
insurance! The lesson to be learned here is that just
because someone calls himself an expert on slate, that
doesnt mean hes not a shyster. Always get
a second opinion in a situation of this magnitude.
In this case, it save the prudent owners nearly half
a million dollars!
roof (with Buckingham slate).
It is very common to run into situations where perfectly
good slate roofs have been condemned as beyond repair
by either roofing contractors or roof professionals.
Usually the contractors are trying to land a re-roof
job, or the professionals are selling materials or
getting a commission on something. However, a properly
informed, honest roof professional can save a client
a lot of money as well as preserve one of Americas
most overlooked treasures slate roofs.