What
You Need to Know to
Keep a Slate Roof Alive
for Centuries
by Joseph Jenkins
Traditional
slate roofs are fabulously successful roofing systems that
can easily function as a waterproof covering on a building
for a century, 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) with wood boards or battens and metal fasteners
(nails). They are simple, low-tech roofing systems that
are beautiful to look at. When they do need to be replaced,
the slates can be discarded as clean fill as opposed
to the toxic waste of petrochemical roofs. As such, slate
roofs are sought after by those who are ecologically
minded.
Despite their reputation as the finest of roofs, 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 beautiful slate roof on a cathedral
in Arkansas that is 120 years old. The cost to install
this ornate 11,700 square foot roof, with a 220 foot
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.
FOUR
MAIN REASONS WHY OLDER SLATE ROOFS FAIL
1) TYPE OF SLATE
One
reason many older slate roofs fail is because of the type
of slate – some types wear
out sooner than others and once they wear out they can’t
be saved. 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, 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, an age that had
been reached, and it was showing a lot of delamination,
softening, and crumbling. The slates had served their useful
life and were now failing. The green slates, on the other
hand, 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, the roof would not have needed replaced
at this time. Yet, because half of the slates were a 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 of 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 in order to have it identified. Presently, in the
US, roofing slate is still being quarried in Virginia,
Pennsylvania, New York, and Vermont. However, a century
ago there were hundreds more American slate quarries
than there are today, including in Maine and Georgia.
The differences between the slates from the various quarries
were sometimes striking, so a knowledge of the history
of slate quarries is also important for people who work
with traditional slate roofs.
2) FLASHING FAILURES
Even
if a slate roof is composed of very long lasting slate,
the metal flashings can wear out and leak before the slate
wears out. These flashings are sheet metal joints that
are installed between the various planes of the roof in
order to prevent water entry, 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,
also 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 in association with institutions and upper-scale
residences, and often sheet lead flashings were used on
older buildings, especially around plumbing
vent pipes. 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 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 is routine work 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 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 are then put back into their original
positions and the repaired roof will look much the same
as it did before the repair, except with new flashings.
The sign of a good repair is one that is invisible to
the layperson.
3)
SLATES ARE BROKEN OR MISSING
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 a slate will
eventually break here and there on the roof. A typical
20 square roof (2,000 square feet), with a typical 10”X20” slate,
will have about 3400 slates. If fifty of them fail after
a century, then the failure rate of the roof is 1.5% per
100 years — phenomenally small. Yet, one missing slate
is all it takes to create a leak, which in turn may cause
someone to shout, “tear it off and replace it!” More
often than not, many people unknowingly lose their good, repairable, slate roof when they could easily have repaired
or restored it.
Faulty slates should simply be removed and replaced. Replacement
slates matching 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.
The nail and bib method
is perhaps the most widely used. This involves nailing the
replacement slate with a nail in the slot between the overlying
slates (see illustration below) and then sliding a “bib” flashing
under the overlying slates and over the nail head. The bib
is often bent slightly in order to friction fit it into place.
It can be composed of aluminum, copper, or other non-corrodible
metal. 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”X7”.
A slate hook is a hard
wire hook made of galvanized steel, copper, or stainless
steel, approximately three inches long. A small exposed loop
hooks the replacement slate in place (see illustration).
This is one instance when an exposed repair device is acceptable
as 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
in the field of the roof.
The tool required for removing slates from a roof is the slate
ripper – a sword-like object that slides up under
the slate and pulls out the two nails that 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,
leaving 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”) are readily cut with a simple
hand-held 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 — this 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, and that means a slate roofer
who will repair any slates he breaks during his moving about.
It is improper to tar or coat the surfaces of slate roofs,
or to use surface tar for repairs. Not only is this unsightly,
but it doesn’t stop leaks permanently and it ruins
the slates.
4)
ABUSE, BAD REPAIRS, AMATEUR WORK, NEGLECT
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,” the ones that
are face-nailed, tarred, repaired with non-matching slates,
coated, or reflashed incorrectly.
Furthermore, roofing contractors 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 roof owner will listen to bad advice when it is the
only advice that can be found. All 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.
ADDITIONAL
CONSIDERATIONS
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 often copied 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
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 it’s 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 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
so no felt was used when the roof was installed. These
roofs do not leak — felt or no felt. 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 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. This means that 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” board roof
deck, and, of course, a slate roof in excellent condition.
Natural roof decks do not need ice
membrane, a fact that has been proven by millions
of century old slate roofs with natural wood decks and
no ice membrane. There is no acceptable reason to downgrade
proven, simple, natural, and fabulously successful traditional
slate roofing methods, and the trend toward membrane-covered
plywood decks under slate is a trend that will create
a whole new set of problems for the slate roofs of the
future. When smart roofers and architects stick with
traditional roofing methods they create for our future
generations one of America’s most overlooked treasures — a
beautiful, long-lasting slate roof.
Author Bio:
Joseph Jenkins, in the preservation trades
since 1968, directs Joseph
Jenkins, Inc. in northwestern Pennsylvania that provides
national slate roof consulting
services, slate and tile roof restoration contracting,
slate roofing publications and
slate roofing tools
and supplies. He has personally worked on over a thousand
slate roofs, many with an average age of one century. Jenkins,
whose company also installs new slate roofs, authored and
self-published The
Slate Roof Bible, which has been recognized in four national
book award competitions
and presented with the National Roofing Contractors Association
Gold Circle Award. The research for this book involved travel
to Newfoundland, Quebec, Spain, Wales, England, Scotland,
Germany, France, Ireland, Italy, and throughout the eastern
United States. Jenkins has been a presenter on the topic
of slate roof restoration at the past eight annual International
Preservation Trades Workshops. He has also conducted slate
roofing presentations at the Natural Building Colloquium
in Maryland, for the Roof Consultants Institute, the Restoration
and Renovation Trade Show, and many other venues. Jenkins
was on the Board of Directors of the National Slate Association
(slateassociation.org) and founded the Slate
Roofing Contractors Association of North America (slateroofers.org).
He maintains a web site at slateroofcentral.com which
provides information on slate and tile roof repair, lists
industry contacts and sources of
materials, sells tools and maintains a message
board on slate, tile and asbestos roofing. His website
at josephjenkins.com provides
information about his other publications.
