green slate date on a Vermont purple slate background.
Slate roofs are the world’s finest roofs.
They’re fireproof, waterproof, natural, will last centuries,
and they have a track record that goes back thousands of
years and spans the entire Earth. They are beautiful, simple
roofs made of rock on wood — ingenious, effective,
and fabulously successful. Yet, in the United States, they
have been under intense and relentless attack for decades,
destroyed by the thousands, cast away and forgotten. The
culprit, in one word, is ignorance — in two words:
roofing contractors. Slate roof owners are now plagued with
two problems: roofing contractors who do not understand slate
roofing systems, or no slate roofing contractors at all.
If a slate roof is in need of repair, maintenance or restoration,
what’s a roof owner to do?
The best way for someone to protect a slate roof is to first
become informed. It is very common for slate roof owners
to face a continuing onslaught of roofing contractors wanting
to tear off their slate roofs. The author of this article,
a slate roof restoration professional
has seen time and time again when a perfectly good slate
roof has been condemned by not one, but several roofing
contractors, who have informed the owner that the roof
must be replaced. These are good, sound, slate roofs that
have fifty or seventy-five years of life remaining, maybe
more. The number-one threat to slate roofs on vintage homes
today is poorly informed, inexperienced roofing contractors
looking for a house on which to staple cheap, asphalt shingles.
In effect, roofing contractors are the predators and slate
roof owners are the prey. Owners can best protect themselves
and their slate roofs by knowing more about the material
that is covering their houses.
slate (above) with a Vermont unfading green slate date, located
near Fair Haven, VT.
This article will, in a nutshell, cover the important points
needed for a basic understanding of slate roofs and how
they work. A good slate roof can easily last 150 years
and maybe a lot longer. Examples of such roofs on simple
buildings as well as high-class homes dot the New England
countryside and are scattered across
the United States and around the world.
The four basic elements of a slate roof include: 1) The slate
itself. This should be a hard variety with a reputation
for longevity. 2) The roof deck. This should be a board
deck (not laminated wood) at least 3/4” in thickness.
3) The flashings (the sheet-metal joints between roof planes).
These should be copper or stainless steel, although the
old “tin” flashings will last quite a long
time if kept painted and are easily replaced when they
wear out. 4) The nails — they
should be copper or stainless steel, but cut steel nails
and hot-dipped galvanized steel nails will easily last
a century or two on a well-maintained slate roof. These
four elements make up a basic American slate roof — stone
slabs attached with nails to a properly sloped wood roof
deck and flashed where necessary with appropriate sheet
metal. Such a roofing system is very simple, yet amazingly
durable and long-lasting. What goes wrong with these roofs?
Slate is stone. It is pulled from the earth in heavy slabs
and worked down into individual shingles, largely by hand.
The fact that slate roofs are rock roofs is the basic reason
why they last so long. However, stone is a natural material
and may have invisible fractures or other imperfections that
are not noticeable when the roof is installed. These can
cause a slate or two to eventually break and come off the
roof. People walking on slate roofs also damage them. This
includes the roofing contractors who install or repair the
slate. A “bigfoot” roofing contractor will crack
slates which can later, after a freeze/thaw cycle or other
environmental pressure, break apart. Broken slates can easily
be replaced, however, as we will discuss below.
Some slate varieties are softer than others and do not last
as long. Such slates will become flaky, and crumbly when
they reach the end of their effective lives, which could
be as soon as 55 years, but more likely around 80-100 years.
These “soft slate” roofs cannot be saved or
restored, but can easily be replaced with new or salvaged
slates. Harder slates, such as most Vermont slates, the
Peach Bottom slates, Buckingham slates, Monson slates,
and others, could conceivable last centuries on a properly
maintained roof. Click here to
see photos of different slate roofs.
Flashings eventually wear out. These are the metal joints
around chimneys, in valleys, alongside dormers, around
pipes that protrude through the roof, etc. They are also
easily replaced by experienced slate roof restoration professionals.
Finally, bad workmanship,
such as improper repairwork, plague many an old slate roof.
This is visible as tar splotches on roofs, mis-matched slates,
metal patchwork, coated roofs, and other mistakes. Once again,
these errors can usually be removed and properly replaced
by someone who knows what he is doing. Coated slate roofs
cannot be remedied, however, and such a practice should be
Figure 3: Standard slate
installation. All slates are the same length and width.
What you see is only the bottom part of each slate — most
of the slate shingle is overlapped by the slates above.
In fact, each slate is overlapped by the second row
above it typically by three inches. This critical overlap is
called the “headlap.”
For a better understanding of how slate roofs work, let’s
take a look at how a slate roof goes together. First, there
is the “standard installation,” which is the
most common and basic way to install slate in the United
States (Figures 3 & 4). Each slate is the same length
and width. Each slate is fastened to the roof with two nails
along a chalk line that marks the top edge of the slate.
Note that each slate overlaps two courses below it. This
is called the “headlap” and is usually three
inches. The headlap is one element of a slate roof that is
essential. Slate sizes, widths, lengths, colors, shapes,
and thicknesses can all vary, but the headlap is a constant
that must be maintained. A minimum three-inch headlap is
standard, but greater headlap is acceptable. Less headlap
can lead to leakage, depending on the slope of the roof (however,
two inch headlap is common on older roofs with adequate slope).
Figure 4: Standard slate installation
showing placement of nails and overlapping of slates.
A simple variation of the standard installation is the “random
width” roof, which utilizes slates of various widths
rather than just the uniform width of the standard installation
(Figure 5). In this case, the “sidelaps,” as
well as the headlaps, are critical. Sidelaps are the lateral
spacing of the side-butts of each slate in relation to the
course above or below (Figure 5A). The sidelaps, like the headlaps, should be a minimum of three inches.
Figure 5 (above): Random width
Figure 5A: Sidelaps.
Slates can vary in both width and length. This is called
a “graduated” roof because the slates graduate
from larger slates at the bottom of the roof to smaller
slates at the top (Figure 6). Again, proper sidelaps and
headlaps must be maintained for the successful functioning
of graduated roofs.
A random width installation can be modified into a “staggered
butt” style, which utilizes slates of two or more lengths,
fastened to the roof along chalk lines that mark the top
of the slate as in the standard installation, but allowing
the extra lengths of the slates to hang down, creating a
unique look. This can be further modified by mixing slates
of different colors and even different thicknesses to fashion
a roof that is a work of art (Figure 7).
Figure 7: Staggered butt
slate roof. Slates vary in widths, and lengths, and,
in this case, colors and thicknesses. The top of the
slates are lined up but the bottom is allowed to hang
In this case, the longer slates simply create
a longer headlap.At no time is the three inch minimum headlap
Finally, the staggered butt style of slating
can be further modified into a “ragged butt” style,
which is simply done by cutting the exposed bottom edges
of the slates randomly to create an inimitable roof (Figure
8). All of these slating styles are variations of the standard
installation; all maintain correct minimum headlaps and sidelaps;
all are leakproof if properly maintained (with or without
underlayment), and all will last as long as the slate itself
lasts, providing an Act of God, or a roofing contractor,
does not destroy the roof first.
Figure 8: Ragged butt
slate roof. Slates vary in widths, and lengths, and,
in this case, colors. The top of the slates are lined
up, but the bottom is allowed to hang down like a staggered
butt roof, but the bottom edges are trimmed at random
to create a unique look.
When a slate breaks after being installed on the roof, how
is it replaced? This is easily done in the same manner
in all cases: either with the “nail and bib” technique,
or the “slate hook” technique.
First, the broken slate must be removed. This is handily
accomplished by using a slate
ripper to hook the nails that fasten the slate to the
roof. The hooked nails are then pulled out and the offending
slate comes out with them (Figure 9).
Figure 9 (above): Using a slate ripper
to remove a broken slate.
A matching replacement slate is then slid into place and
fastened with an appropriate roofing nail in the overlying
slot, then the nail is covered by a bib
flashing (Figure 10 below).
Figure 10: Nail and bib repair
An alternative to this is to use a slate
hook to hold the replacement slate in place (see above). Exposed strap hangers should not be used when replacing
slates (Figure 12). These are unsightly and will open up, flatten,
and allow the replacement slate to slip out if ice or snow
should slide down the roof.
Figure 12: Do not use exposed
The removal and replacement of slates on the roof is also
necessary when replacing flashings on the roof. When flashing
metal deteriorates and must be replaced, the slates that
cover the flashing must be removed in order to allow the
old flashing to be taken out. Once the old metal is completely
out, new metal can be installed, after which the slates
are replaced in their original locations. When the job
is completed, the roof should look like the original roof,
not like a repaired roof. This is routine work for a slate
roof restoration professional.
repairs (see video above) require replacement
slates that match the roof. This is one reason why many
standard roofing contractors are not competent at the repair
or restoration of slate roofs. Most roofing contractors
today make their living by re-roofing, usually with a short-lived
asphalt material such as fiberglass shingles. A good slater
will stockpile a variety of salvaged slates — all
different sizes, shapes and colors, or he will know where
to get what he needs, perhaps from a roof slate salvage
yard. Sources of both new and salvaged roof
slates and roof tiles are listed, for example, on the Slate Roofing Contractors Association
Myths and Misinformation:
1) Your underlayment (felt paper underneath
the slate) has worn out and therefore you have to replace
your roof. Wrong. Felt paper is installed when the slate
is installed in order to keep rain out of the building until
the roof has been completed. Once completed, the felt paper,
now peppered with holes from the roofing nails, serves no
useful purpose other than as a temporary semi-waterproof
layer that will eventually turn to dust. Many century old
slate roofs never had any felt paper installed under them
whatsoever and are still functioning quite well, and leakproof,
today. It is the slate and the flashings, not the underlayment,
that render the roof leakproof. However, you will find that
many of today’s roofing contractors of the asphalt
variety (as well as architects) will argue until they are
blue in the face that the underlayment is the most important
part of the roof.
2) Your slate nails were not copper and
therefore you have to replace your roof. Wrong again. Most
older slate roofs were installed with either hot-dipped galvanized
roofing nails or cut steel nails. On a hard-slate roof, these nails
will still be quite good after a century. On a deteriorating
soft-slate roof, which allows some moisture to penetrate
through the slates to the nails, the nails will be much more
decayed. Soft-slate roofs will need to be replaced anyway.
The roofs made of harder slates will not, regardless of the
type of nail. In rare cases, entirely wrong nails were used
in the first place to install the slate roof. This was the
act of an unscrupulous roofing contractor and there is no
remedy for it other than to replace the roof.
3) Roof slate is an obsolete material and cannot be bought
today. Very wrong. Roof slate is still quarried in Pennsylvania,
Virginia, Vermont and New York as well as around the world
(and imported into the US). It is readily available
new in any size, shape or color desired. There are
huge deposits of it resting in the Earth — deposits
that will never be depleted. There are also companies that
specialize in salvaging older roofing slates. Salvaged
slates can be bought to match just about any existing
4) Nobody repairs slate roofs anymore and competent slaters
cannot be found. There is, unfortunately, some truth to
this. There are many “pretenders” who
will declare themselves slate roofing experts when, in
fact, they are simply blowing it out their wazoo. There
are also many predatory roofing contractors who know that
there is a dearth of competent slaters and they will lunge
upon your house with dollar signs in their eyes knowing
that you are over a barrel. In fact, there are many competent
slate roofing professionals scattered throughout the country
and they are increasing in numbers as more training, information,
tools, equipment and networking become available. There
is now a new professional association,
the Slate Roofing
Contractors Association of North America, which is listing screened, experienced professional slate
roofing contractors on the internet.
If a slate roof owner is armed with the proper knowledge,
understanding and information, she will best be able to
protect and preserve the stone roof covering her home.
With the help of a competent slate roof restoration professional,
there is hardly a slate roofing problem that cannot be
understood and solved.
Joseph Jenkins, in the preservation trades
since 1968, directs Joseph
Jenkins, Inc. in northwestern Pennsylvania that provides
national slate roof consulting
services, 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 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
around the world. Jenkins has been a presenter on the topic
of slate roof restoration at many 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).
Jenkins also 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 SlateExperts.com provides more