about Slate as a Material
|1. Where does
slate come from?|
2. What is a
3. What is the
grading system for slate?
4. What colors
does slate come in?
5. Do slates
change color with age?
Questions about Slate Roofing
|1. How much does a
slate roof cost?|
2. Besides the
low life cycle cost, why else should one install a slate
3. How much does
4. Is there
special framing or structure needed for slate?
5. What is the
lowest pitch that the slate should go on?
6. How are slates
7. What is
8. Can a slate
roof that has been covered over by asphalt shingles be
9. Does a slate
roof need to be cleaned periodically?
10. Does a
slate roof need periodic maintenance?
11. Should a slate
roof be coated for the sake of longevity or to improve its
12. Are slates
available for repairing old slate roofs?
13. Can an old
slate roof be repaired without causing further damage?
14. Is a slate
about Slate as a Material|
|1. Where does slate come from?
Slate is mined all over the world, from the U.S. to Brazil to
China to India and throughout Europe as well as many places in
Africa. Even Australia has recently started to quarry slate. Most of
these areas potentially have good quality slate. The trick is
getting it produced. The question is whether or not the operators
know how to quarry and finish the slate to high quality roofing
slate. It often takes digging very deep down past the soft "top
rock" to get to good stone, and then producing the product is a
skill in itself.
|2. What is a ribbon slate?
Ribbon slates are Pennsylvania black slates that have small bands
running through them. These ribbons contain a high amount of carbon
that deteriorates much faster than the rest of the slate. This
results in the slate becoming separated at the ribbon, allowing
either the slates to fall out or water to penetrate the roof. Even
so, ribbon slate roofs have still been known to last 50 to even 100
years if the roof is steep and well shaded. Once the ribbons become
soft, the roof is virtually unrepairable and must be
|3. Is there a
grading system for slate?
It is very simple. S-1 means the slate will last more than 75
years. S-2 slate is meant to last 45 to 75 years and S-3 slates are
meant to last 45 years or less. However, slates of all three grades
generally do last longer than their required minimums.
|4. What colors does slate come in?
Gray, green, purple, black, and red. Sometimes there is a
mottling effect, mixing a couple of these colors together, the most
common being gray-black, gray-green, and green-purple.
|5. Do slates change color with
Yes. Weathering or semi-weathering green and gray slates can
develop brownish and pinkish tints, due to iron in the slate. At the
time of new installation, the percentage that will turn color is
unpredictable. It can be as little as 5 to 10 percent or nearly
100%. The only way to predict for certain what a roof will look like
(if one wishes to have a weathering slate) is to install a reclaimed
slate which has already undergone the weathering process.
Unfading slates also change colors to some degree. Some can turn
a deeper shade and some can become slightly washed out looking.
Weathering slates will change colors almost immediately after
installation and may continue to change for years, while unfading
slates change colors more gradually and subtly.
about Slate Roofing|
|1. How much does a slate roof cost?
A simple, straightforward slate roof starts out at a little under
$15 per square foot and can run more than $40 per square foot
depending on the type of slate used, the design of the roof, the
height of the building, the pitch of the roof, and so forth. Most
slate roofs run between $15 and $30 per square foot,
|2. Besides the low life cycle cost, why else
should one install a slate roof?
Aesthetics: Slate roofs come in a wide variety
of sizes, colors, and thicknesses. One can create many different
looks to enhance just about any architectural style.
Ecological considerations: Slate is a natural
product that comes out of the ground and can safely go back into the
ground. Petrochemical roofing made of asphalt and fiberglass
typically lasts twenty years. According to statistics, construction
and demolition debris make up 28% (by weight and volume) of the
refuse being placed in landfills. That is more than the waste from
styrofoam, fast-food packaging, disposable diapers, and all plastic
packaging combined! Approximately 20% of all
construction debris is roofing. Though not the only reason,
environmental concern is certainly a good reason for installing
Effective protection: A slate roof is the best
chance you have for getting a leak-free roof. Most leaks in roofs
occur in flat areas and areas with protrusions. Since slate roofs
are installed by craftsmen who are thinking in service life of
decades or even centuries, these details are usually installed with
great care. As a result, the quality of leak protection is much
higher. Thus slate roofs leak much less often than other kinds of
roofing -- especially on houses with complicated roof lines.
Slate roofs are one of the most fireproof roofs.
Slate roofs are much more wind resistant than asphalt roofs.
Slate roofs also do not rely on the underlying roofing felt to
remain leak proof the way many clay and concrete tile roofs do.
Long-term Reliability: Slate has been the
material of choice for a large percentage of the most important
buildings built in the last five centuries. The method of
installation has changed little over five centuries. New roofing
products, such as many of the simulated slates, have come and gone
in less time than asphalt roofs last. Some of these products have
been sold with 50-year warranties, which became worthless when the
businesses went under.
|3. How much does slate weigh?
Standard thickness slate weighs approximately 800 to 1,000 pounds
per square installed. (A square is a 10 foot by 10 foot area.) 3/8”
slate weighs approximately 1,500 pounds per square. Every 1/8” of
additional thickness increases the weight by approximately 500
pounds per square.
|4. Is there special framing or structure needed for
A good solid deck with 16” on center framing is preferred. Most
structures are built to handle wind and snow loads that far exceed
the weight of slate.
However, a weak structure that has too much flex to it will
result in a poorly installed slate roof. Ideally, one would use a ¾”
thick tongue and groove deck on 2” x 12” rafters 16” on center.
However, this is an expensive deck, and would be a rare sight today.
Otherwise, it would be best to have at least 5/8” plywood or ¾”
plywood over 16” on center trusses built to handle slate.
|5. What is the lowest pitch that slate should go
We prefer not to put slate roofs on pitches less than 6/12. We
will consider putting on slate roofs that are 4/12 and 5/12 if we
double felt (30#) and run a 4” headlap on the slate.
|6. How are slates cut?
Slates are cut using a slate cutter or the sharp edge of a slate
hammer. One does not usually use a saw, as a saw will not provide
the attractive beveled edge characteristic of roofing
|7. What is flashing?
Flashing is the sheetmetal that is used at protrusions and roof
transitions due to the fact that slate cannot be bent. It keeps a
roof watertight in these areas. The preferred choices for flashing
are copper, lead-coated copper, and terne-coated stainless steel.
Aluminum should not be used, as it cannot be soldered.
|8. Can a slate roof that has been covered over by
asphalt shingles be salvaged?
No. When asphalt shingles are applied over slate, the nails are
driven through the slates. This method leaves holes in the slate if
the nails and shingles are removed. New slate must be applied to
ensure a watertight roof.
|9. Does a slate roof need to be cleaned
|10. Does a slate roof need periodic
Yes. Even new slate roofs will shed a few slates. It is a good
idea to visually inspect a slate roof at least once a year. The
owner of the property can do this annual inspection. Any slates that
are seen to be missing, sliding, or broken should be replaced. Every
few years, it would be a good idea to have a more careful inspection
from a slate roof expert.
|11. Should a slate roof be coated for the sake of
longevity or to improve its appearance?
A slate roof should not be coated with anything. Coatings will
generally accelerate deterioration of slate.
|12. Are slates available for repairing old slate
Yes. Many contractors have inventories of reclaimed slate with
which to repair old slate roofs. Old slate roofs often have
weathered to a different color than new slate of the same type.
Also, many old slates are thinner than the new slates that are being
quarried today. That is why reclaimed slates are often the best
choice for repairing an existing slate roof.
|13. Can an old slate roof be repaired without causing
Most slate roofs are actually very durable and can withstand the
necessary foot traffic of workmen. Hook ladders and roof planks can
be used to distribute the weight of workmen, and they do help.
However, a few broken slates can be expected and are easily repaired
in the process of repairing the existing roof. If the work is done
carefully by trained workmen, a slate roof will not be ruined by
craftsmen replacing broken slates and damaged flashings.
|14. Is a slate roof fireproof?
Slate itself is fireproof. A slate roof is one of the most fire
resistant roofs that exist. However, the wood deck under the slate
is not fireproof. Often fires will jump from house to house as
sparks hit adjacent roofs, igniting the surface if it is a flammable
material. Many of the raging fires that engulf hundreds of houses
every year would not occur if the houses were covered in slate.