Apart from the
additional cost, are there technical reasons why slate should
not be installed over IWS? I understand about the role of
felt for temporary covering during installation, but in chronic
ice damming climate (where I work) where the usual design
solutions cannot be used on old construction, we apply IWS.
We do not use it as a "prophylactic" for bad craftsmanship!
B.B., Grand Rapids,
MI
Regarding your
article in Fall 2001 "Traditional Roofing" entitled "Why
Slate Roofs Don't Need Ice and Water Membrane", may
I respectfully object? I lived...under a slate roof [that]
leaked in several places. We were able to trace the problems
to some of the valleys.[S]late roofers look[ed] at the roof,
and ...were generally of the opinion that the only way to
completely stop the leaks would be to remove and rebuild
most of the roof slating, widening the flashing. This proved
too costly, and we lived with the leaks. I have kept this
experience in mind during my practice as an architect, and
have always paid close attention to roofing details. I love
slate roofs, and try to talk clients into them whenever possible.
However, I have always been concerned that the nature of
the slate material creates relatively large open channels
between the slates, with the potential to allow water to
be driven up under windy storm conditions. This would probably
not cause leakage in the field of the roof, but at valleys,
hips and ridges, where there is less overlapping coverage,
it seems likely that water can be driven above the top course
of slate and onto the substrate. There is also the whole
concern about ice damming at eaves... Over the years, use
of such membranes has virtually eliminated leakage problems
with roofs on buildings I've been involved with. [Y]ou seem
to be in denial about people's experience with slate roofs.
Unfortunately, I have heard too many people object to my
proposal to use a slate roof on their house because their
experience of slate roofs is that they leak! If I want to
put slate roofs on my buildings, I need to be able to assure
my clients that they will not leak, because modern materials,
like the membranes to which you object, will make such a
thing possible. M.W.
My family has been
in the slate roofing business since my great grandfather
immigrated here from Ireland in the early 1880s. He was
trained in England first as an apprentice and then a roofer.
My grandfather started our slate roofing company in the 1920s;
we have been in operation ever since.
I just spoke to my uncle that is now running our company and
discussed with him ... about the use of ice and water membrane
under slate and have the following comments. Ice and water
membrane under slate as well as other roofing material
becomes brittle and can crack. We assume this happens due
to the high temperatures slate roofs achieves or the expansion
of different materials. Slate as well as roofing shingles
stick to the ice and water and can be difficult to replace
without breakage. Ask any of your asphalt shinglers what
it is like to strip a roof totally covered with ice and
water with the shingles installed directly to the ice and
water. The same happens to slate and the slate can be destroyed
or become unsalvagable.
Some of the places we have observed ice and water membrane
failures are at dormer/ roof deck intersections and chimney/
roof deck intersections. The materials expand and contract
differently and can cause cracking of the ice & water
at these areas. We install ice and water to our felt underlayments
on shingle installations and not to the roof deck. So beware!
We only use two solutions when it comes to ice damming
on a slate roof. First one is a copper ice belt. The second
involves installing two layers of 30 pound felt with a
layer of slaters mastic in between. We have never had a
failure. My uncle spoke to the manufacturer of the failed
ice & water system and they told him they now make
a new improved high temperature ice & water membrane
for slate and metal installations. We don't utilize ice
and water under slate and can not vouch for this new ice & water
membrane. K.S.
Joe Jenkins replies:
I have never used
a square inch of ice and water membrane on a slate roof and never intend
to, basing this position on my own personal experiences over
the past 34 years since I started working on roofs. You readers
can decide for yourselves what to do. Here’s my side
of the story:
Having examined slate roofs around the world under many different
circumstances, it’s obvious to me that ice and water
membrane is not a required element of a slate roof system
and is instead a fad peculiar to the United States and
only appearing within the past 15 or so years. Frankly,
of the thousands of slate roofs I have examined, none have
had ice membrane except those installed in the past decade or two
in the United States by roofers whose primarily business
is to install asphalt shingle roofs on plywood roof decks.
One of my aversions to ice and water membrane is simply that
the manufacturing industry is trying to make it a required
component of every roof, while it has no real purpose at
all with permanent roofs such as slate. Its original roofing
purpose was to prevent plywood from delaminating, and properly
designed non-plywood roofs don't need it. I have never
used ice and water membrane because I have never built
a roof out of plywood; I only use board decking when building
roofs. I also do not install “asphalt” roofing,
which does not make a real roof at all — it’s
a temporary covering for people who can’t afford or
don’t really care about having a real roof. Plywood
decks under temporary roof coverings are all the rage in
the United States, where wasteful throw-away construction
has become the norm. [Read an article about ice membrane and cancer-causing chemicals.]
In about 1984, a neighbor asked me to build a porch on the
front of their 15 year-old ranch-style house. The house
roof was 4:12 and the new gabled porch roof was to run on up
the front roof of the house, with two valleys. I matched the
existing architecture, building a 4:12 porch roof with
1/2” plywood and 20 year fiberglass shingles, as my
neighbor desired. This was purely a favor — I do not
do this kind of work otherwise (although I have done a lot of building) and have not done anything
like this since on a contractual basis. When I removed the shingles from the front
of the house where I had to tie in the new roof, I found
that the bottom three feet of the plywood roof deck — 15
years old — was delaminated. It was shortly after
that when the building supply houses started insisting
on using ice and water membrane along the eaves of roof
decks, and it was obvious to me why — to prevent plywood
delamination. However, as I do not work on plywood roofs,
I do not have the need to prevent plywood delamination,
and have never taken a second look at ice and water membrane.
Traditional slate roofs are built with board decking, not
plywood. Boards don’t delaminate.
Ice build-up along eaves is often an insulation problem.
Uninsulated roofs thaw the ice and snow from the field
of the roof and the water drains down to the eaves where
it freezes again, building up dangerous levels of ice.
You can largely prevent this by insulating the roof. You
have leakage at the eaves anyway? You think the solution
is ice and water membrane? Think of it this way: If the
leakage is being caused by water penetration through the
slates due to ice back-up, then the water penetration should
occur along the entire length of the eaves. This is not
usually the case at all. If, in fact, that is what happens,
more headlap may likely solve the problem (unless the problem
is due to insufficient slope or other design flaw, in which
case you’re beating a dead horse). In any case, water
penetrating a slate roof anywhere, for any reason, means
the roof was not installed properly or the slates or flashing
simply need repaired. When installing a slate roof in an
area known for ice damming problems, install the bottom
three or four feet of the roof along the eaves with 4 inches
of headlap rather than the typical 3 inches and make sure
the roof slope is 8:12 or greater. Architects must learn
that slate roofs have certain design considerations that
are different from other roofs — sufficient slope
being one of them. The object is to create a waterproof
roof, regardless of the underlayment. Underlayment material
is only expected to have a relatively short life as a waterproof
material. To depend on it for the proper functioning of
a 150 year roof is only to fool yourself (or the home-owner). [Read an article about how to install ice-dam resistant eaves.]
What I have found during times of heavy ice build-up and leakage
along eaves is the leakage does not occur along the length
of the eaves at all. It occurs in one spot or two along
the eaves, showing up inside the building in pin-pointed
places. When I have examined the roof closely, I have found
holes, nails, or cracks at those points on the roof. These
minor roof flaws only leak under ice damming situations
and once these points are repaired, the problem is solved.
Here’s an ice dam tip: the roof is leaking at the
eaves during ice damming, but you can’t find where
the water is penetrating. Simply cut copper bib flashings
about 6” wide and long enough so that when you slide
them under the slots (where the roof slates abut at their
sides) the top of the bibs hit the slate nails. Curve the
bibs slightly so they friction fit under the slates. Use
a drop of lifetime clear silicon under the bibs if you
feel the need for extra adhesion. Install the bibs along
the affected area of the eaves. You can do this in a very
short time and it will likely stop the leaking. It has
never failed for me and is a hell of a lot easier and more
permanent than removing the slates and installing new temporary
underlayment.
I don’t use any underlayment under valleys of around chimneys
when I'm replacing the flashings because I know my work will
not leak — and that's 100% guaranteed (although I do use 30 lb.
felt when installing new roofs). If my work leaks one drop, something
was done wrong and it needs to be redone. If it isn't going
to leak a drop, then underlayment isn’t necessary. One
school of thought is that if the roof leaks, then a hefty underlayment
will stop the water. I don’t buy this. If any roof I build
or repair leaks, I want to know it immediately and find the
problem and fix it. I don’t want the problem obscured
by a false sense of security so someone else can worry about
it five years later. The slates and flashings keep the water
out, not the underlayment. When a slate roof is installed properly,
or reflashed properly, not a drop of water will penetrate the
roof, underlayment or no underlayment. I have had a couple
of instances over the decades when a chimney reflashing job
I did leaked. It took me a while to figure out the problem,
but I did, and once I figured it out, repair was simple and
the mistake won’t be made again. A heavy underlayment
would have hidden the problem. I am very grateful when I can
learn from my mistakes — these are the most important
lessons. I think it would be a good practice for all slaters
to have to install a roof without any underlayment at all on
a building where the underside of the roof sheathing is exposed,
in a rainy part of the world. You will either see that the
roof does not leak a drop and thereby gain confidence in your
work, or you will see where you’re making your mistakes
and thereby improve and perfect your work. In the end, underlayment
or none, light rain, heavy downpour, or flood, the roof should
not leak.
Regarding the assertion that valleys and flashings on slate
roofs leave gaps where wind will, by design, drive water
into the roof — that’s totally bogus. The writer
who mentioned leaking valleys that could not be effectively
repaired was suffering from either roofing contractors
who did not know what they were doing, or a faulty roof
design (or both). Having replaced literally miles of valleys
on slate roofs under all sorts of circumstances and never
having a leak, using no underlayment ever, I know that
the assertion that valleys will leak without ice and water
membrane is ridiculous.
Here is one final issue with ice and water membrane: I was
originally attracted to slate roofing systems because they’re
natural roofs — roofs of stone and wood, biodegradable
roofs, recyclable roofs, ecological roofs, environmentally
friendly roofs. They’re simple, low-tech roofing systems
that are fabulously successful when properly installed.
Adding unnecessary asphalt materials to these roofing systems
steers them away from the ecologically friendly roof systems
that they now are and forces them toward the petrochemical
toxic waste roofing systems so popular in America today.
There’s no excuse for that. Architectural and construction
planners would do well to place a high priority on choosing
building materials and methods that are environmentally
friendly.
[Read an article onhow to install a copper snow apron.]
I had 3,000 sq.ft.
of Spanish slate (from Galicia region of Spain) shipped to
my home in NJ. It is roughly 3/16" in thickness. When
it arrived, I was surprised to see no holes in the tiles. G.M., NJ
Joe Jenkins replies:
They don't typically "hole" the
slate in Spain during quarrying like they do here, as much
of the Spanish slate is shipped to Europe where they tend
to install the slate with hooks rather than nails, so no
nail holes are needed. When slates are produced for the American
market, they are holed before shipping (usually). Looks like
you got a shipment that wasn't holed. You'll probably have
to hole them by hand with a slate hammer. [Read an article about nail holes in roofing slates.]
Buying slates from a foreign source can be a huge gamble unless
you’re familiar with the quarry, the slate mill, and
the work ethic of the people who produce the slate. I have
recently heard quite a few horror stories about large,
new, slate roofs that are failing after only 10 years (or
less) due to “bad” slates coming from foreign
sources. Some of these failing jobs have become a huge
liability for the installer or for the company that provided
the slate — nightmare scenarios that you do not want
to experience and don’t need to if you educate youself. [Here is a source list of roofing slate suppliers.]
Not only are slates that are destined for a European market
usually not holed, but they can also be very thin, maybe
1/8” thick. American slates are always holed and are
usually 3/16” to 1/4” standard minimal thickness.
Furthermore, all nail holes in roofing slate must be punched
to allow for a counter-sinking effect on the front of the
slate so the nail head can sit down into the slate. If
the slate is drilled, the drilled holes must also be counter
sunk. Many foreign slates are being shipped to the US without
the counter sinking, and some domestic slates may also
be lacking in the same manner. Slates that have nail holes
without counter sinking will allow the nail heads to rub
against the overlying slates eventually resulting in leaking
holes popping up all over the roof — a disaster.
So anytime you’re buying slates, make sure you’re
getting a) slates with nail holes; b) nail holes that are counter
sunk; c) 3/16” to 1/4” minimum thickness; d) slates
that are high quality with a proven longevity. Color should
never be the first consideration when buying roofing slates;
quality of the rock should be the first consideration. You
can acquire good quality slates in many colors, but do your
homework first. It will be either your money, your home, or
your reputation at stake.
That being said, I have to add that some excellent slate comes
from Spain, including from the Galicia and Leon regions,
and from other foreign sources as well (look for an article
on Spanish slate in a future TR). Take your time and shop
around — your slate roof should last at least a century
if the right slate is used and it’s properly installed,
so why rush into it? Buyer beware.
