Spring,
2005: Issue #4
Top 10 Mistakes Made
When Installing
New Slate Roofs
by Joseph Jenkins
Download
an expanded version of this article as a PDF file
Slate roof installation VIDEO CLIPS.
As a slate
roof consultant, I travel around the country looking
at slate roofs, both old and new. Some are ancient and
just need inspected and evaluated; others are brand new — and
leak. I am currently involved in five slate roof litigation
cases as an expert witness — all residential structures
with roof work ranging in cost from $20,000 to $350,000.
I see the same mistakes over and over on new slate roofs,
and they’re hurting the long-term prospects of
the American slate roofing industry. Here are the top
ten mistakes, in no particular order.
1. Lack of information: The contractors
(and homeowners) have not done their homework. The contractor
blindly bullies ahead with the job without making any effort
whatsoever to do any research. A simple search on the internet
can yield a wealth
of information about slate roofs, sources of correct
tools, materials, supplies and installation techniques.
2. All slate is not the same: You wouldn’t
buy a car without looking at different models and checking
their track record — and cars only last ten years
and are cheaper than slate roofs! A slate roof is an
investment in the future of your building. It will reasonably
last 150 years if constructed correctly. There are many
different types of slate with differing characteristics
and longevities. Why buy a foreign slate with no track
record? Do the research. [Source
list of new roofing slate] [Source
list of salvaged roofing slate]
3. The contract documents are deficient: Every
detail about the slate roof installation should be included
in the contract documents — type, size and origin
of the slate; type, length and gauge of the nails; type
and installation style of underlayment; type and size
of cant strip; headlap; flashing specifications; number
of squares to be installed; slate installation style,
and many other details. A basic contract (“Sample
Slate Roof Installation Proposal”) is posted here.
4. Lack of headlap: This fundamental
detail of any successful slate roof installation is hard
to overlook, but it is ignored by some roofing contractors.
Lack of adequate headlap spells disaster for a slate
roof. I have seen new roofs with inadequate headlap (i.e.
less than 2”), no headlap at all, and even negative
headlap. Do your homework, contractors, or stay away
from slate roof jobs. [An illustration of correct
headlap and of incorrect
headlap. Another example of incorrect
headlap. ] Read an article about headlap.
5. Bad flashing work: There are two
things that keep water from penetrating a slate roof:
the slates and the flashings. Not only must the flashing
metal be of adequate type and gauge, but it must be installed
correctly. This is not rocket science, but it does require
some training and/or experience in order to be done correctly
and to be leakproof. [Source
of good quality flashing material.]
6. No consultant was used on the job: As
a consultant,
I am called on after the work has been completed and
the roof has failed — this is a mistake. Professional
advice should be obtained before the roof is installed
and even before the structure is built, if possible.
However, not all slate roof installations require a consultant.
Homeowners can educate themselves for very little money
by simply reading a copy of the Slate
Roof Bible, reading past Traditional
Roofing articles online at traditionalroofing.com,
and asking questions on the message
board at slateroofcentral.com.
7. Contractors walking on the slate: This
is one of the worst problems with new slate roof installations.
Roof slate is not to be walked on — period. It
is not a floor that is being installed — it is
a roof. The roof must be properly
staged so the roofers are working off roof ladders
and roof scaffolds. If the contractors are walking
all over the slate roof during installation, it’s
because they don’t know what the hell they’re
doing and the property owner will have many headaches
later when the slates start falling off. This is a guarantee.
Good slaters knows how to install slate, and they won’t
walk on a slate roof unless it’s a last resort
in an unusual circumstance. Need
roof jacks?
8. Poor sheathing materials: The roof
decking must last as long as the slate. A good
roof deck should last the life of two slate roofs,
or about 200 to 300 years. In any case, a roof decking
material under slate must have a known longevity of at
least 150 years. Materials that have been tried and proven
for this purpose include lumber boards and battens from
3/4” to 1.5” thick rough-sawn, planed or
tongue-in-grooved from a variety of species of wood.
Plywood, laminated woods and particle boards are sub-standard
roof decking materials for slate roofs and should be
avoided. Yes, you can install slate on laminated or glued
decking materials, but a compromise on longevity is likely
to be the result. If a slate roof is to be built to last,
the roof deck should be solid boards, not glued sheets
of wood.
9. Emphasis on underlayment: This is
a red herring. If a slate roof leaks, it’s because
it was installed improperly, not because of underlayment
or lack of it. Properly installed slate roofs need no
underlayment. The main purpose of the underlayment is
to keep the water out of the building until the slate
and flashings are installed. After that, if you could
magically yank the underlayment out from under the slate,
it wouldn’t make a bit of difference in the functioning
capabilities of the roof. Secondary purposes for underlayment
include providing a good surface for chalk lines during
installation, providing a minimal layer of insulation,
and providing a cushion for when the slates are being
slapped down during installation. Concerned
about ice-damming?
Barn slate roofs in the United States — and there
were thousands and still are quite a few, mostly a century
old or older — were installed without any underlayment
whatsoever. This is true for some institutional buildings
as well. Most of the older homes in the U.S. were installed
with a standard single layer of 30 lb felt under the slate
roofs. These homes are so old now that the felt has deteriorated
to dust, but the roofs are fine. If the slates and flashings
are intact, the roof will not leak, underlayment or no
underlayment, even in a sustained driving rain. This is
a proven fact, not a theory.
If a contractor or architect is insisting upon a beefed-up
underlayment under a new slate roof installation, it
means they believe the new roof will leak and that the
underlayment will delay the entry of the water into the
building. This is flawed logic and reveals a gross misunderstanding
of slate roofs. Architects sometimes confuse slate roofs
with ceramic tile roofs. Although tile roofs may require
a substantial underlayment, slate roofs, properly installed,
do not.
Underlayment does, however, provide a margin of waterproofing
in the event a slate roof is damaged by wind, tree-fall,
or other unusual circumstance. An acceptable slate roof
installation today still typically utilizes a single
layer of 30 lb. felt underlayment, doubled (half-lapped)
when the need for a heavier underlayment is required
(such as when a roof must be left exposed for a period
of time before the slates are installed).
What about ice-damming? Increase
the slate headlap along the eaves to prevent ice-damming,
but do not rely on what’s underneath the slate to
keep the roof from leaking. If the slate and flashings
are installed correctly, the roof will not leak. That is
the beauty of a stone roof.
10. Inexperienced roofing contractors: It
is an unfortunate fact that many contractors cannot be
trusted to give sound and honest advice or information.
This issue is exacerbated by property owners who don’t
get competing bids before initiating a contract; who
don’t educate themselves about the nature of the
work prior to hiring a contractor; and who don’t
insist upon a detailed, coherent and comprehensive contract
document. One major effort that is being made today to
try to screen contractors for slate roofing purposes
is the Slate Roofing
Contractors Association of North America, initiated
on March 1, 2005. It lists contractor members at slateroofers.org. Included with the listing is a Contractor Profile which
reveals details about the contracting firm that the average
consumer would want to know. There is more information
about the SRCA here. Need
the correct tools?
Bad slate roof installations are seriously harming the
slate roofing industry. One university administrator
told me he had slate roofs installed on his dormitories
because he wanted “the best roofs money could buy.” Then,
after five large slate roofs had been installed on his
campus by the same roofing contractor, it was discovered
they had been installed with
only 1.5” headlap, or none at all. The discovery
of this gross deficiency left the administrator stunned,
shocked and disgusted. He never wanted to look at another
slate roof again. Who can blame him?
See also: Avoid These 21 Contractor Errors
For more information: Top
5 Mistakes Made When Installing Slate Starter Courses
Read
About Drip Edges Designed For Slate Roofs - Buy copper drip edges online
Read more about headlap.
Slate roof installation VIDEO CLIPS.
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