Mr.
Jones in a southern state wrote to me to describe the following
ordeal:
“We built
a new home here last year and put a slate roof on it. When
we specified black slate our general contractor obtained
samples from various suppliers and we chose the one that
was the blackest. It was offered by [Company X] Slate,
Inc., and was represented to be ASTM S1 and domestically
mined, and priced to me at $130,000 installed for 110 squares.
It was put on in June 1996, and by August it had large
red rust spots all over it. It got much worse very rapidly
and every time it rains it leaks rust down on my white
stone entrance, walks, etc. When we investigated, it turns
out the slate was shipped from overseas and had large amounts
of pyrite. Independent testing revealed a modulus of rupture
and an absorption rate that were both so poor as to not
even rate this slate as an S3! The slate company is now
expected to replace the entire roof, and the threat of
litigation is coming up repeatedly.” [Photo below]
Mr. Jones could
have saved himself a lot of grief (and probably a lot of
money) if he had consulted with an impartial slate roofing
professional not affiliated with a slate quarry or slate
supplier, before he spent a small fortune on his roof. At the very least, anyone who is going to spend large sums of money on a new roof should do some homework on the internet.
Other examples of poor quality roofing slate:
Bad Chinese slates (above and below).
Chinese slates that were breaking apart (above).
Vermont slates with an iron leaching pyrite inclusion (above).
You cannot rely
on the opinion of your roofing contractor unless he specializes
in slate. The vast majority of roofing contractors as well
as general contractors, know very little about slate roofs,
and will try to throw you the two “curveballs” of
slate roofing:
Line #1:“Your
slate roof is old and must be torn off and replaced (but
not with slate).”
This is a very common
line thrown at the roof-owner no matter how good the roof
actually is. Beautiful roofs that will still last longer
than any person currently alive are judged finished and then
sentenced to death by ignorant roofing contractors as the
duped roof-owner nods in agreement and forks out money to
pay for the destruction. Get a second opinion from a slate
roofing professional, even if it involves a small fee. That
small fee may save you a small fortune.
Line #2:“Sure
we can install a slate roof for you, and heck yes we can
install it on a roof designed for asphalt shingles.”
This is where a
home owner or roof owner can quickly get lost. The specifications
of roof construction are foreign to most people, and if your
roofing contractor tells you the roof must be constructed
a certain way, you want to trust and believe him. Unfortunately,
most roofing contractors don’t know how to construct
a roof suitable for the longevity of slate and will try to stick slates on
a roof designed for asphalt shingles, which is a waste of
good slate (see below).
Plywood,
which consists of thin layers of wood glued together into
four foot by eight foot sheets, became popular as a roof
decking material at about the same time that asphalt shingles
became popular (late 1940’s early 1950’s). Plywood
won the affection of asphalt shingle roofers for three main
reasons: the big sheets will lay down faster than boards,
the material can be less expensive than kiln-died boards, and
plywood is convenient to buy (every lumber yard has it and
most will deliver it to the work site).
After over fifty
years of plywood roof decking, most living roofers today
don’t even remember back when roofs were built of solid
wood. Modern roofers have convinced themselves that plywood
is suitable as a decking material for any roof, mostly because
99.9% of all American roofs installed today are temporary,
disposable roofs which have to be replaced every twenty years,
or less, no matter what. If the plywood is bad after twenty
years it’s simply replaced as well. This presents a
grave problem for people who want a 150-year slate roof correctly
installed. Plywood has not demonstrated the longevity of
slate and should be avoided under any permanent roof (i.e.
slate or tile). Unfortunately, many roofers will tell you:
a) That the use
of roof decking material other than asphalt will raise the cost
of the job sky-high, and
b) That the use
of rough sawn lumber (the traditional material under slate) or solid lumber
will not work and can’t be done, despite the fact
that virtually every older slate roof in America is decked
with rough-sawn lumber or solid lumber.
This leaves the
roofing customer between a rock and a hard spot. On the one
hand, you have your preferred roofing contractor, whom you
want to trust,
convincing you to nail your slate to plywood. His
influence is bolstered by some slate brokers and most architects
who also erroneously recommend plywood under slate. On the
other hand, when you look into the matter, you will find
that plywood, when subjected to heat and moisture, has a tendancy for delamination
(which is why it doesn’t have the longevity of solid
wood) and you know that there’s no way you’ll have
a 100 or 200 year slate roof if you deck the roof with plywood.
The following quote describes a typical scenario:
“Though
the building was only twelve years old, the roof was leaking
at every conceivable point. The original shingles were
guaranteed for twenty years, but poor ventilation had caused
the plywood deck to delaminate in many places, and
shingles to blow off during high winds. The roof was repaired
on a consistent basis, but the efforts were in vain due
to the delamination of the plywood" [emphasis
mine]. - Quoted
from Roofer Magazine, November, 1997: The Roof Doctor’s
Prescription for Success, by Melinda North (page 27).
The above situation
involved the complete replacement of the roof, all 32,000
square feet of it. Ironically, the roof was again sheathed
in plywood, however an improved ventilation system presumably
will enable the new plywood to last as long as the asphalt
shingles (which is not very long when compared to slate).
What
should you use under slate? There are several good options:
OPTION #1)The
standard material to which roofing slate is fastened, traditionally used for generations, is solid lumber.
Any species
of wood will work, including hardwoods such as oak, however,
it’s easier to use hardwoods when they’re green
(which is why a lot of rough sawn lumber is used for roof decking green and
not dried). Once hardwoods such as oak have dried, you can't put a nail through them very easily (the nails bend). Mixed hardwoods can also be used.
The material should
be at least 3/4 inches in thickness, and a full inch (“four quarter”) is even better. Care
should be used in selecting lumber that is accurately and
uniformly sawn by a skilled sawyer. The carpenter or roofer
installing the sheathing lumber should cull out any marginal boards.
Rough sawn lumber
can be purchased wholesale from saw mills, not from retail lumber
yards, and usually must be ordered in advance. Look under “lumber,
wholesale” in the yellow pages of the phone book to
locate a local sawmill, or just ask around. Many of the
smaller sawmills aren’t listed in the phone book. The
cost of rough sawn lumber (“number 2 grade”) has
traditionally always been LESS than the cost of plywood,
and rough-sawn lumber will easily last a century or two. It’s
also easily repaired if damaged. Make sure your rafters are
spaced no more than two feet on center, and use 8 penny nails
to fasten the lumber to the rafters.
OPTION #2)The
rough sawn lumber can be purchased “five quarter” thick
(one and a quarter inches), then planed on both sides down
to one inch in thickness.
This technique is
obviously more time-consuming and expensive but may be preferred
by the die-hard perfectionist who wants boards that are all
exactly the same thickness. Again, moisture content of the
wood isn’t important so long as wood with high moisture
content (“green wood”) is nailed solidly into place,
or previously stickered up to air dry. The most important
consideration regarding moisture content is that once some
hardwoods are completely dry, you won’t be able to drive
a nail through the boards (most of the nails will bend),
so hardwood lumber should be used green.
OPTION #3)In
areas where no local lumber products are available, imported
kiln-dried softwood sheathing lumber, 3/4 inches thick, can be used in place of
rough sawn lumber. This option works fine.
Again, the workers
installing this lumber must cull out the bad boards, or cut
out the bad sections where large knots or other flaws will
weaken the roof deck. Three-quarter inch thick, planed softwood
(spruce, fir, pine) boards will last a century or more under slate.
OPTION #4)Tongue
in groove lumber, kiln dried, is suitable as a roof decking
on slate roofs.
OPTION #5) Slating lath,
or 1X2 to 1X4 strips of wood spaced for nailing the slate,
made of at least 3/4 inch
thick solid wood.
Institutional
buildings, and any building with a heavy slate ot tile could
use nominal 2 inch thick tongue-in-groove roof decking, such
as 2x6 planks. This was commonly used on churches, college buildings,
and other institutions where a roof deck was needed to withstand
the heavy weight of ceramic tiles or graduated
slates.
Here are a few other
mistakes commonly made when a slate roof is installed:
1) WRONG NAILS ARE
USED
When installing
new slate it's a good idea to use copper
roofing nails (an
inch and a half long). When installing used, recycled, vintage,
antique, in other words OLD slate, hot dipped galvanized
roofing nails will do fine. In fact, almost all old slate
roofs in the US were installed with hot dipped galvanized
nails. These nails easily last 100 years. NEVER use electro-galvanized
nails. Electro-galvanized nails are fine for asphalt shingles,
but those roofs are only expected to last 20 years, so a
cheap nail will do on asphalt. Slate roofs last much longer and are
not cheap, so never use a cheap electro-galvanized nail. Stainless steel roofing nails are also good.
2) INADEQUATE HEAD
LAP IS USED
The head lap is
the overlap each slate has with the slates TWO courses below.
That lap should be between two inches and four inches depending
on the slope of the roof (three inches is generally recommended).
Don't waste time
constructing a nice slate roof and then use cheap aluminum
flashing in the valleys, around the chimneys, etc. Copper,
stainless steel, terne coated stainless, lead coated copper,
lead sheet, and in some cases heavy aluminum are all good
with slate.
