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2, Spring 2002
a Graduated Slate Roof
By Barry Smith
Edinburgh, Scotland in November 2000, I was fascinated to
see thousands and thousands of graduated slate roofs. Scottish
slate, it was explained, can't be split into smooth and regular
pieces, so orders were traditionally filled with different
sizes of slate. These would be sorted at the job site and
installed with the larger slates at the bottom and gradually
smaller slates on up the roof. Fortunately for Scotland,
this frugal practice produces a beautiful and distinctive
roof that looks appropriate on both mansions and barns.
Graduated roofs are uncommon in North America because we have
many types of very fine-grained slate that can be used
to produce uniform thickness and sizes of roofing slate.
A uniform slate roof requires less labor to install and
is therefore less expensive, so this has always been the
norm. As a result, the few graduated roofs to be found
tend to be on higher-end buildings. These roofs were installed
to give the building an elegant European look. I was therefore
very interested when contacted by an upscale contractor
in Erie, Pennsylvania to bid on a graduated purple slate
roof for a new garage, being built to match the bay-front
mansion beside it.
After meeting the contractor and home owner at the building
site and discussing their plans, a sampling of measurements
was taken from the slate on the house to determine what
sizes to order. The slates were about 70 years old and
in great shape. They ranged in thickness from 3/8" to ¾",
giving the roof a very textured look.
Matching the roof exactly was going to cost a lot more than
an approximate match. So I had several slate companies
quote on 3 different options, all involving purple slate.
First: 4 different lengths in descending ratios; 40% of
22", 30% of 18", 20% of 16", and10% of 12".
Each of these came in 3 to 5 different widths, and ranged
in thickness from 3/8"-1/2". Second: The same
as above except thinner; ¼" to 3/8". Third:
20" long slates by random; ¼"- 3/8".
The thicker slate was obviously more expensive than the
thinner, but the installation labor was the variable that
was most effected by each of the options above. Finally,
I was able to give them a proposal with the 3 options,
with the closest match being the most expensive. They decided
on the middle option, thinner slate, but still graduated.
The roof was just
over 13 square, but because of valleys and hips, starter
slates, capslates (instead of ridge iron), the need to balance
all of the different sizes, and the need for extras to do
some work on the house, I decided to order on the heavy side.
In the spring of 2001, we took delivery of 19 square of slate,
consisting of 17 different sizes, from Evergreen Slate Co.
For the remainder of this article, I assume that the reader
has read The Slate Roof Bible, especially Chapter 12, or
already has a working knowledge of basic slate installation.
When we arrived at the job site, my employees Mark and Mildred
(don't ask) were ready to pound some nails. However, I
knew that this was going to be a bit trickier to get started
than the normal job. I scrambled to get the calculations
done to keep ahead of them. I counted all of the different
sizes and verified that the above ratios were delivered
and then determined the ratios of the various widths within
With the above numbers, the chalk-line layout could be determined.
I calculated the area that the 22" long slates would
cover (40%) and determined the number of courses of this
size. The chalk line interval for these courses would be
9 ½", assuming a 3" headlap. (22"-
3" headlap, divided by 2 = exposure). There were 5
different widths of 22" long slates that broke down
accordingly; 11" (9%), 12" (29%), 14" (59%),
16" (1%), and 18" (2%). We used these percentages
to guide us when carrying up the slate so that we wouldn't
run short on any of the widths before the roof was completed.
After determining the starter course chalk-line and the required
number of courses of 22" long slates, it was time
to graduate down to 18" slate chalk-lines (7 ½",
which were to cover the next 30% of the roof. The key to
doing this involves a transitional row of 20" long
slates, the length being arrived at by averaging 18" and
22". The top of the transitional slate is treated
like an 18" slate, because it uses the same chalk-line
interval as the 18" long courses (18"- 3"headlap
divided by 2 = 7 ½"exposure); however the bottom
of the transitional slate acts like a 22" slate, because
it has a 9 ½" exposure.
Between the 18" long slates and the 16" slates, the
transitional row is 17" long with a chalk-line interval
of 6 ½", which is the same for the rest of the
16" courses (20% of the roof).
The last transitional row is 14" long, with the chalk-line
interval stepping down to 4 ½". This, as before,
is the interval for the rest of the 12" long slates (10%
of the roof). As you might imagine, this took several attempts
before all of the numbers were worked out. We went through
this process on the roof itself, and once we were satisfied
that we had the right measurements, we each drew up a "key" on
an index card that told us everything we needed to know about
each of the 23 courses; chalk-line measurement, slate length,
and the percentage of each width within each length (see below).
At last we could start nailing! As each slate was installed,
care was taken to maintain at least a 3" lateral lap
relative to the slates being covered below. This slows
the process considerably, compared to installing uniform
size slates. Referring to our keys frequently slowed the
process further. As each side was completed, the remaining
slates would be recounted to make any needed adjustments
to the percentages of widths within each length. The final
product was a very good match, both in color and style,
and the difference in slate thickness on the house and
the new garage was hardly noticeable, making that compromise
seem like a reasonable one. With the addition of copper
half-round gutter and spouting, the new garage was beautiful,
and will remain so for many, many years.
Layout Key below
(course # on left
column, distance from drip edge in center column, size of
slate in right column)
22-- 155.5"-- x 8" (28%)
21-- 151"-- 12" x 9" (37%)
20-- 146.5"-- x 10" (35%)
18 --137.5"-- 14" Transition
16 --126.5"-- x 8" (13%)
15-- 120"-- 16" x 9" (31%)
14-- 113"--.5 x 10" (31%)
13-- 107"-- x 11" (25%)
12-- 100"--.5 17" Transition
10-- 86"--.5 x 9" (26%)
9-- 79"-- 18" x 10" (45%)
8--71.5"-- x 11" (29%)
6-- 56.5"-- 20" Transition
5-- 49"-- x 11" (9%)
4-- 39.5"-- x 12" (29%)
3-- 30"-- 22" x 14" (59%)
2-- 20.5"-- x 16" & 18" (3%)
1-- 10.5"*-- 22" x 12" Starters
* Note that the
headlap on the starter course is only 2 ½" since
a 12 ½" wide slate would be a special order and
is unnecessary [Note: He could have used a 14" starter
Most of the faces had hips on both sides so we started in the
middles and worked our way to the edges. Note how the steep
roof pitch allowed us to lay ground ladders right on the roof,
thereby avoiding the need for any roof planks.
Putting the finishing touches on one side.
This diagram (below) shows the 3" headlap and the detail
for a "transitional" slate, which is necessary when
laying out a graduated roof.
Read another article about graduated slate roofs.
Go to Barry Smith's web site.
more about graduated slate roofs in the Slate
Roof Bible, 2nd edition!
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