Spring,
2005: Issue #4
CONICAL SLATE ROOFS
and How to Install Them
How do you install flat, rectangular
stones on a curved surface? Slate roofs on rounded turrets
and domes are incredibly beautiful. But is it hard to install
these types of roofs? How is it done?
The building shown in the top photo, built in 1889, is
known as the Oberlin (Ohio) Gasholder. Remnants of the
date, inscribed into the Vermont sea green slate roof
with Vermont purple slates, still adorn the roof, but
the roof is in dire need of repair. How many roofing
contractors are up to the task of repairing or replacing
the slate on a round turret?
The new slate roof in the
above photo is located in Stirling, central Scotland, and
installed by Dave O’Hare.
The turret slates are Cupa heavy Spanish slates with
English Burlington blue-grey random slates on the main
roof. Note the “shouldered” slates (i.e.
top corners cut off) and the diminishing widths as the
slate climbs the turret. Rounded slate roofs require
more time and more attention to detail, but they pay
off in beauty and durability.
The turret above is on a new Vermont mottled green and
purple roof in western Pennsylvania installed by the
author. More on this later.
Mr. O’Hare has been involved in about 12 round turrets
in the 20 years he’s been in the trade. He explains, “Once
the turret is felted and assuming the turret rises to a
point, insert a large six-inch nail or similar at the very
top of the point, then hook a string line onto the nail.”
O’Hare has a unique method for aligning the slates
as they curve around the roof. First, he lays the slates
onto a straight board on the ground or scaffolding. A strip
of wood can be nailed onto the bottom edge of the board
to act as a stop for the slates, which are then laid side
by side on the board so that the bottoms of the slates
are tight against the wood strip. Lay as many slates, face
up, as the board will allow (maybe 10). Mark your slates
at either end of the board and snap a line across the exposed
surface of the slates to mark the bottom of the overlying
course. “You are now ready to start slating the turret.”
O’Hare explains, “The bottom course of slates
is the most important as this has to be even round the
turret. If this course isn't level, then the rest of the
courses won't be level. There are many ways to ensure the
slates are getting the correct overhang.” One way
is to use a tape measure and check the overhang on the
starter course as you nail it around the bottom edge of
the turret. O’Hare has done it enough times that
he can simply use his finger as a measure. The author prefers
a 1.5” slate overhang. Others may prefer a 2” overhang.
Instead of a cant strip under the starter slates, O’Hare
finds that the carpenters in Scotland typically build the
fascia board in such a manner that it acts as a cant, lifting
the starter slates enough to give them the angle they need
to lay properly. If a cant strip is used, it must be installed
in short lengths. Such short pieces may need to be drilled
and then screwed into place to prevent splitting of the
wood. For 10” wide slates used as the first course
on the bottom of a rounded turret, 10” long cant
strips will do nicely as will 10” wide starter slates.
For example, the rounded turret shown at left is slated
with 10” x 20” slates, which have an 8 1/2” exposure.
Therefore, the starter slates are 10” wide and 12” high,
thereby creating a 3 1/2” headlap at the bottom of
the turret. The starter slates are staggered so that the
side joint on the first course of slates above the starter
course falls right in the center of the starter slate.
The remaining slates on the turret are all cut from 10”x20” stock.
What about the nail at the top center of the turret? This
is used as an anchor for a string that hangs down over
the turret roof as a slate-trimming guide. You can use
a simple chalk line for this purpose — hook the
end of the string over the nail and let the chalk box
hang down over the eave of the turret. O’Hare explains, “Once
you have positioned the first slate onto the roof, this
is where the string line comes in. Hold the line at either
[bottom] corner of the slate and mark the angle at which
to cut the slate. It is important to try and maintain
half bond [half side-lap] on the slates, but it isn't
always possible. Concentrate on keeping the slates going
round the turret even. I nail all slates three times — one
at the head of the slate and two at either side. As I
progress up the turret and the slates get narrower, I
notch either side of the slate with my slate knife and
[nail] the side nails into the notches. Use the string
line as a guide to achieve a correct angle. We always
cut the bed (rear) of the slates with a slate knife.
Every slate has to be cut so that the slates go round
the turret.”
The above photo shows the
small turret in Pennsylvania as the slate is starting to
be applied. The wooden cant strip has been attached to
the roof sheathing in 10” lengths.
The photo below shows how a chalk box and line are being
used as a trimming guide in order to cut the proper angle
on the sides of the slates. The string is moved around
the turret as the slates are installed. Every slate must
be trimmed in this manner. A GT Professional hand-operated
slate cutter was used for this purpose. Nail holes were
punched as needed using a slate hammer. The holes were
punched in the slate prior to trimming the edges. All
hole punching and cutting was done from the back of the
slate.
The photos below show the
work in progress and the finished turret prior to the installation
of the ridge.
The fact that every slate must be trimmed at an angle on
a small turret such as this indicates that more time
and fiddling around are necessary to get the job done
right. Contractors must allow for this when bidding a
job of this nature. In the end, however, such a slated
turret can last well over a century, impressing people
with its beauty and character, as the Oberlin Gasholder
Building has proven.
[Author’s note: The
Gasholder slate roof was installed in 1889 without any
felt underlayment.]
From Traditional Roofing
#5:
See also: Conical Roofing Simplified (TR7)
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