Slate, laid in multicolored decorative patterns, was particularly
well suited to the Mansard roofs of the Second Empire style,
the steeply pitch roofs of the Gothic Revival and High
Victorian Gothic styles, and the many prominent roof planes
and turrets associated with the Queen Anne style. The Tudor
style imitated the quaint appearance of some English slates which,
because of their granular cleavage, are thick and irregular. These
slates were often laid in a graduated pattern, with the
largest slates at the eaves and the courses diminishing in size
up the roof slope, or a textural pattern. Collegiate Gothic
style buildings, found on many university campuses, were often
roofed with slate laid in a graduated pattern.
The configuration, massing, and style of historic slate roofs
are important design elements that should be preserved. In addition,
several types of historic detailing were often employed to add
visual interest to the roof essentially elevating the roof to
the level of an ornamental architectural element. When repairing
or replacing a slate roof, original details affecting its visual
character should be retained.
Before repairing or replacing an existing slate roof, it is important
to document the existing conditions and detailing of the
roof using written, visual, and physical evidence so that original
features can be identified and preserved. Documentation should
continue through the repair or replacement process as significant
details, long obscured, are often rediscovered while carrying
out these activities. Local histories, building records, old receipts
and ledgers, historic photographs, sketches, and paintings,
shadow lines and nail hole patterns on the roof deck, and bits
of historic material left over from previous interventions (often
found in eave cavities) are all useful sources of information
which can be of help in piecing together the original appearance
of the roof. Size, shape, color, texture, exposure, and coursing
are among the most important characteristics of the original slates
which should be documented and matched when repairing or replacing
an historic slate roof.
Historically, three types of slate roofing standard,
textural, and graduated were available according to the
architectural effect desired. Standard grade slate roofs were
most common. These are characterized by their uniform appearance,
being composed of slates approximately 3/16" (0.5cm) thick,
of consistent length and width, and having a smooth cleavage
surface. Thirty different standard sizes were available, ranging
from 10" (25cm) x 6" to 24" x 14" (15cm
x 61cm x 35cm). The slates were laid to break joints and typically
had square ends and uniform color and exposure. Patterned
and polychromatic roofs were created by laying standard
slates of different colors and shapes on the roof in such a
way as to create sunbursts, flowers, sawtooth and geometric
designs, and even initials and dates (Figure 3). On utilitarian
structures, such as barns and sheds, large gaps were sometimes
left between each slate within a given course to reduce
material and installation costs and provide added ventilation
for the interior.
Textural slate roofs incorporate slates of different thicknesses,
uneven tails, and a rougher texture than standard slates. Textural
slate roofs are perhaps most often associated with Tudor style
buildings where slates of different colors are used to enhance
Graduated slate roofs were frequently installed on large
institutional and ecclesiastical structures. The slates were
graduated according to thickness, size, and exposure, the thickest
and largest slates being laid at the eaves and the thinnest
and smallest at the ridge. Pleasing architectural effects were
achieved by blending sizes and colors.
Detailing at the hips, ridges and valleys provided added
opportunity to ornament a slate roof. Hips and ridges can be
fashioned out of slate according to various traditional schemes
whereby the slates are cut and overlapped to produce a watertight
joint of the desired artistic effect. Traditional slate ridge
details are the saddle ridge, strip saddle ridge and comb ridge,
and for hips, the saddle hip, mitered hip Boston hip, and fantail
hip. A more linear effect was achieved by covering the ridges
and hips with flashing called "cresting" or
"ridge roll" formed out of sheet metal, terra cotta,
or even slate. Snow guards, snow boards, and various
types of gutter and rake treatments also contributed to the
character of historic slate roofs.
Two types of valleys were traditionally employed, the open
valley and the closed valley. The open valley is lined with
metal over which slates lap only at the sides. Closed valleys
are covered with slate and have either a continuous metal lining
or metal flashing built in with each course. Open valleys are
easier to install and maintain, and are generally more watertight
than closed valleys. Round valleys are a type of closed valley
with a concave rather than V-shaped section. Given the broader
sweep of the round valley, it was not uncommon for roofers to
interweave asphalt saturated felts rather than copper sheet
in the coursing in order to cut costs. Although principally
associated with graduated and textural slate roofs, round valleys
were infrequently employed due to the difficulty and expense
of their installation.
Common types of sheathing used include wood boards, wood
battens, and, for fireproof construction on institutional
and government buildings, concrete or steel. Solid wood
sheathing was typically constructed of tongue and groove, square
edged, or shiplapped pine boards of 1" (2.5 cm) or 1-1/4"
(3 cm) nominal thickness. Boards from 6" (15 cm) to 8"
(20 cm) wide and tongue and groove boards were generally preferred
as they were less likely to warp and curl.
Wood battens, or open wood sheathing, consisted of wood strips,
measuring from 2" (5 cm) to 3" (7.5 cm) in width,
nailed to the roof rafters. Spacing of the battens depended
on the length of the slate and equaled the exposure. Slates
were nailed to the batten that transected its midsection. The
upper end of the slate rested at least 1ž2" (1.25 cm) on
the batten next above. Open wood sheathing was employed primarily
on utilitarian, farm, and agricultural structures in the North
and on residential buildings in the South where the insulating
value of solid wood sheathing was not a strict requirement.
To help keep out dust and wind driven rain on residential buildings,
mortar was often placed along the top and bottom edge of each
batten, a practice sometimes referred to as torching.
Steel angles substituted for the wood battens in fireproof
construction. The slates were secured using wire wrapped around
the steel angle, where it was twistedoff tight. Alternately,
any of a variety of special fasteners patented over the years
could have been used to attach the slate to the steel angle
(Figure 10). On roofs with concrete decks, slates were typically
nailed to wood nailing strips embedded in the concrete.
Beginning in the late nineteenth century, asphalt saturated
roofing felt was installed atop solid wood sheathing. The felt
provided a temporary, watertight roof until the slate could
be installed atop it. Felt also served to cushion the slates,
exclude wind driven rain and dust, and ease slight unevenness
between the sheathing boards.
Slate was typically laid in horizontal courses starting at
the eaves with a standard headlap of 3" (7.5 cm) (Figure
10). Headlap was generally reduced to 2" (5 cm) on Mansard
roofs and on particularly steep slopes with more than 20"
(50 cm) of rise per 12" (30 cm) of run. Conversely, headlap
was increased to 4" (10 cm) or more on low pitched roofs
with a rise of 8" (20 cm) or less per 12" (30 cm)
of horizontal run. The minimum roof slope necessary for a slate
roof was 4" (10 cm) of rise per 12" (30 cm) of run.