Historic Preservation - Technical Procedures
- General Information On Slate Shingles
- Procedure code:
- Hspg Prepared For Nps - Sero
- Thermal And Moisture Protection
- Slate Shingles
- Last Modified:
- General Information On Slate Shingles
- Last Modified:
GENERAL INFORMATION ON SLATE SHINGLES
THIS PROCEDURE SHOULD BE USED IN CONJUNCTION WITH 07315-03-R,
REROOFING USING SLATE SHINGLES
1.01 CHARACTERISTICS OF SLATE
A. Slate is a natural stone and can produce a wide range of
effects based on its appearance, color, thickness,
surface texture, and roof texture.
B. A permanent material that is waterproof, fireproof,
resistant to climatic changes, and requires no
preservative coatings or paint, and no cleaning,
resulting in lower insurance premiums, higher property
values, little or no maintenance costs, and a high
C. Some slates have a greater porosity than others and will
eventually begin to spall due to freeze-thaw cycles.
D. The quality and characterics of various slate types
vary greatly among the various quarry sources.
Slate types that are rated as ASTM S-1 are
considered the best quality.
Reference: National Slate Association, www.slateassociation.org
1.02 CAUSES FOR ROOF FAILURE
A. Failure of a slate roof is generally due to poor
1. Nails that are driven too far may cause tension in
the tightly held slate resulting in cracking of
2. Nails not driven quite far enough may cause the
slate in the course above to rest unevenly on the
protruding nail head. This makes the slates more
susceptible to breaking if stepped upon.
3. The use of inappropriate nails can lead to failure
such as rusting. If some slates are letting go
because their nails have rusted through, all the
slates may eventually have to be relaid with the
proper copper nails.
4. If shingle nails rather than large, flat-head wire
nails are used, individual slates can easily slip
off of the nails.
B. Leaks in slate roofs can also be caused by deteriorated
flashings. Flashings gradually erode due to scouring of
rain water running down valleys. Atmospheric conditions
can also cause flashings to corrode.
C. To a lesser extent, slate roofs can fail due to the
deterioration of the slates themselves. If the majority
of the slates are delaminating or crumbling due to
atmospheric conditions reacting with the mineral content
of the slates, it is impossible to save the roof.
A. Butt - the exposed end of a roofing slate.
B. Clear - in regions where slate contains bands of rock
compositionally different from the main body of slate,
"clear" denotes slates which have been trimmed of all
C. Commercial Standard Slate - most common and available
roofing slate. Exact definition varies by region, but
generally this slate is 3/16" thick with varying widths
(8" to 14") and a length between 18" and 24". Each slate
has a bevelled edge and pre-punched nail holes. Quality
is fairly consistent.
D. Comb Ridge - ridge finishing treatment in which the
combing slates on the north or east side are laid
extending 1\16" to 1" over the other side. The grain of
the combing slates may be either vertical or horizontal.
E. Cox Comb Ridge - the combing slates (those projecting at
the top) alternately projecting on either side of the
F. Curb - the line formed by the junction of two different
slopes on one side of a roof--especially on Mansard and
G. Exposure - the length of each slate exposed to the
weather, i.e., not covered by the next above course.
Exposure is expressed in inches. A simple formula is
used to compute the exposure: Deduct 3" (standard lap)
from length of slate and divide by two. For a 24" slate,
usual exposure is 24-3= 21, 21 divided by 2=10-1/2".
H. Freaks - slates having an unusual combination or
variation of color, bought for special effects on special
order. They are thicker than usual--never split under
1/4" and up to 2" or more.
I. Graduated roof - variation on the Standard slate roof
described below. Slates are arranged so that the
thickest and longest are at the eaves, diminishing in
size and thickness to the ridges. Usually this is
combined with other generally more labor-intensive
treatments such as closed valleys.
J. Lap (headlap) - that part of a slate overlaying the slate
two courses below. The standard lap is 3". Roofs with
less slope (flatter) often take a 4" lap; those very
steep need only a 2" lap.
K. Ribbon Stock - slate which contains bands of rock
differing in composition and color from the main body of
stone. It is always labelled as such. Usually from
L. Saddle Ridge - finish in which the regular roofing slates
are extended to the ridge line so that slates on both
sides of roof are butted flush. Then another course of
slates is laid with its grain horizontal (combing slates)
and lapped horizontally to cover the previous combing
slate's nail holes. They are butted flush on either side
of the ridge.
M. Standard Roof - one composed of Commercial Standard Slate
(approx. 3/16 in. thick) of more-or-less uniform standard
width and length, with butts laid to a line, in standard
slate colors. (No color patterns, no freaks.)
Encompasses those slates with butts (exposed ends)
trimmed to have an hexagonal, diamond, or Gothic pattern.
N. Square - number of slates needed to cover 100 square feet
of plain roof surface, when laid with the customary lap
of 3". (Roofs with a flatter slope require only a 4"
lap, so more slates are need to cover 100 sq. ft.; very
steep roofs take a 2" lap, so fewer slates are needed per
square.) Commercial Standard Slate weighs 650-750 lbs.
per normal square.
O. Textural Roof - in between a Standard roof and a
Graduated roof. Generally, such a roof has more visual
interest than the Standard, with use of rough slates
instead of smooth, or with unevenly laid butts, or
variations in the thickness, size, and color of slates.
(Not usually over 3/8" thick.)
P. Unfading - a color designation given to those slates that
do not "weather" appreciably or change color over the
years. (As Unfading Red.)
Q. Weathering - the exposed surface of a shingle, or a
modifying word describing the color characteristic of a
slate. Weathering slates react chemically with the
atmosphere to gradually change hue over the years; does
not affect longevity or hardness of the slate. See
1.04 SYSTEM DESCRIPTION
A. Slate quarried for roofing stock is of dense, sound,
rock, exceedingly tough and durable.
1. Slate, like any other stone, becomes harder and
tougher upon exposure than when first quarried, and
is practically non-absorbent.
2. Many slates split to a smooth, practically even and
uniform surface, while others are somewhat rough
B. The color of slate is determined by its chemical and
mineralogical composition and may be obtained in a
variety of colors and shades.
1. Basic slate colors include: Black, Blue Black,
Grey, Blue Grey, Purple, Mottled Purple and Green,
Green, and Red.
2. These color designations should be preceded by the
word "unfading" or "weathering," according to the
ultimate color effect that may be desired.
C. There are several grades and types of slate, but the most
commonly specified is the Commercial Standard slate,
which has the following properties:
1. Surface: Reasonably smooth straight cleavage full
length of slate both front and back. The maximum
bend should not exceed 1/4" in lengths up to 16",
not exceed 3/8" in lengths from 16" to 24".
2. Texture: Should be free from knots or knurls that
in any way interfere with the safe conveyance or
the laying of the slate on the roof.
3. Corners: Reasonably full corners on exposed ends.
No broken corners on covered ends that would
sacrifice nailing strength, or the laying of a
water tight slate roof.
4. Weight: 600 to 750 lbs. per square, depending on
type, color, and quarry. Allow 8 lbs. per square
foot dead load for combined weight of slates,
nails, and felt.
5. Thickness: Approximately 3/16".
A. Slate shingles:
1. Buckingham-Slate Corporation
(Blue-black. High quality Virginia slate sold
through distributors. Free literature)
2. Evergreen Slate Co.
(Gray-green, purple, green mottled green-purple,
gray black, unfading red, Vermont black. Vermont &
New York slate sold direct. Also slater's tools.
3. Hilltop Slate Co.
(Gray-green, purple, green, mottled green-purple,
gray-black, gray, Vermont black. New York and
Vermont Slate sold direct and through distributors.
4. Midland Engineering Co.
(A major distributor for roofing products including
German clay tiles and Vermont slate, sold through
roofers and direct. Free brochures on all products
- specify your interest)
5. Vermont Slate Company Mr. Slate
6. Penn Big Bed Slate Co.
7. Monson Maine Slate Co.
(Unfading black slate, only on special order)
8. Rising and Nelson Slate Co.
(Green, gray, Vermont black, and gray-black,
purple, mottled green-purple, red. Vermont slate
is not sold direct. Free brochure)
9. Shelton Slate Products Co.
(All typical New York-Vermont colors)
10. Structural Slate Co.
(Pennsylvania slate sold through distributors.
11. Vermont Structural Slate Co.
(Green, gray Vermont black, gray-black, purple,
mottled green-purple, red. Vermont roofing slate
sold direct and through distributors. Free
B. Slate roofing substitutes:
1. Monier Group
(Concrete tiles designed to imitate terra cotta,
wood, and slate tiles are sold through
distributors. Free literature)
2. Vande Hey Raleigh
(Manufactures a broad line of extruded concrete
roofing tiles, including a simulated slate and a
Mission tile. Also has a large stock of recycled
slate, concrete, and clay tiles)
C. Slating tools:
1. Stortz & Sons
2.02 MATERIAL PERFORMANCE REQUIREMENTS
A. Slate: Natural slate roofing units used for replacement
should duplicate existing slate installed on the roof and
match for thickness, color and texture, as well as type,
size and existing, and should be punched for nailing. It
should be noted that slate is always sold by the
"square", or 100 sq. ft. of roof laid with a 3" head lap.
NOTE: THE FOLLOWING INFORMATION SHOULD BE SUPPLIED TO
THE ROOFING CONTRACTOR WHEN DETERMINING QUANTITIES AND
1. Kind and color of slate.
2. Size of slate desired, stating length and "all one
width" or random width.
3. Thickness, as "commercial standard," 1/4", 3/8",
4. Type of roof, as standard, textural, graduated or
5. Kind of nails, as zinc clad, zinc, "yellow metal",
copper clad, or copper.
6. Kind of valleys and flashings.
7. If hip or gable roof.
8. Kind of snow guards, as galvanized, yellow metal or
9. If snow rails, size of pipe and number of rows of
10. Location of job; if in city or vicinity, or out of
11. When job is to be finished.
1. All nails, rivets, and similar fastenings, if any,
used throughout the work should be of best grade
2. Nails should be large flat-head copper wire nails.
DO NOT USE COPPER ROOFING NAILS OR ORDINARY COPPER
3. Nail length should be twice the thickness of the
slates plus 1". Minimum length is 7/8". Sizes: 3d
for commercial standard slates up to 18" in length;
4d for slates over 18"; 6d for ridge and hip
1. All intersections of roofs with vertical surfaces
of every kind and all openings in roof surfaces,
should be properly flashed.
2. Match appearance of original materials. If any
existing flashings are to be reused, new material
must be the same as the original material to
prevent galvanic corrosion.
a. Copper - 16-oz. soft copper; occassionally 20-oz.
required, consult manufacturer.
b. Lead - 2-1/2# to 3#.
c. Terne - 20# or 40# depending on type of
flashing, i.e cap and base flashing, 20# or
vertical and horizontal surfaces, 40#.
d. Galvanized - 24 ga. to 26 ga. depending on
type of flashing, consult manufacturer.
D. Base flashings:
1. Should be at least 4" high.
2. Should project at least 4" out onto the roof.
3. Should be a full 96" in length. On sloping roofs
they should lap longitudinally at least 3".
E. Cap flashings or counterflashings:
1. Should turn down over base flashings not less than
2. When attached to woodwork they should extend up
under exterior coverings not less than 4" above the
roof, and should be nailed along the top edge about
3. When attached to masonry work, the cap flashings
should extend 4" into joints of masonry walls and
have the inner edge turned back on itself 1/2".
4. The sheets should be bent to the required shapes,
and built in with the mason work. No cutting out
of joints for setting flashings will be allowed.
1. Flashings should finish in reglets in the masonry.
2. The flashing should be turned into the reglet the
full depth and should be turned back to form a
hook. After the flashing is in place the reglet
should be filled caulked, using molten lead on flat
surfaces, and lead wool on vertical surfaces.
After caulking the reglet should be made smooth by
filling with elastic cement.
G. Step flashings:
1. Step flashings should be used where vertical
surfaces occur in connection with slopes.
2. They should be formed of separate pieces built into
the masonry as specified for cap flashings in
3. Steps should generally be 3", but should in no case
be less than 2", and should not be soldered. Lap
joints should be vertical.
H. Vent flashings:
1. All pipes passing through roofs should be flashed
2. Base flashings should extend out on the roof not
less than 6". They should be of sufficient length
to cover the slate course next below the pipe and
to extend up under the slate course above as far as
possible without puncture by nails.
I. Open valley flashings:
1. Open valleys should be not less than 4" wide.
NOTE: To determine the proper width for flashing,
start at the top with a width of 4", increase the
width one inch for every 8 feet of length of the
valley. Flashing pieces should be full length
sheets and of sufficient width to cover the open
portion of the valley and extend up under the slate
not less than 4" on each side.
2. Where two valleys of unequal size come together, or
where the areas drained by the valley are unequal,
there should be placed in the valley a "crimp"
angle or tee not less than 1" high. This "crimp"
may be formed in the valley sheet before placing,
or it may be made of a separate piece soldered to
the valley sheet.
J. Closed valley flashings:
1. Flashing pieces, for closed valley should be of
sufficient length to extend 2" above the top of
slate roofing piece and lap the flashing piece
below 3", and of width sufficient to extend up the
sides of the valley far enough to make the valley
2. They should be placed with the slate so that all
pieces are separated by a course of slate. Pieces
should be set so as to lap at least 3" and to be
entirely concealed by the slates. They should be
fastened by the nails at the top edge only.
K. Elastic cement or exterior grade caulk such as "Gutter-
Seal" (Dow), "Roof Sealant" (Alcoa), or approved equal.
1. A sticky, waterproof compound used to secure hip
and ridge slates.
2. It has a high melting point and low freezing point.
END OF SECTION