General Information On Slate Shingles

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Slate Shingles
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    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
         salvage value.

    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, 


    A.   Failure of a slate roof is generally due to poor
         installation methods.

         1.   Nails that are driven too far may cause tension in
              the tightly held slate resulting in cracking of
              individual units.

         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
         such ribbons.

    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
         Gambrel roofs.

    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
         Pennsylvania quarries.

    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


    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
              and uneven.  

    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.
              Free brochure)

        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.
              Free brochure)

         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.

              (Gray black)

         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.
              Free brochure)

         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


    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.

         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.

    B.   Nails:  

         1.   All nails, rivets, and similar fastenings, if any,
              used throughout the work should be of best grade
              hard copper.  

         2.   Nails should be large flat-head copper wire nails.
              WIRE NAILS.

         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

    C.   Flashings:  

         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#.
                   Consult manufacturer.

              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
              every 8".  

         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.

    F.   Reglets:  

         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
              and counterflashed.  

         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
              inches deep.  

         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

Last Reviewed 2014-05-28