Installation Diagram - Window Sills
Basic Specification

Window sills usually refer to exterior, rather than interior slate installations. For exterior applications, Clear grade slate should be used. Slate for exterior use should conform to A.S.T.M. C-629 for Class I slate.

Clear Grade - Clear slate is free of Ribbons and most markings. It is a blue-gray to blue-black color and if sealed, oiled or otherwise treated, it is approximately "jet-black". Clear slate is supplied in Natural Cleft, Machine Gauged, Sand Rubbed or Honed finishes.

Ribbon Grade - Ribbon slate is characterized by an ornamental dark band which, in the case of window sills, usually runs along the length of the sill. A ribbon may be wide, narrow or even double. In a wide sill, there may possibly be more than one ribbon. Note: Although Ribbon grade is not recommended for exterior use, with proper pitch and the ability to dry, a ribbon sill may be serviceable for over twenty-five years or more. Ribbon slate is supplied in Machine Gauged, Sand Rubbed or Honed finishes.

Face Finishes - Natural Cleft Finish is a rough textured, split face finish. The bottom is gauged to an even plane and the thickness, although varying due to the facial deviation of the cleft, is constant within the maximum thickness specified. (NOTE: Natural cleft finish may not be suitable for use in installations where stills are installed in multiple window runs. The variation of facial thickness with regard to the edge plane is more pronounced when seen in a run of sills rather than a single still.) Machine Gauged Finish is a smooth, even finish which approaches the honed finish. However, slight swirl marks are usually visible. Sand Rubbed Finish is a smooth, even finish with a slight stipple equal to about 80 grit sand paper. Honed Finish is a smooth, satin finish equal to about 240 grit.

Edge Finishes - Hone & Arris Edge is a honed front edge and either a 1/8" chamfer or a 1/8" radius on the top front edge to remove the sharpness. This may be specified in conjunction with any of the facial finishes, however in the case of Natural Cleft finish, the 1/8" radius is normally used rather than the chamfer. Arris Edge is the 1/8" chamfer or 1/8" arris on the top front edge without honing the front edge. Sawn Edges are edges directly from the saw and are usually very smooth and sharp as they are cut with a diamond saw. Other Edge Profiles are subject to availability of tooling and should be investigated prior to specifying.

Thickness - Window sills are usually supplied in 3/4", 1", 1 1/4", 1 1/2", & 2" thicknesses. Other thicknesses are available to comply with special field requirements.

Treatments - Sealers usually darken the slate and give a glossy appearance. Natural cleft finish and Sand Rubbed finish may be sealed suitably, however, sealers may not adhere nor give a desirable appearance to Machine gauged or Honed finish. Impregnators usually do not as radically alter the color of the slate and give a more satin finish. As with sealers, Machine gauged and Honed finish may not accept the application of an impregnator. Oiled Finish is usually a mixture of 50% Boiled linseed oil and 50% turpentine. This is suitable for all facial finishes and is renewed only as often as desired. Since slate is very dense and non-absorbing, a finish is only required to facilitate easier maintenance.

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  • The Structural Slate Company
    222 East Main Street
    P.O Box 187
    Pen Argyl, Pennsylvania 18072-01
    1.800.67.SLATE or Fax: 610-863-7016