French slates were usually not larger than the British doubles. It was the part of the slate exposed on the roof, known as the margin or gauge, that determined the effect. In speaking of slate size, the method of application was as important as the quarry-produced size because the margin or exposed part could be anywhere from a third to nearly half the slate length depending on the lap used. Assuming an average lap of three inches, the French slate roof showed four inches to the weather while the British “Duchess” slate exposed a margin of 11 inches. The decorative impact of small slates was to increase the scale of the roof. Practically, the choice of slate size was limited by roof pitch and the scale of the building. The general rule used by roofing contractors in Toronto according to 19th-century encyclopedias of architecture was the steeper the pitch and more exposed the position the smaller slating should be. Conversely a low-sloped roof required large slates, but not if the building was small. The thickness of slates was a second factor which could substantially alter the look of a roof. A quarried block of slate was split with hammer and chisel along the cleavage planes into individual laminate or slates. The thinner the laminate the better the quality of the slate. The best cleavage usually occurred in the darker colored black, blue and grey slates which were split in sizes about 3/16 of an inch. Some Welsh slates were as thin as 1/32 of an inch, but these were not recommended for reasons of breakage. Colored green, purple and red slates were ordinarily coarser, splitting to about 1/4-inch thick or more. Architects who favored texture on a roof preferred the heavier colored slates instead of the thinner, smoother “blue” slates which they compared to metal.
Before the mass production of slate and standardization of thicknesses, builders commonly sorted roofing slate placing the thickest at the lower edge of the roof, the medium towards the centre of the slope and the lightest at the ridge. To achieve the plastic or sculptural appearance of such roofs, 20th-century architects specified slates of diminishing thicknesses for what they labeled the “graduated” slate roof.
A third decorative determinant in slate roofing was colour. Slate colors resulted from variations in mineral composition: chlorite in the greens, the matte in the purples, and iron oxide and hematite in the reds. Specific quarries were usually identified with a particular colour from the grey slates of Angers, the purple of the Ardennes, the blue slates of Bangor in Carnarvon- shire and the esteemed bluish-green of Kendall in Westmorland. The study of slate sources also shows several colors often occurred within the one quarry, or as Vermont’s variegated purple and green slates indicate, within the one slate. This great variety of colours and shades created unlimited possibilities for colour effects from the uniformly coloured slate roof in cold or warm values to the contrasting multi-colored or polychrome surface in lively hues or subtle shadings. To compound the choice, slates could be permanently coloured or “fading” respectively producing the unfading tone or a weathered appearance which, several years after installation, was radically different from its original colour. Even slate luster, while not strictly colour, was used to obtain variation. The 19th-century architect, Viollct-le-Duc thus records how French slaters applied the metallic coloured slate of the Anjou region in diverse ways to produce patterns from the sun’s reflection. The fourth and perhaps most ornamental variable in slate roofing was cut. Cut refers not to the rectangular size but to the shape of the tail or exposed edge of the slate. Slate shapes assumed many forms, some specific to the slating craft itself and others borrowed from the centuries old decorative arts repertoire that drew its inspiration from the feathers, palm-leaves and fish scales of nature. There were about six best known shapes. Plain or common slating referred to tail edges chipped to a straight horizontal line parallel to the upper edge or slate head. Sometimes the corner angles of plain slates were cut away forming broken lines. When the tail was kept straight but the whole slate reduced in size to a geometric square and applied so that only half the square appeared above the slate under it, the result was lozenge or diamond form. The lozenge shape was also obtained by cutting a sharp line to the middle axis of the tail and laying the slate so that the visible part formed a net of diamonds. If the angle formed with the centre was less acute the cut formed a hexagon. Slates were also rounded, an ornamental option that originated in the necessity of covering conical roofs without the leaky and unsightly protrusion of square corners. Variations on this semi-circular shape ranged from the slightly pointed American Cottage to the more acutely pointed Gothic.
All the slate cuts, except the plain, belonged to the decorative arts family of scales which refers to those ornaments composed of symmetrical plates overlapping each other. Imitating the scales that cover fish, scale ornamentation was used by almost every people and at all times by a great number of industries — “the roofer in terracotta, wood or slate, the sculptor, the cabinet-maker and joiner, the zinc and copper worker, the jeweller Bands, borders, squares and other decorative elements were secondary designs developed by the slate roofer and other artisans to break large scaly surfaces as well as accentuate and alternate different cuts. The quincunx (arrangement of five objects in a square or rectangle) and the band or strip of sawtooth scales were just two of the ancient decorative devices that reappeared in the
slating fashions of the 19th century.