Estimating Slate Quantities for Roof Areas


Estimating the amount of slate needed to complete a slate roof installation requires accurate measurements to be taken to calculate the area of the field of the roof. One roofing square is equal to 100 square feet. Once the area of the roof is known, the number of squares can be calculated by dividing the area by 100. A roof area of 4,000 square feet, for example, is equivalent to 40 squares. Slate roofing in North America is sold by the square at a 3-inch headlap, so that enough slate, regardless of the size ordered, is provided to cover 100 square feet of roof area when installed with a 3-inch headlap.

In addition to measuring and calculating the field area, it is also necessary to measure and calculate other specific component portions of the roof that require additional amounts or specific sizes of slate to complete the project. These specific component areas include hip and ridge slates, undereave slates for roof eaves and distinct changes in the roof slope that require a restart of the slate in the field of the roof,as well as wider slates that might be needed adjacent to valleys and hips, and, depending on the size of the field slates, rakes and vertical walls, as well. Dormer cheek walls and other side wall areas scheduled to receive slate shingle cladding must be measured and calculated, as well.

For roofs with multiple facets and angles an additional amount of slate must be calculated for cutting waste at hips and valleys or other angled portions of the roof where a portion of the slate will be cut off and discarded in order to complete the finished roof. A common method is to allow one square of additional slate for every 100 linear feet of cut work. This can vary, however, depending on the physical properties of the slate used.

Some allowance must also be made for breakage in shipping and handling that may occur while transporting the slate from quarry to rooftop. Shipping and handling breakage can vary depending on the physical properties of the slate, the method of shipment and packaging, and the method of transporting the slate from grade to the rooftop. A typical shipping and handling breakage allowance is 5 percent of the total number of squares of field slate and specific component slate, but this allowance can vary down to just a few broken slates, or an amount greater than 5 percent if rough treatment is encountered en route to the rooftop or during installation.

Graduated slate roofs require that the roof areas be calculated in increments from eave to ridge with allowances made for the number of courses of each size of slate to be used. In so doing, allowances should also be made for the additional 1 inch of headlap that occurs when transitioning between lengths of slate in the field.

For hip and ridge slate in a saddle pattern, or similar method of hip and ridge slating, once the size of hip and ridge slate is determined and the number of linear feet of hip and ridge calculated, it is common to allow a 50 percent overlap of the pieces to be installed and allocate the same number of pieces of slate for each side of the hip or ridge line. For instance, 16-inch long hip or ridge slates are commonly assumed to have an 8-inch exposure for estimating purposes.

Undereave slates, also referred to as starter course slates, are commonly turned sideways and installed with their long dimension (length) along the eaves and their short dimension (width) extending up the roof slope. Undereave slates must be at least 3 inches wider than the exposure of the field slates used at the eave line (4 inches wider for slopes less than 8:12; 2 inches wider for slopes of 20:12 or greater) to maintain the minimum required headlap. For an 18-inch slate with a 3-inch headlap and courses exposed at 7-1/2 inches to the weather in the roof field, the undereave slate must be at least 10-1/2 inches wide. This would require the use of 11-inch wide undereave slates, the next standard size produced in North America. The length of undereave slates is related to the size of slate used in the roof field. The length may be the same as the width of the field slate, twice the width of the field slate, or other appropriate length such that the bond lines of the undereave slate are no closer than 3 inches away from the bond lines of the first course of slate.

When ordering slate for a given project, the number of pieces of each size calculated for component parts of the roof (hips, ridges, undereave course(s), wider slates for use adjacent to valleys, hips, etc.) are commonly converted back into squares for the size and coverage of that slate with 3-inch headlaps and orders placed by the square. For example, if 610 undereave slates measuring 18 x 11 inches are required for a project, this is converted to squares by dividing by 175 (the number of pieces of 18 x 11-inch slate in a square when laid with a 3-inch headlap per Table 1.1). Thus, 610/175 = 3.49, which is rounded up to 3.5 squares. Some estimators use this same method to calculate the number of squares of field slate required (in lieu of dividing the roof area by 100 as outlined earlier). Hip and ridge component slates can also be ordered by the piece, or by the linear foot. Undereave slates can be ordered by the linear foot, as well.

When installing slate with either increased or decreased headlaps to accommodate roof slopes of less than 8:12 or over 20:12, it is necessary to add (in the case of 4-inch headlap), or delete (in the case of 2-inch headlap), varying amounts of slate to the quantities calculated for slate installed at a 3-inch headlap. The incremental increase or decrease in the amount of slate required per square varies with the length of the slate. Appendix I provides appropriate multipliers for each standard length of slate produced in North America.

Since it is often difficult to foresee the actual amount of breakage that may occur on a given project or with any given slate, it is good practice to double-check quantities of slate remaining on site as work progresses to ensure enough slate is ordered and on hand for timely completion of the project. Lastly, best practice would suggest, and often construction contract specifications require, that an additional amount of slate be left on site for the owner’s stock to facilitate future repairs with matching slate.