Jeffrey S. Levine
Philadelphia Historic Preservation Corporation
Inspired Fall 1987

SLATE vs. Asphalt: Comparing the Alternatives

A new slate roof may be the most expensive investment a religious building owner will ever face. However, over the long term, it is also one of the most cost-efficient. Slate is the most durable roofing material available. It will outlast several generations of asphalt roofing and require little or no maintenance over its entire life. It is conservatively estimated that Pennsylvania clear black slate will last 60 years, Vermont and New York slate 125 years and Buckingham Virginia slate at least 175 years. When original cost and average life are considered, a Buckingham Virginia slate roof will cost $2.14 per square per year at current prices over the life of the slate. (A square of roofing slate is enough to cover 100 square feet of plain roofing surface when laid with a 3-inch headlap.) This is less than any other roofing material. Premium grade asphalt shingles, on the other hand, will last only 20 years and cost an average of $4.25 per square per year. Hand split wood shingles will last approximately 30 years at a cost of $3.33 per square per year (see Table 1)

TABLE I
AVERAGE COST PER YEAR OF VARIOUS ROOFING MATERIALS

The above cost comparisons are for materials only and do not include the extra maintenance costs or the cost of multiple installations associated with other types of roofing material. Tin and terne plate roofs, for example, require a tremendous amount of maintenance as they must be painted every three years or so. Unlike wood and asphalt, slate is fireproof. And while clay tile roofs will last almost as long as slate, they are from 2 to 3 times heavier than slate and thus require stronger roof construction.

The average cost per square of roofing materials, as presented in Table 1, is a rather simplistic measure of the cost effectiveness of two alternative materials. After all, an asphalt roof will have to be replaced from 5 to 10 times during the life span of one slate roof. Because roofs must be installed and maintained over time, a more accurate method by which to compare the relative value of a slate roof to some other roofing material must take into account the time value of money. That is to say the cost of resources now and in the future. Present value is a numerical expression of this idea and is a way of valuing assets by discounting future cash flows. The present value of $1.00 received I year from now is really less than $1.00 because the dollar today can be invested to start earning interest immediately. We look to rates of return prevailing in the capital markets to determine how much to discount for time and for risk.

Table 2 is a summary of the net present value (NPV) calculations made to compare the cost, over time, of a slate roof to that of an asphalt roof and one covered with an artificial slate product. It was assumed that the slate roof would last 150 years, a standard grade asphalt roof 15 years and an artificial slate roof 40 years. The size of the roof to be covered was taken to be 40 squares. Inflation and the discount factor " or opportunity cost of capital, were assumed to remain constant at 4% and 7% respectively over the full 150 year life of the slate. Further, the cash flows considered include the cost of materials and labor only. Overhead and profit are not taken into consideration and maintenance costs are assumed to be relatively insignificant over time.

TABLE2
NET PRESENT VALUE
SLATE VERSUS ASPHALT AND ARTIFICIAL SLATE
CASH FLOW

As seen in Table 2, the NPV of the slate roof is $30,900 or the cost at the present time.

The NPV of the asphalt roof is $18,171 and that of the artificial slate roof is $31,714. A slate roof thus starts out costing nearly 5 times more than an asphalt roof and 11 / 2 times that of a roof sheathed with an artificial slate: $30,900 as compared to $6,400 and $21,775 respectively. When the fact that 11 asphalt roofs and 3 artificial slate roofs will have to be installed over the life of one slate roof. Taken into consideration along with the time value of money, it is found that a natural slate roof will cost less than an artificial slate roof and less than twice as much will as an asphalt roof will. The added cost over an asphalt roof is, then, often a reasonable price to pay in order to preserve the beauty and character of an historic church or synagogue. Given the significant aesthetic and practical advantages of slate and its reasonable price disadvantage to asphalt roofing, slate should be employed more often.

It is important to prepare today for the replacement of a slate roof 5, 10 or even 20 or 40 years from now. The first step is to properly maintain the roof by cleaning gutters semiannually and repairing broken or missing slates and deteriorated flashings promptly. The next step is to start a slate roof fund. One way to begin: Count the number of slates on your roof and then ask members of your congregation to buy one or two slates a year at $5 or $6 each. Those who buy reds can be so distinguished!