Such a comparison will define very clearly the endurance of the parts which go to make the structure a whole, and will make clear the extent or number of occurrences of repair or renewal which can be expected to be required within the period which may be found to be that due to the most durable and least frequently repaired portion.

Thus if we may assume a building to be composed only of stone and lumber, it would be natural to expect that the former element would exceed the other in durability and also in ability to withstand the effects of repair. Wood is less durable, and, besides, would in all probability be subjected to strains, shocks, and variations of temperatures, and, moreover, the effect of any changes or repairs would be to weaken its general character in a manner which masonry would not feel; and as it is used for a variety of purposes, some of which involve more exposure to wear than others, such as flooring, it would he safe to say if a life were allotted to the lumber it would not exceed 50% of the more durable element in the structure.

Therefore the necessity for reconstitution of the woodwork of the structure would occur at one half the time of life of the more durable element, and during the second period the material would not possess the same durability as before, nor would the combined structure, as a patched or partly remade construction. The mean duration of the two, if equal in extent or value, would therefore be 75% of that of the longer-lived element.

The proportion which the component parts of a building bear to each other may vary greatly, but as the question of provision for their replacement is a financial matter, their relations are most readily expressed in their monetary values, or, in other words, in the proportion of their original cost to that of the whole structure. Thus if the financial value of the two elements in the foregoing instance be taken as two of wood to one of stone, and the life of the masonry alone, if uncombined with lumber, to be one hundred years, then the relative existence of the combined structure becomes:

[(2x50)+(1x100)] / 3 = 66 years

An assignment of the durable life of different building components was made about thirty years ago, by combining and averaging the views and opinions of a number of expert builders in different parts of the country, forming a schedule of great interest, which is still regarded as of authority, and is valuable as a basis for determining the relative durability of various materials in buildings.

These opinions assigned to the most substantial parts, such as brickwork, stonework, and fireproof floors, a life of 66 vears when erected in combinations of ordinary brick and frame constructions, and of 75 years when forming part of substantial brick and metal buildings, and also laid down the life to be accorded to each of a number of other elements entering into such combinations.

Fig. 10. Scale of years

Fig. 10. Scale of years.

By applying these terms of durable existence to the values of parts of a series of buildings, and plotting these values in diagrammatic form, it is found that they fall within the line of a regular curve, indicating the correctness of the assumptions.

In Fig. 10 the two curves A and B are respectively those of the average values of twenty-two frame buildings and of thirty-six brick-and-stone buildings. These values are set off on the vertical scale of depreciation, and their respective anticipated life is set off on the horizontal scale of years. In curve A, the most durable elements were assumed to have a life of 55 years, in B of 65 years, and in C of 75 years.

The curve A of frame buildings, as it is reasonable to expect, indicates earlier and more rapid effects of declining life, and results in a mean life of 34 years. Curve B of the more substantial buildings, by reason of the greater extent and value of the more durable elements in its composition, declines less rapidly at first, though it eventually takes the same characteristic form, and results in a mean life of 44 years.

To these is added curve C, based on the values of the parts of a steel-framed fireproof office building, the mean life of which becomes 56 years. The solidity of general construction is well indicated by the shape of the curve, showing, as it does, a slow rate of early depreciation.

The evident harmony of this system is apparent, and results in the method herein advanced for the determination of average life, viz., to relate the cost values to the existence of each individual material or component of a structure, and therefrom to deduce the financial mean term of the whole, which is the period of physical deterioration to be covered by financial provision or sinking-fund for its amortization.

The method involves the assignment of maximum longevity to the most durable elements of the combination, to which the less durable may be related. By unduly increasing this period, it would appear that the average life could be extended indefinitely, but such a method carries with it its own contradiction, because the physical life cannot be greatly extended beyond the economic or useful period of existence, and the economic usefulness of all types of buildings is clearly bounded by some reasonable lapse of time, and especially so in the case of buildings operated for commercial profit, to which this subject is particularly pertinent. This is shown in the plotting of three periods in Fig. 11, where an office building is plotted on the basis of the life of its most durable parts, of 66, 75, and 100 years. While the two former show harmonious characteristics, the latter is distorted.

Not only the opinions expressed by authorities such as those already quoted, but consideration of the conditions of modern building construction, lead to the conclusion that, even with most careful usage and the best of modern construction, the physical existence of buildings is limited to a period within that usually regarded as two generations.