=9. Estimate of Cost.=—In making an estimate of cost the information should be presented in a readable and easily comprehended manner. It is necessary that the items be clearly defined and that all items be included. The method of determining the costs of doubtful items such as depreciation, interest charges, labor, etc., and the probability of the fluctuation of the costs of certain items should be explained.
The engineer’s estimate may be divided somewhat as follows:
Overhead. This may include construction plant, office expense, supervision, bond, interest on borrowed capital, insurance, transportation, etc. The amount of the item is seldom less than 15 per cent and is usually over 20 per cent of the contract price.
Contingencies. This allowance is usually 10 to 15 per cent of the contract price.
Profit. This should be from 5 to 10 per cent of the sum of the four preceding items.
The contract price is the sum of these items. To this may be added:
Engineering. 2 to 5 per cent of the contract price.
Extra Work. Zero to 15 per cent of the contract price; dependent on the character of the work, the completeness of the preliminary information, the completeness of the plans, etc.
Purchase of land, rights of way, etc., etc.
The cost of the sewer may be stated as so much per linear foot for different sizes of pipe, including all appurtenances such as manholes, catch-basins, etc., or the items may be separated in great detail somewhat as follows:
Earth excavation, per cu. yd. Rock excavation, per cu. yd. Backfill, per cu. yd. Brick manholes, 3 feet by 4 feet, per foot of depth. Vitrified sewer pipe with cement joints, in place, ... inches in diameter, 0 to 6 feet deep 6 to 8 feet deep 8 to 10 feet deep Repaving, macadam per sq. yd. asphalt per sq. yd. Flush-tanks, ... gal. capacity, per tank. Service pipes to flush-tanks, per linear foot., etc., etc.
These methods represent the two extremes of presenting cost estimates. Each method, or modification thereof, may have its use, dependent on circumstances.
Reliable cost data are difficult to obtain. Lists of prices of materials and labor are published in certain engineering and trade periodicals. The Handbook of Cost Data by H. P. Gillette contains lists of the amount of material and labor used on certain specific jobs and types of construction. The price of labor and materials on the local market can be obtained from the local Chamber of Commerce, contractors and other employers of labor, and dealers in the desired commodities. Contract prices for sewerage work published in the construction news sections of engineering periodicals may be a guide to the judgment of the probable cost of proposed work, but are generally dangerous to rely upon as full details are lacking in the description of the work. A wide experience in the collection and use of cost data is the desirable qualification for making estimates of cost. It is possessed by few and is not an infallible aid to the judgment.