No estimator, no matter how careful he may be, will be able to foresee all the material that will be required for a certain operation or who can foretell all items of expense. The caving of a trench which is being dug, damage to pipes by other workmen, breakage of fixtures, split pipes or fittings, defective work of any kind which must be made good, delay waiting for an inspector are all items of possible expense, any one or all of which might turn up to increase the cost of installation. In addition there are sundry items - such as screws, putty, red or white lead, graphite and other small articles, the cost of which is but slight when taken separately, but in the aggregate amounts to a tolerable sum. In order to cover all these items of expense an allowance, usually of a lump sum, is made under the classification of "Incidentals and Sundries."

The amount of the allowance, of course, will depend on the size of the operation, but should not be large for any size of building. All the known items of expense entering into the installation having already been determined a small allowance will do for incidentals, for twenty-five dollars will go a long way in purchasing the small items which have been overlooked in an ordinary-size job. The only reason for considering such materials is the desire of determining as nearly as possible the total net cost of the operation to the contractor. Whatever profit he figures on making he wishes to be free and clear, not cut down to a fraction of what he is entitled to by oversight in not allowing for all materials and contingencies.