Seriuosly, overhead costs will vary wildly between contractors. A couple of examples:
Dale P. operates out of his home. His wife handles the telco and the books. He has one, and sometimes two, helpers. He does 99% local work and his longest commute is about 20 minutes. His truck is also his personal transportation. In November 2002 his OH was calculated at 9.8-9.9%.
T.E.C., in a nearby town, on the other hand runs 8 trucks, an office staff of three and a roving super. T.E.C. has a large tool inventory as well as stock. Last Juns their OH was calced to be 19.7-21.1%.
Some EC's budget a number of man-hours to establish OH. Fall under the budget and OH is not met, over budget and it's profit. There are so many variables in OH that each EC must arrive at his own indirect costs. Tools, rent (or taxes), vehicles, insurance of all types, office equipment and supplies, bank loan interest, depreciation, etc. There is no catch-all percent.