With the site of the basement laid out accurately, either by means of surveying instruments or by using the right triangle method, excavation should proceed to remove all rocks and organic material, such as sod, bushes, trees and roots from within the limits of the proposed building area. The first 12 inches of top soil should stripped and stockpiled to be used later for grading and landscaping the site. Regardless of the extent of the excavation necessary to reach the desired depth of basement, excavations for the footings must extend to firm soil and to at least 6 inches below frost line. The latter requirement should be rigidly adhered to even though firm bearing soil is found at a shallower depth; footings 6 inches below frost line are completely safe against disruptions due to frost upheaval. The walls of the basement will extend from the top of the footings to the level of the first floor. They must be of sufficient strength and thickness to resist lateral pressures from adjacent earth or water and to support the vertical loads imposed with an ample factor of safety. They should extend to at least 8 inches above finished outside grade, be cast at least 8 inches thick, and they should never be of a thickness less than the wall immediately above them. If the house is to be of frame construction, anchor bolts should be cast within the top of the wall; normal practice for attaching wood sills is the use one-half by 8 inch bolts on 4 foot centers. If areaway surround, internal walls, steps or the like are also to be site cast later, dowel rods should be fitted in the external walls to provide the necessary tie in connections. While the basement floor slab is commonly placed directly on the ground, this is by no means the best practice for space that is intended to be livable. The ideal practice is to use essentially the same technique as for slab on ground basementless floors; the result will be a very much more comfortable and efficient floor. First step is then to provide a base course of coarse gravel or stone to act as a capillary breaks. This base course should not be less than 4 inches thick. A membrane vapor barrier should be provided over the base course. The slab itself should be not less than 4 inches thick and preferably cast as one course.