213. The rule for bridging joists is the same as that for single joisting (see Art. 195). They seldom need be more than 2 inches in thickness, except for ground floors, where they are laid upon sleepers; in which case the depth may be found to a breadth of 2 inches, and an inch may be added to the breadth, on account of the situation; as when proper care is not taken to drain and ventilate the under-side of a ground floor, the joists are subject to very rapid decay. It is a good practice to strew smiths' ashes, or even common ashes, under such floors, to prevent the growth of fungi. The ashes and scoriae from a foundry, or any ashes that contain much iron, are the best. Mr. Batson found this an effectual remedy for the dry rot. He filled a space below the floor of 2 feet in depth with anchor-smiths' ashes, and also charred the sleepers.*

Fig. 72.