This chapter is not intended as a comprehensive review of the many external problems that affect buildings, but only draws attention to certain problems of particular interest or importance. Ground levels in relation to internal floor levels are obviously very important. In normal free-standing construction all horizontal damp-proof courses giving protection against rising dampness should be at least 150mm (6") above adjacent external ground level. On a sloping site this is best achieved by excavating into the rising slope to provide a protective clear area around the building, but this may not be possible on a steeply sloping site, and it may be necessary instead to adopt tanking or, in effect, basement construction. Such problems have already been considered in detail in Chapter 4, particularly in sections 4.8, 4.9 and 4.11.