There are two types of drains, those that dispose of surface water (eg from roofs, hard paved areas) and those that dispose of foul waste water (from bathrooms and kitchens). In older buildings, there is often a mixed use system, where the surface water is used to flush away the waste materials from the toilet, bathroom and kitchen. Due to their age, older drainage pipes are often defective and leak. Leaking drains close to foundations can cause a swelling of the nearby sub-soil if it is of a clay type (known as ground heave) which can damage solid ground floors. Leaking drains can also result in localised subsidence if the nearby sub-soil if of a gravel/sandy type (as the finer particles are washed away by the leaking water/liquid). In areas with a chalk sub-soil (which is chemically alkaline), leaking surface water drains can cause localised subsidence, as rainwater is slightly acidic. Leaking drains will also attract the roots form nearby trees/hedges, which can damage the drain further as the roots grow. Even new drains or older drains in good condition can be damaged by the roots of nearby trees/hedges. A CCTV survey of the drains can usually identify any leaks or other defects, with the affected drains being repaired/replaced as deemed necessary.