Common causes of damp
Leaking high-level gutters and/or rainwater pipes - these are a common cause of low dampness in walls, especially in solid external walls.
Gutters and rainwater pipes can get blocked with falling leaves and other debris and should be cleaned out and checked for defects from time to time. The Society for the Protection of Ancient Buildings (SPAB) has a useful guide to guttering maintenance - for all types of building - at www.spab.org.uk/media-centre/press-releases/press-release/?ContentID=273.
High abutting external ground levels - either at or above the line of the original damp proof course (DPC), or at or above the level of the internal floor. As a guide, ground levels externally should be a minimum of 15cm below the DPC.
Blocked external vents to ground floors - the void underneath a suspended floor must be ventilated to prevent a build-up of moisture, as excessive moisture can lead to low-level dampness in the walls and will eventually decay a suspended wooden floor. If you have a suspended floor, check that the air bricks providing ventilation aren’t blocked, obstructed or missing
Chimneys and fire hearths - open chimney pots can allow rainwater to penetrate into the chimney cavity and permeate right down through the building. Also, due to the construction of some buildings, usually pre-Edwardian ones, dampness can be drawn up from the ground below into the chimney breast and reveal walls.
General defects - lots of common defects in buildings can result in internal dampness, including poor or defective mortar courses in external walls, defective sealant around windows and doors, poorly maintained wooden windows and doors, and broken, cracked or defective stone window sills.