The flat jack is a flat flexible envelope which is filled with hydraulic fluid and can be pressurised using a pump and used to generate small movements in structures and apply
substantial forces. Suitable arrangements allow the measurement of compressive stress in members and the local stress strain behaviour with minimal, easily reparable damage of the masonry. The BRE version is described and some calibration work and application trials are discussed. The work indicates that for most purposes provided the ‘jack effective area’ has been measured for each individual size and the hole size is allowed for, all the calibration data for a range of wall sections and materials fits a common curve and can be used interchangeably.