In general terms the fixing densities for Dri Flex remedial wall ties would be the same as new build - 2.5 per m² or 450mm vertically and 900mm horizontally in a domino five pattern. This density would, however, be subject to on site testing to ensure that the required tensile loadings are being achieved. It should also be borne in mind that around all openings - doors and windows - ties should be installed at no more than 300mm vertical centres and no more than 225mm from the edge of the opening. BRE Digest 401 gives more information on the proof load test requirements.
If the masonry is so weak or friable that the required proof test load for standard density fixings cannot be achieved, it is quite acceptable to increase the density of Dri Flex ties to ensure that the overall loading per m² is achieved.
Lowering the installation density below the standard is not recommended.