Rheology is a key characteristic of fresh mortar since how the material behaves when worked with a trowel governs the quality of the work which the bricklayer can achieve. Sands from different parts of the UK have different gradings, fineness and degree and type of day contamination. The objective of this work was to investigate the influence of these variables on the rheology of fresh mortar. Thirty sands were selected from a much larger sampling survey (over 150) of building sands in the UK. Criteria for selection were (i) overall grading expressed as percentage passing a 300um sieve, (ii) degree of fine particle contamination expressed as percentage passing a 63um sieve and (iii) type and activity of clay present expressed as the melhylene blue value. Each sand was used in a standard mortar mix using 1:5.5 masonry cement: sand and the effect of water content on rheology of each mortar tested in the Viskomat. The rheology of the mortar conforms to the Bingham model and the yield stress and plastic viscosity can be determined from the data. All three fineness parameters affect the water required to achieve a given level of workability and the water demand of the sands varied over a two fold range. The effects of fineness interact in a complex way, with overall grading and MBV being dominant.