Calculating the dynamic effects of surface overshadowing is a major part of most thermal analysis engines. It also represents a significant overhead in the analysis process yet, once a run is complete, this information is usually lost and must be entirely recalculated before the next run. However, detailed overshadowing for specific surfaces is valuable information to the designer. It is also useful for many other forms of building performance analysis such as detailed shading design, material selection, daylighting, right-to-light and even solar- access calculations. This paper proposes the widespread use of pre- calculated shading masks in thermal analysis engines. It discusses why this is important, the different techniques for calculating and storing these masks as well as the benefits and disadvantages of different methods. A comparison of both the accuracy and computational overhead of different sky-subdivision techniques is also presented. More importantly, it shows that complex effects such as solar reflection and incidence effects can be accommodated within the masks themselves and how they can also be used in the calculation of average daylight factors, glare potential, radiant exchange and even internal surface solar tracking.