Foundation engineering does not have the same prescion as structural engineering, due to the variability of the soil properties. Therefore th foundations loads need to be reasonably accurate, but not exact. The AISC manual has tables of weights of common building materials and building systems which greatly aid the computation of the weight. However except for the simplest structures, it is still somewhat time consuming. Consider how you are supporting the loads with the foundation. If you are going to do a single mat the it doesn't matter so much. If you are going to use individual spread footings or pile caps, take careful notes on how much each componets weighs and try to break areas up into logical groups that would be supported by the foundtion