Water conservation is seen as an inherent prerequisite to the sustainable provision of utility services within the building envelope; however concerns have been raised as water usage appears to be reducing, that there will not be enough water to keep the drains clean. The concept that ‘dry drains’ could be a probable consequence of water conservation arose in the US where ‘concerned groups’ argue that further reductions in w.c. flush volume should be resisted as current water conservation measures reduce the throughflow in the drains to the extent that ‘drainline carry’ becomes impossible. This question is dealt with in this chapter together with the need for simulation and in-depth analysis of drain line carry in the context of water conservation strategies. The implications of reduced flush operation on drainage network design will also be discussed, using two case studies: one which looks at the implications of increasing collection drain size as a result of a quirk in legislative prudency (changes to building regulations in England and Wales) and the other which looks at the implications of allowing even further reductions in flush volume in building regulations with the goal of improving sustainability and reducing carbon emissions in Scotland. The case studies make extensive use of the simulation program DRAINET to evaluate optimum operating condition. Drain diameter and gradient choice are discussed both within the horizontal branches of the building drainage network and also in the below-ground connections to the sewer network. The importance of wave attenuation, as discussed in Chapter 4 will be reiterated here, and an evaluation of the link between water usage and carbon emissions will be made for the second case study.