Page 45 of Ref. guide states that a strategy for reducing stormwater volumes is to "capture it for reuse within the building". Table 2 on page 48 shows runoff coefficients for several surface types but does not account for rainwater catchment. We propose to create a surface type called "Roof, catchment area" and assign it a coefficient appropriate to the amount of captured water that will be retained in our storage tank. We can show that about 50% will stay in tank (for use in building) and 50% will bypass it at full tank conditions. Please confirm that this calculation method is acceptable.