To ensure the building is adequately protected from water ingress, the rainwater system must be designed to remove all the water that falls on the roof quickly. BS EN 12056 part 3 provides both data and a method for calculating the highest expected rainfall intensity expected for all regions of the UK plus a method for deciding on the safety factor best suited for various scenario’s.
The designer must decide how best to use this data; the initial calculation may give the designer a very high intensity rate if they input either a high safety factor or an excessively high return period, i.e. 100 years.
Using 100 years as the return period for a project in London would give the following results:
Safety Factor Category 2 222mm/Hr
Safety Factor Category 3 270mm/Hr
Safety Factor Category 4 600mm/Hr
Using the highest intensity rate will result in the designer creating an oversized system rarely, if at all, going syphonic having a dramatic effect on the pipe diameters, however the chance that it will be utilised is extremely unlikely, plus in the normal life span of the property it will probably never operate syphonicaly but only as a gravity system loosing all of the advantages of a well designed syphonic system.
