Syphonic outlets are designed to reduce the entry of air into the system, if air reaches more than 40% of the volume of the pipe the syphonic action will stop. To reduce the amount of air entering the system a baffle plate is usually fitted over the orifice of the outlet, this not only reduces the amount of air being pulled into the outlet opening, it also stop a vortices forming that will draw air into the system rapidly. The best outlets have been tested and approved by the British Board of Agre ment. Outlets should also be designed to easily accommodate the water membrane of the roof, which comes in many forms.