Leaky overflow pipe coming from loft

The overflow pipe frm my loft is leaking out the back of my house. How can I stop this?

check the float valve in the tank is shutting fully

You'll probably find you have 2 tanks in your loft, one to supply cold water to your hot water tank and one to act as a fill and expension tank for your central heating / hot water; first you need to find out which one is causing the overflow. As seco says, it could just be the ball valve not closing properly, inwhich case it may need replacing. It may be that the ball arm just needs adjusting so it cuts off the tank replenishment earlier. If the ball valve is the cause, does pushing up gently on the ball arm stop the flow from the valve? If your F & E tank is overflowing it could be coming from system vent pipe which hangs over this tank. The F & E tank will be the smaller of the two.