I want to move my washing machine out of the kitchen and into the garage where the tumble dryer is already located. The garage is directly below the bathroom and the cast iron soil pipe comes down the garage wall. My question - can the waste from the washing machine be run directly into the soil stack? And if so, would it be relatively straightforward for a plumber to cut into the cast iron pipe to attach it? Would it need any sort of one-way valve or just a simple trap to avoid any risk of, er, toilet waste backing into the washing machine? 
