In addition to 1/4 inch slope in 1 foot, you will need a vent to go up to the existing vent system to divert sewer gasses up to the roof. Every drain has a corresponding vent to maintain a neutral air pressure inside the system. If you drain your sink and one gallon of water goes down, then it will displace one gallon of air. This creats a high pressure area ahead of the water and a low pressure area behind it. If the water drains and the pressure is not relieved, it will drain the trap completely and sewer gasses will come back up the drain into your room. Just like when you stick a hose in a bucket and syphon water by maintaining the bucket higher than the outlet, the same effect will take place on a waste system leaving the trap dry.