In a sauce pan mix the water and flour. Stir well and combine them. Make sure that there are no lumps.
Turn on the heat and allow the mixture to cook on medium heat stirring continuously until the roux thickens and you see traces in the mixture as you stir.
Turn the heat off at this point. It will take only 2-3 minutes approximately.
The mixture should be cooked at medium heat. If using thermometer the temperature should be 149°F or 65°C.
Transfer to a dry bowl and cover it with cling wrap, making sure the cling wrap sticks onto the surface of the mixture. This is to prevent a film from forming on the surface.
Allow to cool down completely before adding the the dough mixture.
If not using the tangzhong immediately, cover it and refrigerate. This can be used within 2-3 days.