Bring water to a boil in a large pot over medium high heat. Peel and cut the potatoes and rutabaga in 1 inch dices, and boil for 8-10 minutes, until tender. Strain.
Press the potatoes and rutabaga in a potato press. Alternatively, push them through a sieve using the backside of a large spoon. (Do not use a food mill or blender, as this will turn the mash rubbery.) Return to pot and stir in the butter.