An out-door altar is preferable to one indoors. But in my country of England the climate is obviously against it. My altar is a small table, I can move it outside in good weather, or indoors when it is too cold to be outside. It is the same with rituals. Best practise is when a witch is skyclad;naked.Not always possible in bad weather! If you have an altar out-doors make sure the area of your circle is clean! (By the way, that is what a witches broom is for; sweeping!) It is not so much what the altar is made of. It is what's on the altar that counts!