Pre heat oven to 350 degrees.
Add margarine into flour and add enough cold water to bind and form a ball.
Divide it into two.
Keep chilled in the fridge for 10 mins.
Meanwhile, mix together apple, sugar, cinnamon, salt and flour.
Roll one part and line the pie tin.
Layer the uncooked apple mix on top.
Dot the butter on top.
Roll the other part of the dough and place on top.
Seal the edge.
Make few cuts on top of the pastry to let the steam out.
Bake at 350 degrees for 35 mins.
Keep the pie in the tin for 10 mins, before cutting.