The Simple Stack tool lets you stack stairs repeatedly without having to position yourself in the right orientation.
Consider a row of stairs used in roofs, depicted below:
The tool lets you stack stairs in the same orientation as the existing stairs, but from this angle:
To make the tool, name a Blaze Rod "Simple Stack" and right click the stairs on the face that you want to extend the stairs in. You will need stairs in your inventory.
SKCraft