How do you create a chimney roof for a house with a difference of a rectangle and triangle in Inkscape?

The linked document is part of a larger package which is available at: Lesson 2 Paths united (teachcomputing.org) (free registration required). Looking at it I can't see that you could do a roof with a chimney by difference of a rectangle and triangle. What does work is to do it by union (my attempt is below). Union of shapes is covered by sheet A4, difference by A5, though both are simply a case of ensuring both shapes are selected and then using the path menu to choose how to combine them.