Are your walls on the right layer? The cutting feature uses the layer to identify the entity type so that it doesn't cut the floor instead. The walls have to be on the WALLS layer for the cutting to work.

Also, it is an advantage to turn off snap when working with the cutting symbols, especially if you are trying to hit angled walls or walls not exactly on the snap grid.

I've got UNION ready, but haven't gotten to DIFFERENCE yet.

If they are partially overlapping, the simplest way to merge them is with the TRACED command. This will create a new poly, so delete the other two afterwards, and change properties on the new one if required.

The link below shows how to use TRACED. The example uses a single bitmap image, but it works just as well on a set of polys.

Check this FAQ entry. It contains a pair of video links in addition to the text.

Probably best to use one of the roof tile fills from one of the city styles?