Can this be mounted to the top of a building and directed downwards?

You may mount Star Shower at any place and adjust the angle of laser towards the intended surface. You should be able to do this without any problem.

If you have in mind something like a long house or a storefront you could consider buying two units, but take into consideration the total covered area is 625 sq ft. Divide 625 by your intended length to find out how far away you should place it. As a rule of thumb, if you end up with a number under 10, you might consider more christmas light projector units.