I'm quite new to Archicad so still getting used to it's quirks and terminology. This issue had been bothering me for ages but I've managed to find a workaround with help from my tutor. In the Floor Plan Cut Plane Settings I've chosen a height which is above the level of my roof and I've also set Relative Floor Plan Range to be e.g. 2 stories above. With these settings and a cover fill I can now see my roof on my site plan. Hope this help anybody coming to this post.
Student Building Designer
AC23 • MacBook Pro 2.7GHz | 16 gb ram • OSX 10.13.6