Select the roofslab and look inside the Selection Info palette, there you should be able to adjust thickness, change to edit mode and Select it again, then you can activate Edit Pivot, selct the edge you want to use and set the slope or Rise/run inside Selection Info palette.
Then you press the Shift key and click on the Pivot edge and there will come a menu in Inspector bar where you can choose changing between Plumb, Squere or what angle you want
V18, V19, V20, V21, 2015/ 16/ 17 Pro. Platinum
Deluxe V20, V21, 2015/ 16/ 17
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB