Hi Simon,
You have added your roofing material to the drawing, so it's available to use. Click your roof slab, right-click and select "Edit Roof Slab Style". Go to the Materials tab, where you will see two headings, Component and Material. Currently the Component Roof shows that the Standard material is being used. You have to click on the word Standard in order to access the drop down listing. Select the Spanish.Red that you have added to the drawing, click all OKs and you will see your roofing material applied. If the Spanish tile is showing on the sides and bottom of your roof, repeat Edit Roof Slab, again go to the Materials tab, but this time click the icon in the upper right (Edit Material), the click the icon in the upper right (Edit Display Properties), uncheck all but TOP, and under Mapping, select "Same as Surface Hatch".
I notice that you have edited the Roof Slab Edge Style with no success. Here is a quick trick you may want to try. Copy your existing roof slab up the same distance as it's thickness. From an isometric view, you can simply copy from the lower endpoint of an edge to the upper endpoint of the same edge. Now, edit the properties of the copy, and change the thickness to something closer to the thickness of the tiles alone. Since the thickness will decrease from the bottom, you will then need to move the new slab back down (using an Isometric view and endpoint snaps).
Now, select your original slab, right click "Edit Object Display", go to the Materials Tab, and select a material that is already in your drawing to see how it looks (one of the painted white ones will work). The "Edit Object Display" changes ONLY the object you select, so you will have to repeat the command for the other slab.