I think I do not understand this business of what is a seamless texture, what is a tileable and seamless texture and how you make them.
In this context "seamless" and "tileable" mean the same thing. When you place multiple copies of the texture next to each other, in any direction (tiling), they should appear as one large texture. No seams. Strictly speaking, all textures can be tiled, but not all of them will appear seamless. Hope that's not just making things more confusing.
Texture files you create for use in SU have a size (obviously), but once you create a Material in SU using a texture, SU also assigns that texture a size ratio. This ratio can be adjusted by you to make the material easier to place and adjust.
In the first pic you can see a building I created in SU. The Edit tab of the Materials window is open. (1) shows the 64 x 64 seamless texture I used to create the material. (2) shows the height and width ratio assigned to it by SU. It seems to use .10 by default. The material has been applied to the building, but the building looks grey rather than corrugated like the texture. This is because the .10 ratio makes each tile, each instance of the texture, so tiny that it just blurs into grey. No good.
In the second pic I've changed the ratio to 10. Now you can see the stripes of the texture, but they are way too big. In the third pic I've changed the ratio to 3, and that looks just right to me.
If your roof tiles are too big when you place them on your building, just change the ratio of that material to something smaller. Once the ratio is adjusted you can apply the material to any number of surfaces and the tiles will always be exactly the same size...no need to adjust for each surface.
Hope this helps.