If icing is crumbly and not smoothing properly, it may need to be thinned down with a small amount of water.
For a smooth finish, a spatula is ideal. An angled spatula is best for keeping fingers away from the icing. You can use other kitchen tools like a butter knife or a spreader, but the serrated portion of the knife will leave marks.
Candies and sprinkles can be placed directly on an iced roof, no extra icing needed.