Many model buildings have domed roofs, including models of Roman buildings, and buildings and churches for Christmas villages. There are several ways to easily make domed roofs for a range of scale buildings. They are easiest to build from gesso or paperclay applied to a solid base. Some suppliers produce domes in scale, or parts to make domes including Hirst Fantasy Architecture who create molds for blocks which can be used to build a wide range of domes with several diameters.
The simplest way to make small domes for gaming buildings, or towers for Christmas villages, is to shape paper clay or some other type of air dry clay over a shape strong enough to support the clay layer without collapsing or bending. The dome in the photo above is two inches in diameter at the base and is made from paperclay shaped over a half of a hollow plastic ball. The ball is a commonly available cake decoration available from cake decorating suppliers.
To make the small dome shown, the decoration was pulled apart and one side was coated with glue, to which a layer of paperclay was applied, pressed into shape and trimmed. The rounded side of a sculpting tool was pressed into the clay to make distinctive ridges for sections of the dome, and a wooden bead was glued to the center of the dome. When the paperclay has dried, it can be sanded and coated with a layer of gesso or paint to protect the paperclay.
Remember that there are a wide range of domed buildings that can be created for miniatures. Igloos, African huts and other structures can all have domed shapes or roofs.