Keystones have traditionally been trapezoid-shaped with smaller “wings” along the sides, it’s a shape that is easy to make out of concrete and is widely recognized as being that of a keystone. Materials such as polyurethane however, are formed in a mold and can be made to take on any shape.
Care does need to be taken when installing polyurethane products, as they tend to expand and contract more than regular cement-covered polystyrene trim. A combination of a chemical adhesive (such as PL Premium) and a mechanical adhesive (such as a screw) is typically recommended.
Decorative keystones from Decoramould are sent pre-primed and are ready to be painted (using an exterior-grade masonry paint) in the colour of your choice.
Home owners typically select the “classic keystone” shape for the majority of their windows and doors. Unique, eye-catching polyurethane keystones are reserved for the main entrance on a home. It provides a way of distinguishing that entrance from the others.
The more ornamental keystones can also be used on large bay windows, where the size is appropriate. The description can be deceptive, a 10″ or 12″ keystone may not sound like much but can actually be much larger than originally anticipated once depth and width are taken into consideration. Since keystones are inserted into the top of trim, the trim is usually cut on site to accommodate the keystone and exact measurements are usually not required.