A trim element such as a door pilaster or frieze board, for attachment to a building wall, having front and back longitudinal faces, first and second longitudinal edges, and a longitudinal groove in the first edge. A mounting bracket for the wall has a forward strip that extends laterally and is selectively friction fit into the groove, a back strip that extends laterally on the back face of the element, and a connecting web between the forward and back strip that is flush with the first edge when the forward strip is fully inserted in the groove. A mounting flange for the wall extends laterally from the back face of the element, beyond the second longitudinal edge, and has a longitudinal channel along the second edge with a width that is effective to receive an edge of wall siding.