Abstract: A sill for supporting a wall panel such as window panels for a modular solarium comprises a forward or outer sill, an intermediate rail, an inner channel, and an inner flange. The intermediate rail extends laterally inward from the outer sill. The rail includes a ramp and a raised surface. The ramp defines an outermost portion of the rail and extends upwardly from the outer sill to the raised surface. The rail terminates at the channel. The channel extends laterally between the rail and the flange. The bottom of a wall panel fits over the rail and has an inner leg which extends into the channel. Fasteners secure the inner leg on the wall panel to the inner flange.