A fenestration system includes a back angle anchor providing a first anchor portion and a second anchor portion extending from the first anchor portion and providing an attachment receptor, the back angle anchor being securable to an underlying substrate at the first anchor portion, a subsill providing a first subsill portion and a second subsill portion extending from the first subsill portion, and an anchor hook extending from the second subsill portion and defining a subsill cavity sized to receive the attachment receptor. The subsill is attached to the back angle anchor using one or more mechanical fasteners penetrating the anchor hook and received within the attachment receptor, and the mechanical fasteners do not penetrate the first or second subsill portions.