Mulling system for plural fenestration units with stiffener member and cover member

A fenestration assembly comprising:

• a first fenestration unit including a first corner member that attaches together a pair of first frame members of the first fenestration unit at a first corner arrangement thereof;
• a second fenestration unit including a second frame member;
• a mulling system that mulls together the first fenestration unit and the second fenestration unit at a mull joint, the fenestration assembly including a first side and a second side that are cooperatively defined by the first fenestration unit and the second fenestration unit, the mulling system including:
• a stiffener member that is layered within the mull joint along a first axis extending between one of the pair of first frame members of the first fenestration unit and the second frame member, the stiffener member extending along a second axis between the first side and the second side, the stiffener member including a seating strip abutting the one of the pair of first frame members and abutting the second frame member, the seating strip having a solid cross section in a plane defined by the first axis and the second axis;
• a cover member that attaches to the stiffener member on the first side to at least partly conceal the mull joint on the first side;
• and
• a fastener that attaches the first corner member, the one of the pair of first frame members, the stiffener member, and the second frame member, the fastener having a single fastener body that extends through the first corner member, the one of the pair of first frame members, the stiffener member, and the second frame member.

A method of mulling together a first fenestration unit and a second fenestration unit at a mull joint to define a fenestration assembly, the fenestration assembly including a first side and a second side that are cooperatively defined by the first fenestration unit and the second fenestration unit, the method comprising:

• attaching together, with a first corner member, a pair of first frame members of the first fenestration unit at a first corner arrangement thereof;
• providing a second frame member of the second fenestration unit;
• attaching together, with a second corner member, the second frame member and another second frame member of the second fenestration at a second corner arrangement thereof;
• layering a stiffener member between one of the pair of first frame members of the first fenestration unit and the second frame member, the stiffener member abutting the one of the pair of first frame members and the second frame member;
• attaching a cover member to the stiffener member on the first side to at least partly conceal the mull joint on the first side;
• attaching, with a fastener, the first corner member, the one of the pair of first frame members, the stiffener member, the second frame member, and the second corner member, including extending a single body of the fastener through the first corner member, through the one of the pair of the first frame members, through the stiffener member, through the second frame member, and through the second corner member.