J Channel - Metal edging used on drywall to give the edge a better finished appearance when a wall is not "wrapped" Generally, basement stairway walls have drywall only on the stair side. J Channel is used on the vertical edge of the last drywall sheet.

Jack Post - are  columns made up of steel cylinders topped by a couple of steel plates joined by a large threaded rod. This threaded rod allows the overall height of the Jack Post assembly to be adjusted by turning the rod. Jack Posts are installed underneath the beams that support floor joists, which make them an integral part of the floor structure. (See Telepost)

Jack Rafter - A rafter that spans the distance from the wall plate to a hip, or from a valley to a ridge.

Jamb - The side and head lining of a doorway, window, or other opening. Includes studs as well as the frame and trim.

Joint - The location between the touching surfaces of two members or components joined and held together by nails, glue, cement, mortar, or other means.

Joint Cement or Joint Compound - A powder that is usually mixed with water and used for joint treatment in gypsum-wallboard finish. Often called "spackle" or drywall mud.

Joint Tenancy - A form of ownership in which the tenants own a property equally. If one dies, the other automatically inherits the entire property.

Joint Trench - When the electric company and telephone company dig one trench and "drop" both of their service lines in.

Joist - Wooden 2 X 8's, 10's, or 12's that run parallel to one another and support a floor or ceiling, and supported in turn by larger beams, girders, or bearing walls.

Joist Hanger - A metal "U" shaped item used to support the end of a floor joist and attached with hardened nails to another bearing joist or beam.

Joist "Sistering” - Sistering a joist simply means attaching more material to the side of the joist. This can mean a new joist of the same size and length is screwed or nailed firmly to the old joist, or it can be a smaller structural member. Sistering could also involve sandwiching the old joist with new material on both sides. Typically this involves framing lumber, but it could involve engineered lumber, structural steel or formed steel joists that are made of heavy gauge sheet metal.

Junction Box - A box which protects splices in electrical wires and provides access. Switches, outlets, and boxes for light fixtures are junction boxes.