Jack post- A type of structural support made of metal, which can be raised or lowered through a series of pins and a screw to meet the height required. Basically used as a replacement for an old supporting member in a building. See Monopost.

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.

Jumpers- Water pipe installed in a water meter pit (before the water meter is installed), or electric wire that is installed in the electric house panel meter socket before the meter is installed. This is sometimes illegal.