Header tank:

A cold water tank, usually located above a hot water storage tank or in a roof space, which re-fills the system when water is drawn off from a tap.

Hermetically sealed double glazing:

A factory made double glazed window, sealed to prevent moisture or air entering the void between the glazed panels.

Hip:

The external angle formed by the meeting of two roof slopes.

Hipped roof:

A roof which slopes down to the top of the walls, usually on three sides.

Hopper head:

A small, open-topped collection vessel receiving rain or waste water from two or more pipes, connected to a single pipe below.