Balanced flue:

A flue outlet from a boiler or room sealed heating appliance which uses natural convection to draw the air from outside the building for combustion and expels it back to the outside through a separate compartment of the flue, avoiding the need for a long flue pipe or chimney.

Ball valve (or ball cock):

A device which controls the flow of water from the water supply line into a gravity-operated toilet tank or other cistern, controlled by a float mechanism.

Balustrade:

The protective structure alongside a staircase or around a landing, usually incorporating a series of spindles (or balusters) supporting a handrail.

Barge board:

Timber (or plastic) boarding applied to the verge or gable end of a roof.

Bituminous / mineralised felt:

A traditional covering for a flat roof, usually applied in several layers, the top layer often incorporating very small stone or mineral chippings embedded in the felt, as a reflective layer.