Cavity walls consist of two skins and a gap in between. However, most walls of this type would be now described as composite as their cavity is filled with insulation. Composite walls usually have brickwork as an outer skin and either further brickwork or concrete blockwork inside, with insulation in the blockwork’s holes in modern homes.

With cavities, every metre along the wall a wall tie connected the skins with a row of ties every 450mm up the wall. Fish tail ties are common, named after their shape, and are bonded into the mortar joints.

Usually, and especially in modern buildings, insulation backed plasterboard is applied onto the inner skin in sheets, cut to size from the 2.4m x 1.2m standard size, and bonded by plaster dabbing on the wall.

Where there is a door way in duel skinned buildings, a Cavity Tray (effectively plastic sheeting) is put across the cavity to prevent water moving down. Some vertical joints should then be left in the outer skin, to let any water collected in the Tray be discarded forwards and out of the structure.