The metal has rusted. As metal rusts it expands to as much as nine times its original size.

The image to the left taken on a terrace house in Macclesfield shows:

An injected damp proof course with the tell tale drill holes capped. The silicon based injection fluid fills the air holes in the brick, thus preventing osmosis action by water being sucked into the microscopic pores.

Above is an osmotic damp proof course, formed by a copper wire laid bare against the brickwork and inserted running a small electrical charge to reverse the polarity of the brick work, thus in effect preventing the action of osmosis and rising damp.

The solid brickwork has been rendered over in the past to ground level, which would have bridged any original damp proof course and increased the rising damp.

The render requires a drip edge forming to prevent rainwater running down the render into the exposed brickwork