Common structural defects in houses, and what an engineer looks for

Foundations are usually hidden, so we read them from the signs they leave above ground: stepped cracks that follow the mortar joints, doors and windows that have gone out of square, and floors that slope towards one corner.

Settlement is a building bedding in. Subsidence is the ground below moving down, most often because clay has dried out or a leaking drain has washed soil away. Heave is the opposite, when clay swells. Where the cause is not clear from a visual inspection, we say what further investigation is needed.

Most cracks in walls are not structural. We look at the width, the direction and whether a crack tapers, because a diagonal crack that is wider at the top tells a different story from a hairline crack in plaster. Our guide to cracks in walls explains the damage categories.

We also check whether walls are bowing out of plumb, whether mortar has washed out or bed joints have cracked and slipped, and whether the lintels over windows and doors still carry the brickwork above.

From the loft we look at the timbers that hold the roof up. Rafters can sag between their supports. Purlins, the beams that support the rafters part way up the slope, can be overloaded, often after heavy concrete tiles have replaced slates. A roof that is not tied together can push the tops of the walls outwards: roof spread.

The loft and roof structure are part of a Comprehensive General Structural Inspection, not a General Structural Inspection.

A floor that bounces, dips in the middle or slopes towards one wall can mean undersized or notched joists, a wall taken out below, or a support that has settled. We measure the slope and the dip, and look at what is carrying the floor.

Chimney stacks take the weather on every side. We look for stacks that lean, often because the mortar on one side has expanded, cracked bed joints, gaps where the stack meets the roof, and bricks that are spalling or losing their mortar.

Inside, we check whether a chimney breast below has been removed and, if so, what is now holding the stack up.

Much of London is built on London Clay, which shrinks as it dries and swells when it is wet. Trees draw water from the clay, so a house near a large tree can move with the seasons.

We assess nearby trees against NHBC guidance (Chapter 4.2, Building near trees), which links the foundation depth a house needs to the distance and height of a tree, and check the ground in British Geological Survey records. See subsidence surveys for how we investigate movement.

When a load-bearing wall comes out, a steel beam takes its place, sitting on padstones that spread the load into the walls or piers at each end, and down to the foundations.

We size the beam and its supports and produce the calculations Building Control asks for. See wall removal and steel beam calculations.