When to call an engineer

What it looks like: Cracking that runs diagonally, that is wider at one end than the other, that steps through mortar joints, or that continues through a lintel or above and below an opening. Width that changes with the seasons, or that has changed since it was first noticed.

Why it is structural: A crack in a load-bearing element is the element telling you that something in the load path has moved. Whether that movement has stopped, and whether the element is still carrying what it was built to carry, cannot be judged from the crack's appearance — and both of those questions need somebody who can see the structure above and below it.

What it looks like: A visible dip along a beam, a door or window that has gone out of square beneath one, brickwork cracking above an opening, or a floor that slopes toward the middle of a span.

Why it is structural: Deflection is the member reporting its own condition under load, and the question is whether it is within what the design allowed or beyond it. That needs the member's size, span, material and actual loading — none of which is visible from the sag.

What it looks like: A lean away from the retained soil, a bulge in the middle of its height, horizontal cracking, wet ground at the base, or soil and water coming through the face.

Why it is structural: A retaining wall holds back a mass that does not stop pushing, and its failure is usually sudden rather than progressive once it starts. Drainage behind it, the loading on the ground above it and how it is founded all govern, and none is visible from the front.

What it looks like: Noticeable movement when somebody walks across, a slope you can see with a ball or a level, springiness near the middle of a span, or a gap opening between the floor and a skirting.

Why it is structural: Bounce is a stiffness question and a slope is a support question, and they have different answers. Both depend on the joists, their span, their spacing, their condition and what is beneath them, which means opening up or measuring rather than looking.

What it looks like: A lean visible against a vertical edge, mortar that can be raked out by hand, displaced or spalled brickwork, or cracking where the stack passes through the roof.

Why it is structural: A stack is a tall unrestrained element in the most exposed position on the building, and it is heavy. The failure mode is that it comes down. Its condition below the visible part, and the state of its restraint, are the parts that matter.

What it looks like: Doors that catch or swing on their own, windows that will not close, gaps that taper along a frame, or cracking at the corners of openings — appearing over months rather than all at once.

Why it is structural: Openings are the weakest points in a wall and they distort first, so they act as an early indicator of movement elsewhere. Whether that movement is seasonal, historic and finished, or ongoing is the whole question, and it is answered by monitoring rather than by inspection on one day.

What it looks like: A bulge visible when sighting along the wall, a gap opening between a wall and a floor or ceiling, cracking that follows the bulge, or bricks that have moved relative to their neighbours.

Why it is structural: A wall carries load by being straight; a bow means it is no longer doing that, and the capacity falls sharply as the bow grows. Whether the wall is still restrained by the floors and roof is the question, and it needs somebody inside the structure.

What it looks like: Cracking that was not there before an excavation, a demolition, a basement, a new build next door, or heavy plant on the road — and that appeared over days or weeks rather than years.

Why it is structural: Timing tied to an external cause changes what the cracking means and who may be responsible for it. Recording the condition promptly and having it assessed matters for both the building and any claim, and the record is worth more the earlier it is made.

What it looks like: A visible dip, a bulge, cracking in a line, or plaster that sounds hollow over an area. Sometimes preceded by a water stain.

Why it is structural: A ceiling coming down is a falling hazard with weight behind it, particularly lath and plaster. Whether the failure is the finish losing its key or the structure above losing capacity is the question, and the second is why it is on this page.

What it looks like: Cracks wider than a hairline, cracks with a step or a level difference across them, cracks that leak water, or cracking accompanied by settlement of what sits on the slab.

Why it is structural: Shrinkage cracking in a slab is normal and expected; cracking with displacement across it is not, and the difference is what the assessment is for. What is beneath the slab, and whether it is still moving, decides the answer.

What it looks like: Timber that is soft to a probe, crumbling, discoloured, carrying fungal growth, or showing exit holes and frass. Commonly at a bearing end, a wall plate, or anywhere that has been wet.

Why it is structural: Decay reduces the section that is actually carrying load, and the worst of it is usually at the bearing — the point the member depends on most and the point least visible. How much section remains is a measurement, not an impression.