Walk away from a building when you see walls leaning more than 5 degrees, foundation cracks wider than 1 cm, a sagging roofline, bulging walls or wood charred deeper than 2 cm, or when you hear creaking, popping or falling debris, because each means load is moving along a path the structure was not built to carry. A plumb line shows lean: a 3 m wall leaning 5 degrees has its top about 26 cm out from its base.
Part of Building Material Recovery
A structure shows distress before it fails, in its cracks, its lines and its sounds, but the signs are easy to miss from inside a room. Each one points to a particular way the load has been lost.
Each Sign Tells You Which Load Path Has Failed
The figures below are the parent article’s rules of thumb, not engineering limits. A building showing none of them can still be unsafe, so use them as reasons to leave, never as proof of safety.
- Foundation cracks wider than 1 cm. The building has shifted and may continue to move.
- Walls leaning more than 5 degrees from vertical. Lean grows on itself, because gravity pulls the centre of mass further off the foundation.
- Sagging rooflines. Rafters or trusses are broken or overloaded. A sagging roof can drop onto the floor below.
- Bulging walls. An outward bulge means the wall has separated from the floor or roof framing and is no longer braced.
- Sounds. Creaking, popping, grinding, or debris falling from inside means the structure is still settling, and so still moving.
- Recent rain or snow on a damaged roof. Water weighs 1 kg per litre, and a pool on a partly collapsed roof can bring it down suddenly.
- Fire damage. Wood charred more than 2 cm deep has lost much of its structural capacity. Steel that has been through a fire may have warped or lost its temper.
What You Need
- A plumb bob or a weight on a string, see Plumb Bob.
- A tape or marked stick for crack widths.
- A partner and a clear line of retreat.
Method: Measure The Signs, Then Apply The Rule
Step 1: Measure lean. From the outside, hold the string at the top of a wall so the weight hangs down beside it. Measure the gap between the string and the wall at the base. Then compare it with the wall height.
Step 2: Measure cracks. Compare crack width with your marked stick. A crack that widens from bottom to top, or is stepped along mortar joints, points to movement.
Step 3: Look at lines. Sight along the roof ridge, the eaves and the corner lines. A visible sag or bow is a reason to stay out.
Step 4: Probe the char. Push an awl into charred structural wood: if it penetrates deeper than 2 cm, the member is compromised.
Step 5: Listen for a minute. Stand still, outside, and listen.
Step 6: Decide. One clear sign is enough to stay out. If you have already entered and a new sign appears, leave by the nearest exit without gathering tools.
Worked Example
A 3 m wall leaning 5 degrees has its top offset from its base by 3 × tan 5°. Using tan 5° = 0.0875, that is 3 × 0.0875 = 0.26 m, or about 26 cm. So if a string hung from the top of a 3 m wall lands more than about 26 cm from the base, the wall leans more than 5 degrees. For a taller wall scale the offset in proportion.
Common Mistakes
- Measuring lean by eye. Eyes are fooled by neighbouring buildings; use a string.
- Trusting a wall because it is not leaning. Cracks, bulges and sags are separate signs.
- Ignoring a small new sound. It is the structure moving.
- Re-entering for “one more load.” The last trip is the dangerous one.
Safety
A collapsing structure gives seconds of warning
If you see any of these signs or hear new sounds while inside, leave at once by the nearest exit and do not stop for tools. Keep a clear route to the outside, never work in a building that has shifted off its foundation, and never go above the ground floor of a compromised building. Do not stand beside a leaning or bulging wall, because it can fall outward. See also Structural Safety Assessment.
What’s Next
- Learn next: Personal Safety During Demolition — working with a partner and an escape route
- Combine with: Asbestos, Lead Paint & Other Hazards — the dangers that do not show as cracks