Which cracks should I worry about?

Answer

Worry about cracks in beams, columns or slabs; diagonal or stepped cracks, especially from the corners of doors and windows; cracks wider at one end than the other; cracks that are widening or reappear soon after repair; cracks with rust stains or bulging; and cracks accompanied by sagging ceilings, sloping floors, or doors and windows that start to stick. These need a Professional Engineer's assessment.

From the RRD Field Guide, checked against the sources cited in Cracks in Walls, Beams and Ceilings in Singapore: Which Are Structural and When to Worry. General information, not advice for your project.

The fuller picture

Most hairline cracks in plaster, paint and at wall-to-beam joints are non-structural, caused by shrinkage and thermal movement. Worry when cracks are wide or widening, diagonal or stepped, tapered, recurring, accompanied by rust stains, sagging or sticking doors, or appear in beams, columns or slabs. Monitor with dated tell-tales and call a Professional Engineer for those signs.

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