When we show resale communities in Taichung, the most common question is a client pointing at a wall and asking whether a crack matters. The short answer: most wall cracks are shrinkage in the plaster or brickwork. The ones to worry about are diagonal 45-degree or X-shaped shear cracks (剪力裂縫), and what matters is which structural member they are on. Some people call them “shear lines” or “shear fractures”; it is the same thing. This guide gives a table to classify cracks, a rough severity test, where to look on a viewing, and who to call.
Three numbers to remember: shear cracks run at 45 degrees, diagonally or crossing in an X; cracks in RC walls of 0.2 cm or wider should be assessed by an engineer, per the Taichung Urban Development Bureau’s self-inspection handbook; and the order of danger is column, then beam, then shear wall, then partition wall. Based on the Bureau’s Simple Handbook for Preliminary Self-Inspection of Building Structures after Earthquakes (updated 2026-09-20).
What is a shear crack?
A shear crack is a diagonal 45-degree or X-shaped crack that forms in a column, beam or wall after it takes the horizontal force of an earthquake. It appears at the top and bottom of columns, the ends of beams, on shear walls or at window corners, and it means the member has been loaded. As the quake rocks back and forth, the member cracks diagonally one way, then the other; the two together form an X.
It differs from a plaster shrinkage hairline in three ways: the direction is always diagonal, the two sides of the crack are often offset or spalled, and it appears on a load-bearing member rather than in the middle of a decorative wall.
Five crack types: classify by direction
Direction gives a first classification. Diagonal 45-degree or X-shaped cracks are shear cracks. Horizontal cracks appear mostly at column tops and bottoms or in walls. Vertical cracks are usually shrinkage or settlement. Map cracking and hairlines are usually plaster.
| Appearance | Common location | Usually means | Action |
|---|---|---|---|
| Diagonal 45 degrees, one or more | Column top or bottom, beam ends, window corners, shear walls | Shear crack; the member has taken earthquake force | Professional assessment |
| X-shaped crossing | Columns, short columns, shear walls, beside openings in RC external walls | Shear crack from repeated loading; the most serious | Professional assessment |
| Single horizontal | Column top or bottom, RC wall, brick wall | Column end offset or wall bending; RC wall cracks 0.2 cm or wider need assessment | Depends on width and location |
| Single vertical | Mid-span of long beams, brick wall to column junction, lintels, below window sills | Mostly shrinkage, uneven settlement or brick separation; vertical cracks at mid-span of long beams need assessment | Usually self-repairable, beams excepted |
| Map cracking, hairlines | Whole walls, ceilings | Plaster or paint shrinkage, no structural effect | Repair yourself |
Based on the Taichung Urban Development Bureau handbook; an engineer’s assessment always governs (updated 2026-09-20).
Rough severity test
Judge severity by three things: width, whether it goes through or exposes rebar, and whether it keeps widening. The Bureau’s handbook uses 0.2 cm on RC walls as the threshold for professional assessment; engineers in practice often use 0.3 mm to separate shrinkage cracks from those needing injection repair.
- Width. Try inserting a business card (about 0.3 mm thick). If it will not go in, it is probably plaster. Diagonal, X-shaped or horizontal cracks in RC walls of 0.2 cm (2 mm) or wider are listed in the handbook for professional assessment. The 0.3 mm figure is the practical threshold for epoxy injection, not a legal number.
- Through-cracks and exposed rebar. If the crack is visible on both faces of a wall, lets light through, or concrete has spalled to expose rebar, it needs assessment regardless of width.
- Growth. Mark the end of the crack in pencil with the date and check in a month or two whether it has passed the mark. A growing crack is more dangerous than a wide one.
- Offset. If the two sides are no longer flush, one proud and one recessed, the member has slipped.
Our view: The rough test only decides whether to call an engineer, not whether to buy. If you see a diagonal crack on a column or beam, skip the measuring and go straight to the “what to do” section below.
Where is most dangerous? Column, beam, shear wall, partition
The closer the member is to the main structure, the more dangerous the crack. Columns hold up the building, beams hold up the floors, shear walls resist earthquakes, partition walls only divide space.
- Columns: the top, the bottom and the columns beside doors and windows crack most often. A “short column” pinned by a window sill wall concentrates force in a quake and X-shaped cracks are especially common.
- Beams: 45-degree cracks near the ends beside the columns need assessment; fine spider-web cracks at mid-span can be repaired yourself.
- Shear walls: the thick RC walls around the lift and stair cores. Diagonal or X-shaped cracks of 0.2 cm or wider need assessment.
- Partition walls: cracks at brick lintels, window sills and below air-conditioner openings can usually be repaired yourself. But ground-floor partitions are the second line of seismic defence; do not remove them just because “they are not structural”.
Where to look on a viewing
Look where decoration cannot hide things: stairwells, basement columns, front and rear balconies, the roof, and RC walls beside service shafts. Nobody repaints cracks there.
- Stairwells: on each floor, look at the stairwell walls and landings for diagonal cracks or horizontal breaks along the treads.
- Basement and ground-floor columns: car park columns are the most honest. Look at the tops and bottoms for 45-degree cracks, spalling or exposed rebar.
- Boxed-in interiors: new joinery, wallpaper and fitted cabinets hide walls. If an old home has just been repainted, ask why.
- Disclosure statement: the property disclosure statement (不動產說明書) for an existing home must state whether beams and columns have visible gaps or cracks, with position, length and width. Read what the seller filled in first.
For the rest see the viewing checklist. For communities built before 1999, also ask about damage records from the 921 earthquake. A pre-handover inspection company does a whole-home check; see the home inspection checklist.
Found a suspicious crack? What to do
Photograph it, measure it, watch it, then have a structural or civil engineers’ association assess it. Fees are quoted by building size and scope; there is no fixed price.
- Photograph: place a coin or ruler for scale, take wide and close shots, note the floor and position.
- Measure: a business card, crack gauge or callipers all work; record the widest point.
- Watch: mark the crack end with the date. If you cannot wait, go straight to step 4.
- Engineer’s assessment: the Taichung structural engineers’ and civil engineers’ associations both offer assessment services. Eligible communities can have the management committee apply for a preliminary seismic assessment, subsidised up to NT$12,000 to NT$15,000 per building (Article 3 of the subsidy rules).
If you have already decided on a home, do not pay earnest money before the result, or make “no major structural defect on assessment” a condition in the offer and the sale contract. See the buying process.
Our view: The crack itself is not frightening. What is frightening is everyone seeing it and nobody asking an engineer. A preliminary assessment applied for by the committee costs far less per household than commissioning your own.
FAQ
Are shear cracks and shear lines the same thing?
Yes. Both mean the diagonal 45-degree or X-shaped cracks a member develops after taking horizontal earthquake force. “Shear line” is the colloquial term.
How wide must a crack be before I call an engineer?
The Taichung handbook says diagonal, X-shaped or horizontal cracks in RC walls of 0.2 cm or wider should be professionally assessed. Diagonal cracks on columns and beams need assessment regardless of width. In practice engineers inject cracks of 0.3 mm and above.
Is a 45-degree crack in a partition wall serious?
Cracks in brick partitions can usually be repaired yourself, but if the sides are offset or the whole wall shows a wide X, have it assessed. Check the columns, beams and shear walls of the same building at the same time.
Can I buy a home with cracks?
Whether to buy depends on the engineer’s assessment, not the number of cracks. Plaster cracking has no effect. If columns or beams have shear cracks and no repair record can be produced, do not pay earnest money before the assessment.
Conclusion
Look at the direction first (diagonal and X-shaped are the ones to watch), then the member (columns and beams first), and use width only as a supporting clue. If you cannot tell, photograph it and contact us; we will help you decide whether an engineer is needed. Related: seismic ratings and RC, SRC and SC structures.