Skip to content

Façade cracks: bridge, reinforce or repair – how to decide

5 min read

“How wide is the crack?” is usually the first question and rarely the one that decides. Three others matter more: where does the crack come from, is it still moving, and is water getting in through it? Only then is it clear whether a coat of paint is enough, a reinforcing mesh is needed, or the cause calls for building work.

Crack types and their causes

The crack pattern often reveals the cause. The table summarises what Baunetz Wissen and the Fraunhofer IBP (Fraunhofer Institute for Building Physics) record on this.

Crack pattern Typical cause Assessment
Map cracking in the render of an external thermal insulation composite system (ETICS) Render and insulation board expand differently in sunlight; the reinforcement fails through defective material or insufficient embedding Reinforcement should normally absorb such cracks (Baunetz)
Cracks along the insulation board joints Marked post-shrinkage of the insulation boards According to the IBP system-specific, but merely a visual impairment
Notch cracks at window and door corners, window sills, roller shutter boxes Stress peaks at openings According to the IBP not system-specific, generally to be assessed visually
Vertical and horizontal cracks between wall and ceiling Differences in deformation through shrinkage, heating or swelling Avoidable through expansion joints (Baunetz)
Blisters, cracks, spalling in the render Insufficient adhesion to the substrate, differing thermal expansion, chemical reactions Clarify the cause in the render build-up, do not paint over
Blisters and map cracking after a coat of paint Unsuitable paint on the system IBP: example of the importance of choosing system-compatible materials

The IBP statements on window corners and board joints come from inspections of residential buildings with rigid polystyrene foam ETICS and synthetic resin render (1975/76, 1983, 1995). According to the IBP, the results are transferable to mineral systems.

First establish: has the crack come to rest?

For masonry cracks, Baunetz Wissen recommends a graduated approach. A crack that has formed can be closed provisionally with joint sealant. Where cracks have existed for a very long time and will no longer change, the existing render should be replaced with fabric-reinforced render.

The criterion is therefore movement, not width. Whether a crack is still working can only be established by repeated measurement at marked points. How long this takes in the individual case depends on the cause.

Three routes: bridge, reinforce, repair

Route When it fits What is documented
Bridge Stable, fine cracks without functional damage, for example board joint cracks after post-shrinkage IBP: Board joint cracks that initially arose from post-shrinkage can be permanently covered by a crack-filling paint. Crack bridging denotes the ability of a material to bridge substrate cracks without function or integrity suffering.
Reinforce Stable cracks that are larger or more frequent; blisters and detachment of the coating Baunetz: For long-standing, unchanged cracks, replace the render with fabric-reinforced render. The IBP lists additional reinforced render layers and reinforced coatings as maintenance measures; in three cases, however, a coat of paint would also have sufficed. Reinforcement is therefore not a mandatory measure.
Repair Cracks with a moving cause or lack of adhesion of the render Baunetz: Expansion joints between wall and ceiling avoid differences in deformation; a crack that has formed can be closed provisionally with joint sealant. Where adhesion is defective, the cause in the render build-up must be clarified before coating.

Which crack movement a coating bridges is a property of the system. The test method for this is described in DIN EN 1062-7 “Bestimmung der rissüberbrückenden Eigenschaften” (determination of crack-bridging properties) (edition 2004-08). The result belongs in the data sheet of the respective system; how to read such information is shown in the article Reading the data sheet: V and W classes.

Why crack widths apply only to a given system

Figures for crack width always relate to a system. An example from a DIBt approval for a coating system on reinforced concrete in containment basins (not for façades): the system permanently bridges cracks up to 0.4 mm wide; existing cracks over 0.2 mm must be closed or repaired before coating, once their causes have been eliminated. Two limits, then: one that the system achieves, and one above which preparation is needed.

These values do not apply to façade coatings. They show only that both limits must be looked up in the data sheet or approval of the chosen system, rather than applying a blanket figure in millimetres.

When water becomes a problem

DIN 4108-3 puts rain protection under driving rain exposure in the foreground, so that water does not get from the surface into the construction through cracks or capillary suction. Baunetz Wissen describes three exposure groups; external renders without special requirements for driving rain exposure can be used only in regions of group I, while for groups II and III a water-repellent render must be used. This argues for including the exposure group of the location in the assessment of a crack.

In the polystyrene ETICS examined, according to the IBP, render cracks did not affect the moisture content of the insulation layer because the insulation material is neither hygroscopic nor absorbent; consequential damage was not to be expected. With absorbent substrates or insulation materials, this may be different.

What to record before coating

  • Record the course and position of the crack (window corners, board joints, surface) photographically.
  • Check crack movement and note the result.
  • Name the cause: shrinkage, thermal expansion, adhesion, reinforcement.
  • Justify the chosen measure and compare it with the data sheet.
  • Notify concerns in writing about a coat of paint on moving cracks demanded by the client, if possible before work begins. The article Checking the substrate under VOB and BFS describes the form.

In many cases the question remains open of how long a crack must be observed before it is classed as stable. There is no fixed rule for this; it depends on the cause, the building element and the season, and therefore belongs in the documentation of the decision.

Sources

→ Planning facade protection in old buildings economically

Read more

More practical knowledge from the same subject area.