A crack in the exterior plaster can be a small thing and may look harmless. But if the crack penetrates the entire plaster, it loses its protective effect against wind and weather. The masonry is at least partially at the mercy of environmental influences. Other cracks reveal serious structural defects that absolutely must be repaired. Reason enough to take a closer look at the facade.
A crack in the plaster is relatively commonplace and need not be a big deal. If it goes too deep, the plaster loses its protective function at this point and moisture can penetrate, but such minor cracks can be repaired quite easily. However, a crack in the plaster can also be a symptom of deeper serious damage in the structure.
Ultimately, then, there are two types of cracks: In the case of so-called plaster-related cracks, the damage is only on the surface and it can be repaired by do-it-yourselfers. Dynamic cracks, on the other hand, are often larger and wider. In this case, the damage is deeper, namely in the structure itself, and this cannot be easily repaired.
Detect cracks due to plaster
Surface cracks, which only affect the plaster itself but not the structure, are only a few millimeters deep and usually spread over a wide area, which is why they are also called hairline or net cracks. They can be easily made visible by wetting them with clear water from a spray bottle. Their pattern then emerges clearly. They are often harmless, but some plaster-related cracks can penetrate all layers of plaster and extend to the substrate, i.e. the wall. Moisture can then penetrate at this point and cause damage.
Builders distinguish several types of plaster cracks, whose designation already suggests the cause:
- Shrinkage cracks: They are probably the most common type. They occur when plaster layers dry out too quickly. This happens especially on dry and sunny days.
- Shrinkage cracks, often called stress cracks: They often occur months after plastering work has been completed. The cause is usually tension between the plaster layers, which can also cause the plaster to detach from the substrate along the crack. They can be recognized by their usually net-like structure (also called craquelure).
- Bag cracks: These occur when plaster has been applied too thickly, for example to compensate for an unevenness in the substrate. In a sense, the plaster sags away at the bottom, thickening there while cracking at the top.
- Notch cracks: These occur primarily at the corners of wall openings such as windows and continue diagonally from there. They are usually the result of stresses in the masonry – which is, however, intact. Notch cracks also occur in composite thermal insulation systems when the plaster reinforcement at the corners of windows and doors is inadequate or non-existent.
- Age damage: Some cracks in the exterior plaster of a building are purely the result of age. For example, the plaster loses its adhesion and hollow spots develop – or the plaster becomes porous and crumbles away.
Treating plaster cracks
Because of their many causes, there is no universal recipe for optimally repairing plaster cracks. In principle, the do-it-yourselfer willing to renovate has two options:
- covers the cracks found
- fills them
Suitable materials are available for both in DIY stores. The decisive factor is ultimately the extent of the work required. A single bag crack is quickly repaired. Large shrinkage cracks, numerous age spots and a heavily tarnished appearance, on the other hand, require more of an area-wide approach.
How to cover cracks
This is an overplastering of part or all of the facade. In other words, a new layer of plaster is simply applied. The requirements for this are simple, but mandatory: the old plaster adheres firmly and does not sand off.
If you plan to wrap your house up warm anyway, you can combine crack repair and thermal insulation with a composite thermal insulation system. The insulation boards themselves bridge the cracks, and the reinforcing fabric, which is necessary anyway, does the rest.
If, by mistake, the damage is not caused by plastering at all but by a dynamic crack, there will be profound damage to the structure, which can also destroy the composite thermal insulation system. So before DIYers apply an ETICS, they should be very sure.
Repair plaster and fill cracks
It is somewhat more complicated to fill cracks. The first step is always to clean the affected areas from dirt and loose particles. Since water-bound building materials are usually used, it is advisable to wet the areas around the cracks. Smaller cracks can then be filled.
DIY stores carry various ready-to-use fillers, also in small package sizes. Fillers are pasty, which is why they are well suited for shrinkage cracks.
Special facade fillers are available in widths from 15 centimeters and at prices starting at around ten euros. For repair and renovation, manufacturers offer special fillers called rubber mortars. They are very flexible because they have been tempered with rubber and fibers, and are therefore well suited for wide cracks.
Cracks that go deeper and are wider than one to two millimeters should be filled in two or more work steps. The reason: A layer of fine putty must not be thicker than four millimeters and must dry out before the next one follows.
Since putties are usually white, the repaired areas unfortunately stand out on colored facades. It is therefore advisable to color the compounds accordingly with a dash of tinting paste before spreading.
Filling cracks and plastering the exterior wall
However, the classic method of repairing stronger cracks is plastering. To do this, you first need to enlarge the cracks. Those who have a good command of a parting grinder, take it to help. But equally useful are a hammer and chisel. The cracks should be laid out in a V-shape, because this gives the edges more adhesive surface. The widened cracks are now painted with deep primer and then plastered.
The repair compound may have to be applied in two layers, so the do-it-yourselfer makes a base coat and a top coat. In between, you can lay strips of reinforcing fabric along the cracks. They strengthen the plaster because they offer considerably more resistance to tensile forces.
Lime-cement plaster is available in 25-kilo bags, but smaller quantities are rarely purchased. Instead, you can make a small amount yourself. It consists of two parts hydrated lime, one part cement, six to eight parts fine sand and water. Add water gradually until the mortar has a slight sheen.
Detect Dynamic Cracks
Dynamic cracks are usually due to unwanted movement in the building. Reasons for this can be, for example:
- settlement in the substrate or in the masonry
- excessively large joints in the masonry of the shell
- rough dimensional deviations that were simply plastered over thickly later
- inadequate preparation of the plaster base or the base plaster
- non-observance of service lives (processing periods) of the materials used
There are some indications with which even the construction layman can recognize dynamic cracks. First of all, their location can be a clue. Neuralgic points are, above all, wall openings such as windows or doors, as well as places where different building components are brought together. For example, extensions, gables or where an interior wall joins an exterior wall. Furthermore, the shape and course of the cracks can provide a clue:
- The crack has a conspicuously straight course. Then it possibly follows a horizontal joint.
- The crack is formed in steps, i.e. it continues across horizontal and vertical joints or runs along the joints of masonry units.
- The crack is located at the corners of wall openings and moves away diagonally.
- The crack is significantly wider and deeper than ordinary hairline cracks.
Observing: Here, the goal is to determine if cracks are getting even longer and, more importantly, wider. The easiest way to do this is to mark both ends of the suspected crack with a strong pencil stroke. After some time, you can then see whether the crack has grown beyond the line. As a rule, it will also have widened. If you want to be on the safe side, you can do the plaster test. To do this, the crack is scratched out in one spot, preferably the center, i.e. it is clearly widened. A blob of freshly mixed plaster is then placed on this spot and smoothed out. After one or two weeks, the plaster is inspected. If it has cracked, there is disturbing movement in the building.
In sunshine, the plaster spread should be shaded so that it does not dry too quickly and possibly crack due to heat exposure.
Act on dynamic cracks
Anyone who is reasonably certain that dynamic cracks are involved should initially refrain from treating them and instead consult an expert, architect or structural engineer. These professionals can definitively assess what phenomenon is actually behind the cracks observed. The experts will help clarify the following questions in particular:
- Is the crack harmless or is it due to a design or planning error?
- Is it possibly a case of botched construction?
- Is there an unexpected movement in the foundation soil?
- What measures are recommended to a builder or owner?