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Why Does Plaster Crack?

Cracks can be a sign of plaster damage, and it’s true that cracks are part of the history of buildings. They are, however, also signs of structural weakness that can lead to further problems if not fixed. So why does plaster crack? Are there ways to prevent them? Are they fixable?

These are some of the points this article will explore.

Why does plaster crack?

Why does plaster crack
Plaster cracks on walls. Source: Pixabay

The first thing to understand about plastering is that it does not have the most flexible composition. While you can get pretty creative with it (especially with texture), plaster tends to crack easily. Such cracks are typically caused by rapid changes in home temperature, expansion, or contraction. However, there are some things you can do to prevent this from happening in your plaster.

Cracking plaster is a common problem with older homes. There are several reasons why this happens. Cracks can be the result of thermal movement, the settling of a foundation or sill work, or even exposure to moisture for an extended period of time. The extent of the crack can also vary across different plastering types.

But there’s more to it. Some of the common reasons for plaster to crack are:

  • Shrinkage of plastering
  • Dry shrinkage
  • Structural cracks
  • Debonding
  • Grinning
  • Lack of hardness
  • Popping
  • Expansion of plastering

Let’s look at each in more detail.

Shrinkage of plastering

Plastic shrinkage cracking is the name given to a condition that occurs in walls and ceilings where moisture levels drop drastically in a short time. The process of capturing and extracting water is known as dehumidification.

Dry Shrinkage

Dry-shrinkage cracks occur when the liquid in a plaster or stucco material evaporates. They happen because plant-based materials—like cement and lime—shrink when they dry out. These cracks are stable but should be filled with filler before you paint.

Structural cracks

Structural cracks are caused by foundation movement, moisture expansion, brick wall shrinkage, and roof slab expansion. These types of cracks usually form in straight, vertical lines or diagonal lines.

Debonding

The problem of debonding is that the inner layer shrinks faster when it’s dry and the outer layer shrinks faster when it has good ventilation. The plastered wall often sounds hollow when tapped, indicating the presence of a cavity. The difference in the thermal expansion of the outer layer of plaster and the interior plaster causes the air pocket to form, which produces a hollow sound.

Grinning

 The appearance of plaster walls when the position of mortar joints are clearly visible through the plastering is known as grinning. It is mainly caused due to the difference in suction capacity between brick walls and the cement mortar used for plastering.

Lack of hardness

Poor quality of cement mortar leads to cracking and less hardness which is due to excess amount of sand and dust in it. This happens mainly due to mixing of poor quality water and addition of extra water after first mixing.  

Popping

Popping is formed from the plaster wall that are formed by the presence of contaminants in the mix. The popping forms conical fragments that break out of the surface of plaster leaving holes that vary in size. Caused by moisture it usually occurs and can be removed by application of cement filler and paint to cover.

Expansion of plastering

The plaster expansion is a moisture related problem which can be prevented in the initial stages. The cause of moisture is due to wet wall coverings, inadequate circulation of air or consistently high ambient humidity. If this damage has already occurred, removal of plaster and application of a breathable plaster are two processes that can correct the problem.

Why does plaster crack when drying?

Moisture control is key when it comes to plaster. If your project is experiencing cracking, then there’s a good chance that the issue stems from an imbalance in moisture.

There are few reasons why your plaster cracks when drying:

  • Sand or grain is poorly mixed – You need to have a professional grade mixer to ensure there are no particles that are of a chunky texture within your plaster mixture. This can cause them to separate which eventually leads to cracks in the plaster.
  • Direct sunlight – when the plaster is directly exposed to the sunlight, the oxidation of the plaster can take place faster. The heat will dry out the moisture at a faster pace which will lead to cracks.
  • Contact to wind – Continuous contact with wind can cause a drastic aeration, which causes the plasters to dry prematurely.
  • Moisture absorption – the bricks surrounding the material can sometimes absorb moisture out of the plaster which causes drying at a faster pace. Since bricks are a porous material, the chances of this sucking out all the moisture is highly likely.
  • Painting the surface before drying – It’s important to not paint over a surface until you are completely sure that the layer is completely dry. Doing so cuts off the air supply causing many cracks that separate the paint layers too.

Steps to fill a crack with a sealant

Plaster is not a perfect medium, and inevitably, it will crack. If a small crack appears in your plaster, you may be able to repair it yourself with these simple steps.

  • Clear out any dust particles or debris around the crack
  • Apply some adhesive within the crack
  • Spread the caulk over the crack to properly cover it.
  • Give it some time to try. The times can vary based on the sealant that you choose. 
  • Layer as needed to smooth out the crack.
  • If the layer protrudes outwards, you can sand it down with a sandpaper.

You must note that it will take a while to finish, but if you don’t panic you can fill cracks and make a smooth finish.

Tips to ensure plaster does not crack

There are a few things that you can consider to get the most out of the plaster application. These include:

  • Use of high quality paint – Generally, if the quality of paint is low, it can cause poor adhesion. It’s important that you do not combine different brands as this can lead to separation in the mixture as well. 
  • Use of professional mixers – it’s important that you properly mix sand and small particles evenly . As such, using a professional mixer can help you with the same.
  • Work with smaller layers – you would need to patiently add layers with small incremental coats instead of lopping on the plaster. This will help dry faster, make them smoother, reduce cracking and absorb fewer oxygen bubbles.

Final thoughts

It’s important to do your prep work before plastering or you’ll find yourself patching up the cracks! Now follow these steps and you’ll have a nice base to plaster and paint. If you’re still unsure on why plaster cracks or why they don’t last longer, you can get in touch with professional plasterers at Ralph Plastering.

We offer free consultation and no obligation quotations for all your plastering needs. Get in touch with us for more information.

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