Why Is My Plaster Cracking?

Discover the reasons why your plaster is cracking and how to tackle the problem.
Written By: David White | Updated:
Why Does Plaster Crack
David White/DIY Works

Whether you live in a new build or an old house, cracks in your plaster are more common than you may think. From a hairline crack that’s less than 5mm wide to a much larger crack that’s 10mm wide, there are many reasons why it could’ve happened.

To help you understand why your plaster has cracked, below we walk you through the main reasons and how to fix the cracks yourself with the use of filler and a bit of decorating.

Why Does Plaster Crack?


1. Shrinkage

Plaster is prone to shrinking, and it can occur from excessive amounts of water evaporating in the first few hours after it has been applied. This is often referred to as “plastic shrinking” in plaster, and it’s fairly common and easy to fix.

If the plaster is already dry, moisture can still evaporate due to the plant-based materials within the plaster mix. This type of shrinkage is referred to as “dry shrinking” in plaster, and it’s just as common and not something to worry about.

2. Debonding

Debonding of the plaster is where the inner layer shrinks at a different rate than the outer layer, and it’s mostly caused by a thick layer of plaster being applied. As well as cracking of the plaster, debonding also causes a hollow sound when you tap the surface, indicating a cavity underneath the plaster.

3. Structural Cracks

Structural cracks in plaster are also very common and although it sounds worrying, it can be caused by the slightest foundation movement, brick wall shrinkage or roof slab expansion. Usually, these types of plaster cracks form vertical or diagonal lines across walls and ceilings.

cracks in plaster

David White/DIY Works

4. Grinning

If the mortar joints are visible through the plaster and causing it to crack, this is referred to as “grinning”. The cracks are usually very visible due to the grinning, and they are caused by the difference in suction capacity between cement mortar and brick walls. Although it isn’t aesthetically pleasing to look at, it won’t cause further cracking that’s already present, and most of the time, it can be easily fixed using filler and redecorating.

5. Poor Quality Mix

Using a poor-quality mix can cause a host of issues that lead to plaster cracks. From the lack of hardness due to insufficient amounts of cement in the mix to contaminant particles in the mix that react with the moisture and cause “popping” to occur, the quality of the mix is crucial in order to prevent cracks in the plaster.

6. Expansion

Expansion of plaster can be caused by wet wall coverings, excess humidity or poor air circulation. Along with the cracking of the plaster, it can lead to swelling and softening of the plaster.

7. Dried Too Quickly

It’s crucial that you allow plaster enough time to dry because attempting to speed up the process can cause cracks to appear. Whether you’ve turned on your central heating or used a dehumidifier, being too impatient will lead to cracking. With regards to drying plaster, you should allow it to dry naturally without any intervention.

8. Exposure To Wind Or Sun

Exposure to the outdoor elements isn’t recommended when drying plaster for two main reasons. Firstly, continuous contact with wind can cause excess aeration and the plaster to dry too quickly. Secondly, direct sunlight will cause oxidation of the plaster, and moisture will dry out of the plaster at a fast rate, leading to cracking.

9. Painted Before It Was Dry

Although fresh plaster isn’t the most aesthetically pleasing surface to look at, it’s important that you don’t become impatient and paint it too early. Painting the plaster before it has had time to dry can cut off the ventilation it needs, leading to cracks.

If you are wondering when the right time to paint plaster is, we wrote a detailed guide on painting new plaster.

10. Poor Preparation

Preparation is key to a flawless finish, and if the surface isn’t prepared appropriately before plastering, it can lead to cracks appearing. For example, not removing all the wallpaper on the wall beforehand is a common mistake.

how to fix cracks in plaster walls

David White/DIY Works

How To Repair Cracks In Plaster

From our experience, plaster that’s cracking and is less than 5mm wide can be easily fixed yourself with filler and a bit of redecorating. However, if the crack is over 5mm wide, we would recommend asking a plasterer to check it out.

Alternatively, if the crack is over 15mm wide, you may have structural damage that requires expert advice (i.e. you will want to talk with a structural engineer regarding the cracks).

To fix cracks in plaster walls or ceilings that are less than 5mm wide, follow the below steps:

  • Remove any loose plaster, dust and debris (vacuum afterwards)
  • Apply a plaster filler into the crack
  • Spread the filler across the crack to achieve a smooth finish
  • Apply additional layers of the filler if required
  • Allow time for the filler to dry
  • Sand down the filler if it’s protruding outwards

It’s important to note that the above method is only effective upon hairline cracks less than 5mm wide. For larger cracks, you’ll need a professional, and it’s not something we would attempt to try and fix as a beginner DIY’er.

Conclusion

Cracks in plaster walls and ceilings are completely normal, and depending upon the crack’s size, it’s nothing to worry about. Hopefully, our guide on why plaster cracks has answered all your questions and given you the confidence to repair small cracks yourself. However, if you require further information regarding plaster cracking in your home, feel free to get in touch, and we will try to provide our assistance where possible.

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