Ask a building inspector – Why is my plaster cracking?
Hi, I have these cracks in my walls that look like spider webs or something. I have heard that cracks are serious and because these cracks are on all the walls I am concerned my house is going to collapse. Please can you tell me how serious it is and how to fix them? – Andrew
Dear Andrew, the patterns of the plaster cracks must be identified because they will give you the reasons for the cracks.
Because you have not sent me a photograph, I am going to give you three options of what it could possibly be. In my book, The Proud Home Owner I have included a chapter on cracks and I discuss 11 different types of cracks.
I want to put your mind at ease that your house will not collapse if these are plaster cracks, only if you have structural cracks. The short answer on how to identify structural cracks, is that they are normally bigger on the one side than the other
If the cracks are on your walls, they could be one of the following three types of cracks.
As the name indicates, it is a network of small fine cracks and could be in a hexagon pattern. They are the result of over trowelling a rich mix (high cement content) and it usually happens within a few hours after the mixture was applied. Over trowelling means that the trowelling action was carried on for too long, which resulted in all the moisture being removed from the surface.
Do not open these cracks to fill them with a crack filler. It is not serious because they are shallow and not more than 2mm deep in the plaster; a good paint should take care of it. If they are spotted the same day, then it can be “floated” close with a little water and a trowel.
The bricks should be thoroughly dry when they arrive on the building site and should have reached their required strength. If not, then they will continue to dry and shrink which could cause the shrinkage cracks in the plaster.
Clay bricks must be damp when the plaster is applied, but cement bricks must be dry.
It has the same pattern as that of crazing but the hexagon patterns are larger in size and the cracks can go through the plaster to the bricks.
It is standard practice to mix one very large batch of plaster mix in the morning, and then to add water again later when it has dried out.
This is totally wrong and should not be allowed to happen. Another bad habit is to reuse the “droppings” from the floor and add water or cement.
This is the result of moisture lost after the plaster has hardened. Plaster with a high cement content, or made with poor-quality sand will tend to develop a few large cracks.
How to repair cracks
You should get acquainted with the different filler products because of the wide variety available. It is important that you select the most suitable product for your specific application.
Not all products are waterproof agents, and not all products can be painted over. Some products have a better adhesive quality than others. Some will shrink and could crack when it is dry.
These cracks are not more than 2mm or 3mm deep and about 1mm wide. Do not try to open them further as you will only emphasise the crack even more. The secret is to use a product with a very fine texture which can get on the inside of the crack. Pressure must also be applied to force the filling into the gap, using your thumb and a putty knife. I recommend Pollyfilla Mendall 90 to fill these cracks. You can also use it to fill the gaps between the cornice and the wall or ceiling. It has good flexibility and adhesion qualities, and will not fall out.
If you are unsure about your type of cracks and how to repair them, let Gauteng Home Inspections do a crack inspection for you.
Got a building-related question? Email firstname.lastname@example.org and we’ll be sure to assist you
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Who is Albert van Wyk?
Albert van Wyk of Gauteng Home Inspections and author of The Proud Home Owner has been in the building industry for more than 38 years, having built many luxurious houses on difficult stands to the satisfaction of very meticulous clients as well as office blocks, factories, cluster developments and townhouse developments. Many of the homes were featured in SA Home Owner and on the TV program, Top Billing.
Albert has extensive knowledge of the National Building Regulations and the NHBRC requirements, and is one of only 40 ITC-certified Roof Inspectors in the country.