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Cracks in House Walls

What causes cracks in the walls of buildings?

The walls of buildings often crack particularly during the first year after a building has been built. Often it is just the new building settling onto the ground but sometimes they are caused by something more serious especially if you live in an earthquake area. Let us look at what causes cracks, how you can tell if they are serious and how you can repair them.

Let us look at some of the causes of cracks in walls.

Building Settlement

Buildings are heavy things, funny that. In the months or even years after you have been piling up rocks, concrete, steel, tiles and wood on a piece of land the ground might be feeling a bit overloaded and it may have a distinct tendency to settle a bit. This settlement may cause cracks in that pile of masonry you call a building and these often appear in non structural parts of the building particularly through walls. Movement is usually minor and cracks are normally "hairline" - very fine cracks that may run in any direction (horizontally, vertically or diagonally across the wall). Cracks may be visible on both sides of walls. To repair settlement cracks simply fill them and paint over.

Movement of the Building

If Billy Smart's circus goes past and they have rather a lot of elephants or if the earth moves for the lady next door then these may well cause cracks in your walls. If you have the unfortunate but predictable experience of having a wonderful neighbour who suddenly decides one day to build a 3 storey apartment building on the 200 square metre block next to yours the enormous weight is likely to compress the ground and may cause some rather severe cracks in your house as a couple of people I know can testify.

Earthquakes and ground movement

Earthquakes are another matter and can cause the sort of cracks that are larger than your walls. Be grateful if you still have walls.

If you suspect there has been ground movement have a good look around. Has the house been built on a former rice field? Has there been a landslide nearby, has a river nearby eroded and destabilised the ground or has some idiot started excavating the land next door? These are all fairly common threats to buildings in Bali.

Are the cracks serious? Should I worry?

Minor cracks are usually nothing to worry about but larger cracks may suggest building failure. Have a look to see if cracks go right through walls. Check if there are cracks or movement in your floors, this may suggest foundation damage.

These sort of cracks can be serious and may damage the structural integrity of the building particularly if they go through or near structural columns or beams. Remember that if the house is weakened in this way the house has lost a lot of its ability to resist an earthquake.

Care is needed and if in doubt you should call someone who knows what they are doing and that you trust. Building structural repairs require specialist design and execution and is a subject little understood here in Indonesia. Failures usually start from the foundations and, as you can imagine, repairing foundations requires both expertise and experience.

Cracks where electrical cables and air conditioner pipes have been installed in the wall

When electrical cables and air conditioner pipes are installed in walls channels are chiselled out of the wall, the cables and pipes are installed and the channels refilled with cement. If installed badly cracks can form where the channels have been cut. These usually run vertically down the wall from the ceiling to electrical switches and sockets. These cracks do not affect the structural strength of the building. To repair cracks should be widened a little then filled with a cement rich filler.

Cracks in the surface plaster on the wall.

Many buildings in Indonesia suffer from "crazed" walls.

I recently saw a local health clinic building, only a year old, and inside every wall was covered with cracks like large crazy paving. It can look very worrying but this form of cracking is usually only the skim of plaster on the surface of the wall.

The first thing to know is that throughout Indonesia internal walls are not plastered in the way that walls in Western countries are. Here they don't use plaster which is relatively soft, they use a thin layer of cement "render" which is made by mixing water, sand and cement and is usually harder than Arnold Schwarzenegger's head. Try banging a nail into your wall to hang "Moaning Lisa," that masterpiece created by Komang Da Vinci of Sukawati, and you will soon know what I mean.

If the cement render on your walls has cracked this is because it has shrunk as it set. Normally this shouldn't happen however if your tukang in his real job is a rat catcher from Bangli then he perhaps would not know that, once in place, cement should be kept cool and damp so it can cure and dry out slowly.

Poor quality sand

Joe The Bangli Ratcatcher would also not know much about sand. Sand makes up the major component in cement for render and for mortar. Sand is, of course, made up of small particles of rock, a fact that is fairly obvious to us but not necessarily to an erstwhile hunter of small rodents.

If you buy sand from Mr Dodgy Building Products Limited (and of course the word Limited refers to the quality of his products which in turn is a result of his limited liability status which means he doesn't have to take responsibility for anything he might sell you) then it may well be that the "sand" has rather a lot of mud or clay particles in it and you have a problem

As the cement render is mixed the clay or mud in the sand soaks up water and, unlike rock, swells when it gets wet. You apply the cement render to the wall and as it cures and dries out the mud or clay shrinks and you end up with cracks all over your beautiful wall.

So how do you test your tukang's sand to make sure it doesn't have mud or clay in it? Find a tall slender glass. Put some of the sand in the glass then add water. If the sand is all rock you will find the level of the sand doesn't alter as the water is added. If there is mud in the sand it will absorb water and swell and the level of the sand will rise. You will also find that the water turns muddy.

If you find that Joe The Ratcatcher has bought sand with mud or clay in it there is something you can do to get rid of the mud. Use a cement mixer to wash the sand several times in water. The mud or clay will dissolve in the water leaving much cleaner sand.

The shape of the sand particles also makes a difference. The best sand for building is "sharp" sand where the individual particles have sharp angular shapes. This helps the particles to key together within the cement. If you look at beach sand you will find the particles are rounded, like small pebbles. This is round sand and comes a poor second for building purposes. Beach sand also has a problem in that it has salt in it which can encourage damaging corrosion in steel reinforcing bars.

OK so you have crazy paving walls, what can you do about it?

How to repair cracks and crazed plaster on walls

The first thing to do is to check the sticktion (that's a good word isn't it) between the render and the body of the wall, to do this tap the walls with your knuckles to see of the cement render is loose. If it sounds hollow then you will know that the render has come away from the wall behind. In this case you will have to remove the cement render and replace it (sorry).

If the cement render is solid and is still well adhered to the wall then you can simply fill the cracks and repaint the wall. Remember the render is only for appearance and so, as long as it is not going to fall off when little Jimmy wants to play football in the living room or granny throws a pan at you, it will be alright.

In external situations cement render often cracks away from the wall beneath as a result of expansion and contraction due to exposure to the sun. It is important that when the cement is applied to the wall the cement mix is correct and that the surface it is being applied to is clean, free of dust and is rough so the render can "key" onto it. As it is applied the cement needs to make very good contact with the wall behind without air gaps. Good plasterers usually wet the wall first then "work" the plaster onto the wall to make sure it keys properly. With the advent of pasterboard real plasterers are a dying breed I'm afraid.

If the cohesion between the cement render and the wall is poor then the heat from the sun will tend to heat the surface render to a higher temperature than the solid wall beneath, the render will expand more than the cooler wall and will break loose.

If you see telltale cracks in those beautiful cement finished surfaces on the outside of your house once again you can tap the walls to see if the render is loose. If it is loose you will need to knock it all off and start again.

Copyright © Phil Wilson December 2012
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