Why Does Drywall Mud Crack?

Drywall comes in smooth, flat sheets that, with proper installation, will remain smooth and unblemished for a very long time.  For most walls, however, more than one sheet of drywall is needed, and the small gaps between these sheets must be filled with mud and tape.  These joints are often to blame for the ugly cracks that sometimes appear in our walls.  Is there anything that can be done to prevent this?  Why does drywall mud crack in the first place?

Most drywall mud contains water.  It dries as the water evaporates out of the compound, causing slight shrinkage.  When the exposed surface of the mud dries faster than the innermost part of the layer, cracks often form. Because of this unavoidable physical property of drywall mud, proper application is the first and most important step in preventing cracking.

Drywall mud should be applied in layers.  The first layer should be the thickest, and each subsequent layer should be a little thinner.  Small cracks may form in the first layer, but they should be shallow enough to be easily covered by the next layer.  Each layer should be allowed to fully dry before applying another layer.  Generally, a total of three layers is sufficient for a strong but smooth finish.

Using the correct type of drywall mud for each layer can also help prevent cracking.  For the first layer, a dense and highly adhesive mix of mud should be used.  Each layer after the first should be a more lightweight mix than the previous layer.  The type of tape used matters also.  Paper tape is stronger than mesh tape and is therefore recommended in most situations.  Mesh tape can produce great results as long as it is used with the correct types of mud - using all purpose or lightweight mud with mesh tape is generally not recommended.

If your first layer of mud cracks, all is not lost.  Most small cracks can be covered with properly applied subsequent layers.  If cracks in the initial layer are deep or crumbling, however, it is best to scrape away the layer and start fresh.  To patch a small cracked area, sand down the offending section after the mud has completely dried and scrape out any crumbling mud.  Cover the area with new tape, being sure to keep the tape perfectly smooth.  Then apply a thin layer of mud over the tape, feathering it out at the edges so it is level with the existing mud and drywall. 

Proper application of drywall mud and tape will prevent most cracks.  While some cracks are caused by settling or environmental conditions outside of anyone’s control, a good mudding and taping job will make your walls more resistant to even these cracks.  Take your time in selecting and applying mud and tape to your drywall seams, and the chances of unsightly cracks popping up in the future will be kept to a minimum!

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