There are numerous reasons why the plaster along the exterior walls of your home will crack making the interior walls look very unsightly. It is important to determine the cause of any cracks you find so that you can repair them effectively. Some of the most important causes of these cracks are listed below.

1. Structural Cracks

Structural cracks, as the name implies, are caused by structural weaknesses in a building which are worn further by certain conditions, such as the bathroom shower. A steam shower or steam bath enclosure should be properly fortified with tiling.

The most important structural cracks include:

• Settlement resulting from inadequate or improperly located footings, the use of undersized or improperly spaced members, omission of bracing, or shrinkage of lumber

• These cracks are usually large and well defined, extending across the surface and through the plaster

• They may start near the corner of a door or window, or run up and down the corner where two walls join, or along the joints between walls and ceiling

2. Map and Shrinkage Cracks

Inferior workmanship and the use of poor quality of plastering materials are the main causes of what are known as "map cracks and "shrinkage cracks;" that is, shrinkage in the plaster itself. There are ways to distinguish between these two types of plaster cracks:

• Map cracks are usually caused by improper bonding between the plaster and the base

• They are less noticeable than structural cracks and go through the plaster, but do not extend entirely across the surface, as do the latter

• They are made up of a series of cracks running at various angles and embracing areas usually 6 inches or more across

• Shrinkage cracks, on the other hand resemble map cracks, except that the cracks themselves and the areas they enclose are much smaller

• They differ from the map cracks inasmuch as they do not go entirely through the plaster and are usually confined to the finish coat

• Careless workmanship is usually the cause of these cracks

• Steam from a sauna can dilate these cracks; an infrared sauna unit is the best way to go

3. Loose Plaster

Sometimes the keys or clinches that hold the plaster to the base break off or become loosened and cause the plaster to bulge and crack.

On ceilings, around plumbing, bathroom fixtures and kitchen fixtures, it will often hang in this condition for a long time before falling off, being held together by the hair or fiber in the plaster.

Occasionally the nails or fastenings used to hold lath in place may corrode and break, allowing that part of the plaster covering the loosened lath, or laths, to sag and crack.

Tools Needed for the Repairs

For cracks and small holes a small diamond-shaped mason's trowel or broad-bladed putty knife, for plastering larger areas a plasterer's trowel and a shallow pan.

Materials Needed

• Plaster of Paris or commercial patching plaster

• A small amount of ordinary glue if necessary

• Clean water

Mixing the Plaster

• All mixing boxes and utensils should be clean and clean water from your glass sink, bathroom vanity, or bathtub should be used in the mix

•Particular care should be taken that no traces of old set plaster have been left in the mixing box

• The water should be placed in the mixing box before the dry plaster is sifted into the water

• The mix should then be stirred thoroughly to dissolve all lumps

• It should be of such a consistency that the putty may be picked up on a broad-bladed knife and forced into the crack or break in the wall

Using Plaster of Paris

If only a small amount of fresh plaster is needed, plaster of Paris alone may be used. Plaster of Paris sets very quickly. If it is to be used without a retarder, only so much should be mixed at one time as can be put in place in 10 minutes or less.