Oh, they did occur already, but the previous homeowner simply filled them with joint compound and then cleaned the excess joint compound off with a damp sponge, leaving the small cracks filled with joint compound and you wouldn't have noticed those small white cracks against the surrounding white paint.
Typically, if the cracking occurs at a 45 degree angle eminating from the two top corners of doorways in interior walls, it's because of movement of the house due to frost heave.
The reason for this is simple to understand. A doorway through an interior wall represents a deep "notch" in the wall. Consequently, the weakest point in the plaster on the wall occurs at that "notch" or doorway.
Typically, when they build a house, they pour the footings that support the jack posts (that support any beams that support the floors) and the basement concrete walls first.
Once the footings under any jack posts and the basement walls are fully cured, they pour a floating slab onto the basement floor. So, the basement floor and the basement walls can move relative to one another.
In winter, water percolating up from deeper in the ground will freeze when it reaches the "frost line" which is anywhere from a few inches to a few feet deep in winter, depending on how far north or south you live. As that water freezes, it expands, and continued percolating of water up from deeper in the ground can cause an "ice lens" to form inside the ground. They're called ice "lenses" because their shape is similar to the "convex on both sides" shape of the glass lens of a magnifying glass. If the ground is frozen to the foundation of your house, and an ice lens forms at the frost line, then the H2O can lift your house slightly on one side or the other as it freezes and expands.
When that happens, the foundation walls can be pushed upward a little, but the footings under any jack posts in your basement won't be pushed up because they are below the frost line. (The moisture in the ground under them doesn't freeze in winter.) The result is that any interior walls will get "bent" as one end of the wall gets pushed upward, but the middle of the wall remains in the same position resting on the jack post. And, that wall will do as it's expected to; break at it's weakest spot, which will be at the doorways through those walls.
Buildings with shallow foundations (of only two or three feet in height) are particularily vunerable to frost heave because they don't have foundations that extend below the frost line to anchor the building down and minimize it's up and down movement due to frost heave. In these types of buildings they prevent movement of the building by both:
1) attaching thick styrofoam insulation to the sides of the foundation of the building, and
2) extending that thick styrofoam insulation laterally outward a few feet under the ground all the way around the building.
Thus, even if the ground freezes to the styrofoam in winter, that styrofoam is soft enough to allow the outside of it to move up and down without causing the house to be lifted. That is, the styrofoam acts as a "slip plane" between the moving frozen ground and the building's foundation walls.
The lateral insulation spreading outward under the ground actually insulates the ground below it from the cold of winter above. Geothermal heat below the 4 inches or so of foam insulation buried around the building keeps the ground around the building from freezing.
You could protect your own house from moving due to frost heave by insulating around your house to a depth of 3 or 4 feet with 3 or 4 inch foam insulation, and then laying 3 or 4 inch foam insulation down on the bottom of the trench you've dug around the house before filling it back in with soil. The vertical insulation would act as a slip plane between the moving frozen ground and your basement walls, and the horizontal insulation would prevent the soil below it from freezing in winter (and therefore eliminate any movement of the basement walls below it).
Google "frost protection shallow foundation" without the quotation marks and you should find quite a few web sites dealing with this.
Last edited by Nestor_Kelebay; 06-28-2009 at 11:39 PM.