Im reading two different things. Can you tell me what im missing?
Diagonal Cracks
Diagonal cracks that grow in width, especially ones that are wider at the bottom than at the top, indicate settlement. Diagonal cracks over windowsindicate a weak header. Diagonal cracks in a poured concrete foundation that are fairly uniform in width or are hairline-type are caused by shrinkage and, though they may allow water entry, do not constitute a structural defect.
Tiny cracks that follow the plaster surface that look like spider veins may be indicative of a moisture problem behind the plaster itself. Diagonal cracks can be associated with settlement or shifting of foundational elements. Cracks over doors and windows can be attributed to the expansion and contraction of wooden framing components, and are generally not considered structural defects.
PS, Is there a place for students to ask questions? I though I found it but it said im not allowed to post anything.
Some times the spider veins is the paint cracking. Hit it with a green scrubbie gently and water and if you get to stucco and no crack then it is the paint. I do not know if the same is true with true Plaster.
One section is describing cracking in structural concrete. The other section is about cracking in stucco veneer or plaster over framing. Two completely different problems. Stucco over framing cracks a lot, especially if not cured right after installation or not enough control/expansion joints.
Most shrinkage cracks occur at predictable locations in concrete walls. Concrete shrinks uniformly in all three directions, height, length and thickness as it cures. The effects of shrinkage in the wall thickness direction can be ignored, just concentrate on shrinkage effects on the height and length of a foundation wall. As concrete cures the shrinkage forces start to grow in all directions until it reaches the tensile strength of concrete, which is very weak compared to compression strength. Concrete as it shrinks naturally will crack at uniform intervals unless the geometry of the wall or slab causes stress concentrations to occur. In walls, door and window openings, abrupt changes in cross section like in a step down foundation wall all create stress at the corners. The first two pictures shows shrinkage cracks occurring uniformly at locations where the walls are sectioned into blocks where the length is equal to the height of the wall. Once wall openings or changes in wall height occur stress concentrated at the corners, highlighted in yellow, will force shrinkage cracks to occur at these locations first. There are other factors such as reinforcing and interaction between the wall and footing joint that will impact the generic locations I described, but in general you can expect cracking as I outlined. As a side note once a shrinkage crack occurs any future foundation settlement will likely cause an existing shrinkage crack to widen (typically at the top), basically the wall was already broke.
