In above diagram, we have a typical expansion joint. Normally a
filler material is used first. Then, the more expensive sealant is applied
over the filler material. At this point, everything looks fine. Expansion and control joints are engineered into buildings to compensate for anticipated movement. The cause of the movement may range from:
Expansion and control joints allow segments of a structure to move independently of each other while retaining the integrity of the structure. It is imperative that the joints be large enough so that:
However, joint design is more than just the allowance of space between segments of buildings. The space must be "sealed" to keep weather out and the artificially controlled environment in. This is the area where most problems are created.
In above diagram, we have a typical expansion joint. Normally a
filler material is used first. Then, the more expensive sealant is applied
over the filler material. At this point, everything looks fine.
A small crack that has just appeared is of more concern than a large crack that hasn't moved for fifty years. Both the length and width of a crack change as parts of the house move. This is why it's important to chart the rate of movement - you'll want to know if it is getting faster or slowing down.
Cracks that don't move are clearly not a structural problem. Often the best way to treat them is to leave them alone. The same is true of decelerating cracks, those that move more slowly as time goes by. Eventually a decelerating crack will stop moving and then it can be patched. A crack that is moving at a constant rate is more difficult to deal with. If it is moving so slowly that it won't become dangerous for one hundred years, then it is often best to leave it alone.
Sometimes the movement of the building is spread over the length
of the wall or floor rather than just happening at one spot. When this
occurs, many small cracks will appear rather than one large crack. You can
assume the building is still moving if new cracks appear parallel to the
old ones. The total width of all these small cracks should be used to
chart the rate of movement.
However, if the crack is lengthening and/or widening rapidly, try to discover the reason for the crack and take corrective action. Accelerating cracks indicate that the structural stability of the house is being threatened and that the problem will continue to get worse. Action should be taken before the acceleration approaches that of a falling object (for example, the plaster ceiling in the dining room).
A fourth type of movement is cyclical, where the crack opens and closes in different seasons. The solution here again is to do as little as possible since this type of crack rarely causes structural problems.
The direction of a crack is another good clue to its cause. Horizontal and vertical cracks of small size (C-3 or less; see chart) are rarely any cause for concern. Diagonal cracks always indicate that the house is, or has been, in movement - one part of the house has shifted relative to another part.
A diagonal crack in an old house is almost always at a 45-degree angle to the floor or wall. Diagonal cracks indicate a phenomenon called shear, which results from either tension or compression. When a solid material is pulled or pushed enough, it shears or breaks along a 45-degree angle and slides to a new position. Brick walls, plaster-covered stud walls, and plaster ceilings all can act in this way.
By observing which way diagonal cracks slant, and which end of vertical or horizontal cracks is wider, you can determine the part of the house that has moved from its original position. The key to finding the cause of a crack is discovering the movement pattern of the house.
Though it's hard to imagine a wall being pulled apart, tension
shear cracks are very common in old houses. Tension results from one part
of the building staying in place while another part sinks. For example, a
column may rot at its base and sink while an outside wall stays in place.
One end of the wall above will then drop relative to the other end. Since
the plaster on the wall can't move far without breaking, it shears along
the familiar 45-degree angle.
The top of a tension shear crack points toward the end of a wall that has dropped from its original position. If, instead of the column sinking, the outside wall had settled, the crack would have slanted the opposite way.
Tension cracks may also appear because of lintel failure in brick or stone walls. If the arch or piece of stone or wood at the top of a door or window stops doing its job, such as the mortar weakening, the brick above the lintel starts to drop. As the brick is pulled down by gravity, the wall on the side of the opening is pushing up. This tension creates two shear cracks that run from the top corners of the opening to form a triangle above the door or window. Since the mortar is often softer than the bricks, the cracks tend to follow the pattern of the brickwork, creating a stepped pyramid appearance.
When one part of the building pushes down on another part, compression shear cracks appear. Less common than tension shear cracks, they are easily identified because there will be some crushing of the material along the line of the crack.
One kind of compression shear crack occurs when the outside of a building settles into the ground while an interior bearing wall or column stays in its original position. In this case, the column is pushing up while the exterior walls are pushing down. A wall that is above the column will shear in compression, causing 45-degree diagonal cracks whose tops slant toward the column.
Both tension and compression shear cracks set up an overall pattern that can be readily observed. Frequently these cracks come in groups that literally point to the cause of the problem. Since houses are usually divided up into many rooms, it is not possible to see the entire pattern at one time. The direction of cracks should be noted in each room and mentally added together like a jigsaw puzzle. Start at the top of a house and walk down floor by floor, checking rooms and halls on the way. One of the best places to look for cracks is in closets, since they are often not patched and painted with the other spaces.
When your mental picture of the crack pattern is complete, you should be able to determine the location of the cause. Sometimes the cracks will only be found in the corner of one room. This would indicate a localized abuse, such as a rotten beam end. Other times every room in the house may have been affected by cracking.