Second, if it truly is leaning; is it in equilibrium or is it moving? This is difficult to tell at a single point in time. You will likely need to measure the lean on several occasions over a period of time to determine the rate of movement. However, it is likely that someone repaired the gaps made when the wall started to move. So did the gap/crack re-open since the last repair? Does anyone know when the repair was made? If it was some time ago the materials used may not be readily available and that can be used to date the repair and subsequent movement.
[Crack diagnosis is a topic for another blog]
Third, how thick is this wall? Forth, how homogeneous is the wall? What condition is it in? The real question is; will it pretty much act as a unit or is it going to collapse into a pile of rubble? If the masonry units are rectangular and reasonably sound individually, and there is any sort of interlocking bonding between the wythes, it needs to be in really terrible condition to slide apart and become a pile of rubble.
Veneers that have become unbonded can potentially act independently from the rest of the wall.
About this time you are really wondering is this wall going to fall on me while I am measuring things? Is it going to fall into the street? Do we need to close Pennsylvania Ave.?
Lets start with basic physics. On Earth an object is stable, if its center of mass/gravity lies within its base. You can tip it and if its center of mass still lies within its base it will right itself [return to its original stable position] if released. If it is tipped far enough that its center of mass lies outside its base, it will topple over, if released. So what you really need to know is whether the center of mass of the wall is still within its base. Let’s look at a simple diagram.
First lets note that the center of mass of a uniform object is its physical center. The center of a rectangle is the point where its diagonals cross.
The drawing figure I shows a typical 3 wythe wall that is 12in. thick. The dotted position is leaning approx. 2in. The center of mass moves from A to B as it is tipped. The solid lines show the upright position but also provide verticals from the wall’s base. Obviously the center of mass is still well within the base at a 2in. lean. So what is the toppling point? In figure II the wall is tipped until the center of mass C is on the line/boundary of the base. As you can see the wall must tip the full thickness of the wall at the top to become unstable and topple. On this typical 12in. thick wall it must lean a full 12ins.
That is way more that one would expect, but try an experiment with a brick or short length of a 2×4, Watch this YouTube video.
Set it on end and lean it either direction [long or short dimension] and see that the full top must be out of plumb for it to fall over when you let go. Now take two bricks or blocks on end and when you tip them by leaning just one against the other, but not holding the second one, the second one will topple when its tip gets outside the base. However if you connect them together [wrap duct tape around them] it requires the top width of both.
This means that if the veneer course has become unbonded then it will only take a lean of 4in. for the veneer to fall away. That does not mean the building will fall down but a 4in. column of unbonded bricks can break away and can do quite a bit of damage to anything below.
While any masonry wall that is leaning anywhere near its topple point should be considered very dangerous; one should also note that, as with the experiment, the brick, regardless of its lean, has to be released to topple. If the wall is restrained by joists or bonded at the corners then it has not been released, but is only being held by friction. However, if a section of wall has cracked and is thereby released, you can expect it to topple when its center of mass gets outside of its base. Don’t play under it!