The Rule of 45 is a technical term used in physics and has a very practical application in Structural Engineering as it pertains to swimming pool construction. For all you homeowners interested in contracting out your own pool, you would do well to understand not only how its used but a little bit of the background as well.
Back when I built my pool, I knew how this rule was applied, but it wasn't until an associate physics professor from New York wrote to me explaining the background of the rule did I understand how it was derived.
My Structural Engineer explained to me that the edge of my house had to be a distance of at least the depth of the pool away from the edge of the pool. If it wasn't, then special engineering would be required to calculate how much extra steel would be required in the pool's frame to offset surcharging effects from the house's weight on the pool's shell.
Mark D. Shattuck, a physics professor from New York who specializes in granular media and soil mechanics, wrote to me a while back after stumbling across my site one day.
He explained in his email that "the rule of 45 refers to 45 degrees, the maximum angle from the bottom of the pool that must be considered to calculate surcharge effects. If you draw a line from the bottom of the vertical wall of the pool at 45 degrees, then anything inside that line at the surface must be considered for surcharge effects.
In the diagram above, the area highlighted in red must be considered for surcharging effects if the edge of the house encroaches into the distance L from the pool's edge. So if a house's edge is at least a distance L away from the pool's edge (where L=D, where D is the pool's depth), then no special engineering is required.
What this practically means is that when you are designing your pool drawing, you should (if at all possible) insure that the location of your pool's edge at least the depth of your pool away from the house's edge when determining where to dig. If you can't insure that due to unforseen circumstances, then your project will require special engineering to beef up the pool structure. This will invariably mean more steel/rebar, which will require more $$$. Most cities will have regulations about how close the pool can be to the house at a minimum, so be sure to check with your city's developmental services office to make sure you are in compliance.
If you are using a pool design service for your pool drawing, be sure to ask them what kind of set back codes apply to your area. Your structural engineer can also help you with this if you are designing your pool on your own.