PVC must not be heated, bent or stressed in any manner.
In order to insure that DWV fittings will not be confused with pressure fittings DWV fittings are listed by a fraction of a circle while pressure fittings are listed by the degree of the angle.
To determine the angle of a DWV fitting divide 360deg by the fraction:
1/4 bend = 90deg. (360 / 4 = 90)
1/8 bend = 45 deg. (360 / 8 = 45)
1/16 bend = 22.5 deg. (360 / 16 = 22.5)
If you end up with an odd bend one of your pipes is either too long or too short and the length must be corrected accordingly.
To mathematically compute the length of an offset begin by visualizing it as a triangle. (SEE ILLUSTRATION BELOW)
In all cases the sum of all the angles will equal 180 degress and one angle will always be 90 deg. therefore if you have a 1/8th bend (45deg DWV fitting) on one end of the offset the opposite end would be:
180 - (90 + 45) = 180 - 135 = 45 deg.
If you have a 1/16 bend (22.5 DWV fitting) on one end the opposite end would require:
180 - (90 + 22.5)=
180 - 112.5 = 67.5 deg.
There is no 67.5 deg fitting so we combine a 1/8th bend (45 deg) and a 1/16 bend (22.5deg) ...45 + 22.5 = 67.5 deg.
We can then compute the length of the offset from the Pythegorian Therum for a triange.
The Hypotaneue equals the square root of Side A squared plus Side B squared.
Once we know the hypotaneus of the triangle we must then deduct the fitting allowance.
The fitting allowance is the overal distance from the center of the fitting to the base of the hub.
once we know the hypotaneus of the triangle and deduct the total combined fitting allowance for all fittings on both ends of the offset, the resultant is the length of the offset pipe.