Before setting a fall, route the pipe as normal and dimension it. At any stage in
this process, you can use Set Fall 
to define the magnitude of the change in elevation.
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Select the straight section of pipe to which to apply the fall.
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Select Dimension > Set Fall Command
to display the Set Fall Dialog.
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Type a value for the fall. In the example below, a value of 1.0 is entered in the Angle in Degrees box.
The software calculates the remaining four values and enters them automatically into their respective boxes.
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In the Fall to the section, specify which end of the pipe is higher. In the example above, the pipe will fall to the south making the north end higher.
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Click OK.
The software updates the model to show a small triangular Fall symbol, indicating that a fall is applied to this straight section of pipe.
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The fall that you specify is applied to all components of the straight section. It is only necessary to select one piece of pipe on the same straight for the fall to be applied to all the pipe and components.
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The Existing dimensions options are available only if the pipe is already dimensioned. These two options allow you to specify whether the existing dimension is to be taken as the orthogonal or hypotenuse dimension:
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If you select Make Orthogonal, then the dimensions of existing straights in the pipe will change (component lengths are fixed).