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Old 07-09-2008, 09:39 AM
Plynck Plynck is offline
Join Date: Feb 2004
Location: Duchy of Grand Fenwick
Posts: 2,139
Originally Posted by robby
Civil engineer here. I've never heard the grade of the road defined as anything but the rise over the run expressed as a percentage.
Damn. Scooped by robby yet again. Always a bridesmaid...

Okay, let's find something to add here. When designing a roadway, a percentage is used to define the profile (longitudinal section) of the road. The cross pitch (cross section) is usually also defined as a percentage (say 2%), but is occasionally referred to as inches/foot (say 1/4"/ft, which is close enough to 2% as to make no difference).

A similar thing happens with piping. When our plumbing engineer is running gravity pipes (sewer or drain) inside the building, he uses inches/foot. When I pick them up outside, I use percentages.

This is purely a guess, but as far as I can determine the designation generally follows the scale of the drawing. Plumbing drawings are drawn in architectural scale (say 1/4"=1'-0"), and use inches/foot for slopes. Site plans use engineering scale (1"=20') and use percentages. However, site details such as cross sections are typically drawn using architectural scale, which may explain the residual use of inches/foot.

When expressing the grading of slopes, we get into an odd situation. You're probably familiar with roof pitches expressed as rise/run (say a 4/12 roof, which rises 4 inches in 12 inches). Well, for some reason slope grading reverses those, so a 3:1 slope rises 1 foot in 3 feet). Go figure.

So, using the examples above, the following are all equal:

100% slope
45 degree angle
12/12 pitch
1:1 slope