ANSWER

Slope is a measurement of how much an elevation is changing. Just remember rise over run. How far are you going up (or down) divided by how far you’re going to the side.

You may be asked about slope as it relates to sewer pipes, site drainage for pervious and impervious surfaces, drainage on parking lots, parking ramps, ADA and IBC ramp maximum slopes, roof slopes and more.

To fully understand the slope you need to know the rise (vertical change) and the run (horizontal change). If you have these you can write the slope in a number of different ways, usually it’s a percentage or a ratio.

Slope as a percentage = rise/run * 100

Slope as a ratio is shown as rise: run, though the run is usually shown in a fixed unit, like 12 or 100. If you’re given 4:12 it means that for every 12 units you go horizontally, you move 4 units up. This could be feet or inches or yards, just make sure to pay attention to units. Often on the exam you will be given a question with one unit of measurement and asked for an answer with a different unit. A slope question might give you a slope ratio, a horizontal distance in feet and you will have to provide an answer in inches.

Some of the more common slopes are:

Sewers

• 1/4” : 12” (2%) for 2” pipes
• 1/8” : 12” (1% for 3-6” pipes)

Site Drainage

• 5% min away from buildings
• 2% min for impervious surfaces
• 1.5% - 10% for grass areas

Paved Parking Lots

• 2% - 3%

ADA Ramps

• Not greater than 1:12 (8.33%)
  • See 2010 ADA Section 405

IBC Ramps

• Not greater than 1:12 (8.33%) when it’s a component in a means of egress
• Not greater than 1:8 (12.5%) elsewhere
  

Slope answers and math below.

For the last example, I'm just demonstrating how rounding at different stages in the process can change your answer. You will always be told how much to round, but I usually make sure to not round until the final answer. 1/8:12 is about a 1% slope, but if you round to 1% too early it can throw your answer off.