Reasons to calculate slope:

• To determine whether parking spaces are level.

• To determine whether an incline meets the criteria for a ramp.

• To determine whether the slope of an existing ramp meets the criteria for safe and effective use.

• To ensure that the pathway is not higher on one side than the other (cross slope).

Reference points:

• Cross slope maximum: 1:48 (1” rise for every 48” length) = 2.08% = 1.19°

• Ramp minimum: 1:20 (1” rise for every 20” length) = 5.0% = 2.86°

• Ramp maximum: 1:12 (1” rise for every 12” length) = 8.3% = 4.76°
  

Slope Ratio Formula
Slope = Rise ÷ Length/Run (e.g., 6” ÷ 72” = 1:12)

Method #1 for computing slope:

• Use the Surface Level component on the Bubble Level app to determine the degree of cross and run slope.

• For slope, set your smart device on edge. Use the chart above to convert the degrees to determine whether it meets requirements.
  

Method #2 for computing slope:

• Measure height of the rise in inches. The ramp needs to be at least that number of feet long to meet the 1:12 standard.

• Convert the target length measurement to inches.

• Measure the actual length of the ramp and compare to the target minimum length to determine whether it meets requirements.
  

Other methods for computing slope:

• Use an instrument like a protractor or goniometer with moveable arms to calculate slope against level. This article has technical details.

• For most audits, it is fine to state whether it meets the standard (i.e., “less steep than 1:12”) or not (“steeper than 1:12”) without exact slope calculation.

• Quick screening option for parking spaces: eyeball to see if it seems even. Use a marble or something that rolls to double-check.
  

The Bottom Line:

• If the surface is steeper than 1” rise for every 20” length of pathway, it must be treated as a ramp and meet criteria such as having railings.

• If the sidewalk or path is less steep than 1:20, it is not a ramp and doesn’t need railings.

• If a ramp is between 1:20 and 1:12, it is ADA compliant.

• If it is steeper than 1:12, it is too steep to be ADA compliant and would be difficult and unsafe to use.

Deaconess Lynn Swedberg, 4/6/24