The ADA guidelines provide specific ratios for accessibility ramp measurements that are ensured to optimize safe use. ADA requires that all business and public use ramps adhere to a 1:12 slope ratio, meaning that for every inch of vertical rise there must be 12 horizontal inches (one foot) of ramp. If, for example, you had a stairway with a vertical rise of 21″, you would need 21′ of ramp. Additionally, ADA stipulates that no ramp can run longer than 30 consecutive feet without a rest or turn platform.
Residential ramp applications are not required to follow the commercial code, instead they are suggested to feature a slope up to a 2:12 ratio, or one foot of ramp for every 2 inches of vertical rise.
Why the difference? ADA laws are designed to govern public accommodation rather than private. It is up to the homeowner or property owner to maintain a proper ramp incline for guest access.
Rise is determined by measuring each stair height, in inches, and adding them together. Example: you have three stairs leading to the front door. Each stair measures 7″ in height – that means your rise is 21″.