Slope
The Americans with Disabilities Act (ADA) requires a 1:12 slope for wheelchairs and scooters for business and public use. The ADA does not apply to residential construction.
UK guidelines as recommended by the DDA are a max of 1:12 for ramps (with exceptions for existing buildings) "Ramps should be as shallow as possible. The maximum permissible gradient is 1:12 (see Figures 12, 13 and 14), with the occasional exception in the case of short, steeper ramps when refitting existing buildings."
In Hong Kong, wheelchair ramp standard should not over 1:12 slope for wheelchairs except some of the situations under the Barrier Free Access (BFA) terms.
Read more about this topic: Wheelchair Ramp