Hi @ylouka ,
I think it can depend on the specifics. 5% is typically a good rule of thumb maximum for a parking lot as 5% (1:20) is the maximum slope for an accessible route that is not considered a ramp requiring a handrail per ADA & IIC 117.1 .
Per the IBC section 406.3 shown in my previous response, if you’re designing a parking garage with ramp that will also be used for parking the max is 1:15 or 6.67% - I think this was likely the 6% Joey was mentioning.
@coachchrishopstock can you please look into the 20% on the Black Spectacles question and see if you can help explain?
I’ve heard 20% thrown around colloquially as the max for a vehicular ramp, not used for parking, but my understanding is that the max vehicular slope can vary based on municipality.