When you have such low slopes (which I agree with Mike...that's the minimum), you need to make sure you have joints that do not allow water intrusion or you'll have a significant pumping problem.
We've all heard various minimums-0.5%, 1%, even 2%, but it all depends on what is the proposed use, climate, size, & other variables, and what are the consequences of surface toleranes, or movement over fime (e.g. rutting and frost heave).
Minimum slope can be 0% - for certain small areas, that can stand to have a little standing water on it after a rain. 0% is "constructible", since floor slab contractors routinely construct level and smooth concrete. Aircraft aprons are required by FAA criteria to be 1% maximum. I've seen large areas paved (asphalt) to 1%, and large concrete hardstands at 1% and less, without any obvious problems. Of course more care needs to be taken to achieve that with minimal birdbaths.
If a slope which promotes rapid runoff isn't possible, other methods could be used to prevent excessive ponding during rainfall, such as trench drains.
Depending on the size of the parking lot, 1% is pretty flat. Usually a parking facility of any size has peaks and valleys that drain into catch basins (inlets) that lead to an outfall, storm sewer etc.
If the pavement is flexible in material(s) it would not be wise to have standing water...that would deteriorate the pavement over time.
1% is an industry standard on asphalt pavement but when you are using concrete, 0.5% is acceptable. This is because of the smoother surface (lower roughness coefficient) of concrete versus asphalt. With concrete, velocities are slightly higher, the surface tends to be more impervious and runoff doesn't collect as it does on pourous asphalt pavement.
CarlB lists some good variables that should be taken into consideration.
depending on your location, 2% is also an "industry standard". Also, I would not get too hung up on roughness or velocities, because construction tolerances resulting in bird baths is a primary consideration.
Note that runoff will tend to pond for longer periods on concrete because it is not pervious. Asphalt is more pervious and any ponding rainwater will tend to infiltrate through the pavement and saturate the subgrade, possibly causing subgrade failure.
I would also add that it's more important to avoid bird baths on asphalt in areas where vehicles will actually drive. I've seen flat areas on asphalt where water ponds, and some of these areas actually hold up OK. But if vehicles are driving on this flat/pond area, the asphalt surface (and subgrade) will begin to deteriorate much quicker. But concrete seems to hold up better than asphalt under these same circumstances.
