How can I slope outdoor rounded steps?
Since they are concrete steps, slope them from the radius point. All the risers will be the same and the only difference will be at grade, which would be the same problem without sloping the treads. For that matter, the whole assembly can or could be pitched a little or all of the 5% slope to minimize the difference at the first riser.



I have not checked to confirm if it still holds true, but in the U.S., outside steps are required to have slope, but not to exceed 1/4" over 12".

To help picture this in words, if to make every tread level, the riser needs to be
20 cm tall. To make each one slope, reduce the height of the bending form to make the riser 5-8 mm shorter and raising the concrete form so the top of the forms are at finished height. This will automatically pitch the treads.