Okay I'd try and hobble the duckling so the legs are sitting properly under it's body.
http://www.poultrypedia.com/poultry-podiatry scroll down until you come to Hobbles it will show you how to do it and other info also. The duckling will need a niacin supplement you can use Nutritional yeast 1 tab per cup of feed or niacin tablets crushed up onto feed just not flush free or time-released or liquid B complex which is fastest to help. TSC carries one that many of us use it will be in the cattle section and say injectable but we use it orally. 1/2 ml in a small amount of drinking water or over a tiny tasty treat. probably at this age since it's probably not eating much giving in water or tiny drops into the bill is better for doing liquid.
In NC there is Carolina Waterfowl rescue also.
Reason i asked how the duckling got off the roof was just wondering if the jumped maybe the duckling got hurt?
both legs look normal no swelling or abnormal heat? usually once you hobble if there is nothing else going on like an injury they get their legs going normal in a short amount of time.