A rug is a good idea. Would help, if it had a rubber backing.I've placed many ladders on outdoor rugs. Usually they are pretty stuck in place, be it on concrete or deck boards. Curious how many other painters find this to be an acceptable ladder placement?
It’s all a calculated risk. Can’t be too safe.
In addition to trying to create a “grippier” surface, we also put a sandbag behind the ladder feet, or some other nearby weights (landscaping boulders, full five gallon buckets, etc…) to prevent any chance of sliding back. Composite decking and some asphalts can be really slippery.