also...I know about decent rucksacks and all that, just want to know if wearing one while cycling on a road bike can be damaging, as due to the position of the bike, the weight of the pack will be on the back, not shoulders/waist.
But the info is greatly appreciated

But the info is greatly appreciated

Don't want that on my back.
- so converted to panniers and heat rash has gone. Never had it in my life before so haven't a clue as to whether I really was overheating, or it was the number of showers I was taking or some sort of allergy to soap powder compounded by the amount of sweat. I used a Deuter rucksack with airflow pads and hi-vis cover.
listen to your body. If your back aches when you get off the bike after getting said good rucksack, then yes, i would say it was damaging. If you feel fine then i would say you are fine to carry on.