I used to use a specific laptop backpack with built in laptop padding and separate zippered sections but found when riding it would sit too high up and potentially obscure my view when doing over shoulder head checks.
Fortunately I have tiny little Toshiba Portege (1.3kg, size of A4 sheet and only about 1cm thick) so now I use a Deuter backpack with the airflow mesh at the back and put the laptop inside a padded sleeve.
There isn't a huge amount of room, but enough for what I need (rainjacket, gloves, pump, food, odds n sods). All my clothes, shoes, toiletries are stored at work
I also backup files that can't afford to be lost to the LAN/USB as they get revised (surprised how many people I know that use laptops do not back anything up at all