>>2555759>It'll be fine particulates in the air that are radioactive and you breathe it inActually you should be more concerned about fallout on your skin/clothes than breathing it in. Sure, cover your airways but if improvising a respirator would mean you spent 5 more minutes in contaminated clothing you should skip it.
>preferably 2-3 months before moving out of that buildingNot necessary as fallout dust loses most of it's radioactivity after 24-48 hours. If you got caught in fallout zone you should shelter in place for at least that long (of course if you have supplies you should wait for rescuers longer)
You are right about getting as much space and matter between your and source of radioactivity (fallout dust) as possible. It will be in places where snow falls. Roofs and ground. If you can get underground, do it. Even shallow cellar is better than staying on ground level. If there is no underground you should shelter in the middle of a tall building. If there are no solid walls around you should create them of watever material is available eg. books. As soon as the fallout settles it will start "microwaving" everything in straight lines, either you or the cover you're behind. It won't bounce of the ceilings, air won't become radioactive by itself, once filtered of the fine dust it's safe to breathe.
This is decent dramatized survival guide to urban nuclear attack:
https://www.dailymotion.com/video/x54sxft 