It is explained that when Voldemort tried to kill Harry as a baby, his Avada Kedavra rebounded due to Lily's sacrifice.
This caused two (important) things to happen:
- Voldemort got blasted into an unknown state of 'undeadness'
- The Potter's house got blown to pieces
How did Harry not get crushed in the events?
My thoughts are that the destruction was caused by Voldemort, and therefore Harry was protected by Lily's sacrifice.
It's just a funny thought that there could be this little baby with a massive (house) beam on top of him.
I seem to remember that the roof got blown-off, so probably not much debris that could have hurt him?
Most of the cottage was still standing, though entirely covered in dark ivy and snow, but the right side of the top floor had been blown apart; that, Harry was sure, was where the curse had backfired.
-Harry Potter and the Deathly Hallows (Bathilda's Secret).