Probably part of the plot to move into the forest but it was a stupid move.
They were safer in the house and with a bit of work could have fixed up the roof and dried the place out - no way they would have survived in that little tree stump with a baby for long.
Apart from that it was a well paced but ultimately lost something half way through.
Its uncle Frank Kirsty!
Its time to play!
I agree it was stupid. They could have repaired it or salvaged far more, and of course shouldn't have let it get so bad in the first place. The books alone can mean live and death for them. The frigging solar panels and water filtration etc. Burning the house gives them nothing except a little more concealment from that single predator / rapist.
But after watching "World War Z" I realized something: Diversity can be important. They chose to start to live like primitives in the woods. Maybe through some unforeseen circumstances or events that decision leads to them surviving while all others fail.
The individual isn't that important here - the two have a higher chance of dying in the forest, but humans can repopulate incredibly fast. It's far more important that ENOUGH survive. And diverse cultures helps with that.
Think of mono cultures vs biodiversity.