On Aug 18, 1:34*pm, "dennis@home"
wrote:
"The Natural Philosopher" wrote in ...

Don't assume. Rafters are largely in compression.

If rafters are in compression what keeps the walls from spreading out?

Mostly it's down to the fact that the walls are thick enough and heavy
enough that the thrust line doesn't go outside the walls. The tension
in the joists can help, or you can add flying buttresses, or the walls
can fall over after a while.

.... but rafters /are/ in compression.