Translation:The two families live under the same roof.
Why is "lo stesso tetto" and not "il stesso tetto" when tetto comes with definite article il? Does somebody know?
Is "lo" because you have to use always "lo" when the next word begins with 's' or 'z', as in 'stesso'.
Although not necessarily. For you tu use "lo" the word must be masculine (for example you say "la scatola" rather than "lo scatola") and if the first letter's "s" then the following must also be a consonant for it to need the use of "lo".
Nope, because "to live under the same roof" is an idiom that implies sharing a dwelling with someone (living in a close relationship).