The likeliest explanation is this: In areas where bread was a staple, communal ovens were traditionally fired up only once or twice a week.

Today many people have gas stoves or propane cooktops at home, and the communal ovens are disappearing.

In later periods, large communal ovens appear to have been built on these rooftops.

A baker made big loaves in a communal oven.

Thorvin had gone off to see about their turn for baking bread in the communal ovens.

These communal ovens with time evolved into the modern bakery.

Children rush past on their way home with palletes of fragrant bread fresh from communal ovens.

Many of them are also carrying dough, taking it to the communal oven.

So, they would take their prepared foods to a communal oven for baking.

Indeed, for a smaller fee, he would supervise a gathering of neighbors in building a communal oven, drawing on old traditions.