How do Joseph's children escape the house?

It is a terrifying couple of steps in the V between the chimney and the roof ridge. They slide into the ridge and cannot see what is going on below. All of the roof ridges in the area are connected, which is lucky: they would not have gotten away otherwise. They climb for about a hundred yards; then they hear a magnificent explosion. They see their house billow up into fire and smoke.

Edek calls for them to keep moving and they finally descend a twisted fire escape. They hurry far away from the fire. The pale dawn breaks and they shelter in a cellar of a bombed house.