Christophe wrote:At the level of the profiles there is a "triangle" whose base is "embedded" in the attic floor. I don't know if that's a clue for you ?.
Yes now it's very clear, your beams (that of the floor), most likely come to rest on the central IPN (that of the triangle) which is about 160 / 180mm thick. Knowing this, we can assume that there is also an IPN at the level of each pinion and having the same function. But you would have to discover the paneling below to be sure.
Regarding the chaining, citro answered you, I think you see what it is now. I would just add that a chaining serves to support and distribute loads, that of a floor slab for example.