8.2.1 relative pronouns referencing nouns
If we talk about relative pronouns which refer a noun of the main clause we talk about something like that.
The jacket (
) I bought yesterday was expensive.
We built a house
roof is a meadow.
In this case we don't talk about a relative pronoun which refers to an idea, we don' t talk about something like that.
He lost his job what
was really bad, because for this reason he couldn' t pay the rent.
refers to the whole sentence, to an idea, not to a noun.