This paper explores the discourse status of English causal clauses introduced by since. Tests for non-at-issueness demonstrate that neither the relation (between the subordinate and the superordinate clause) expressed by since nor the content of the subordinate clause is at-issue. Other diagnostics further show that these two not-at-issue contents triggered by since belong to two different classes of projective content. This can be accounted for by attributing two different sources to their non-at-issueness: the relation expressed by since is not-at-issue for structural reasons, i.e. because since-clauses modify high evidential or speech act phrases, which are not-at-issue; the content of the subordinate clause is not-at-issue because since lexically selects factive clauses. More generally, this study (and future comparative studies on other subordinators) promises to shed further light on the constraints on different contents projected by the same trigger and the role played by structure in non-atissueness.