Collaborative writing is the process by which more than one author contributes to the content of a document. Multi-synchronous collaboration is very efficient for reducing task completion time but is known to produce inconsistent documents. Most existing collaborative writing environments do not really check the semantic consistency of documents. They rely on authors to verify the coherence of the document. This introduces a severe overhead for authors to achieve efficient collaboration. To address this lack, we use semantic web technologies and a discourse theory called Rhetorical Structure Theory (RST) to reduce the overhead of consistency checking. We develop OntoReST, an ontology based on RST that helps detect incoherent texts automatically. OntoReST also provides authors with valuable information about the semantic structure of texts which contributes towards more coherent documents.