Since topoi were introduced, there have been efforts putting mathematics into the context of topoi. Amongst known topoi, the topoi of sheaves or presheaves over a small category are of special interest. We have here as the base topos that of sheaves over a monoid [Formula: see text] as a one object category. By means of closure operators we then obtain categories of sheaves related to the right ideals of [Formula: see text]. These categories have already been studied but we give these categories a more thorough treatment and reveal some additional properties. Namely, for a weak topology determined by a right ideal [Formula: see text] of [Formula: see text], we show that the category of sheaves associated to this topology is a subtopos of [Formula: see text] (the presheaves over [Formula: see text]) and determine the Lawvere–Tierney topology yielding the same subtopos, which is the Lawvere–Tierney topology associated to the idempotent hull of the (not necessarily idempotent) closure operator associated to [Formula: see text]. We will then find conditions under which the subcategory of separated objects turns out to be a topos, and in the last section, we find conditions under which the category of sheaves becomes a De Morgan topos.