PurposeAs social infrastructures, public libraries are increasingly recognised as providing more than access to books and information; librarians’ work is importantly centred around practices of care. However, the ways in which they provide care is poorly researched, let alone conceptualised. This paper explores how this important part of librarians’ daily work is practiced through the lens of infrastructuring.Design/methodology/approachThe paper first theoretically discusses the concepts of social infrastructuring, care and tinkering. Then, it turns to ethnographic research conducted in the public library networks of three European cities: Vienna (Austria), Rotterdam (the Netherlands) and Malmö (Sweden). The paper comprises empirical materials from all three countries and unpacks 16 librarians’ daily working routines of care through participant observations.FindingsThe empirical analysis resulted in three modes of social infrastructuring in public libraries: (1) maintaining, (2) building connections and (3) drawing boundaries. Practices of care are prominent in each of these infrastructuring modes: librarians infrastructure the library with and via their care practices. Whilst care practices are difficult to quantify and verbalise, they are valuable for library patrons. By using the concept of tinkering, the article conceptualises librarians’ infrastructuring enactments as crucial community-building aspects of libraries.Originality/valueBy focusing on the enactment of social infrastructuring, the paper goes beyond a descriptive approach to understanding public libraries as important social infrastructures. Rather, the paper unpacks how libraries come into being as infrastructuring agencies by highlighting what librarians do and say. Our international study articulates the importance of care practices in public libraries across different national contexts.