This paper concentrates on the proper use of fountain codes for the transmission of sporadic data in a wireless sensor network (WSN). Fountain codes offer great perspectives for the self-organization of WSNs: they self adapt to the channel error rate without control packets. Deploying fountain codes in a WSN raises two problems. First, the size of the data transmitted by a sensor is small in comparison to the size usually considered with fountain codes. Second, WSNs mostly rely on multi-hop transmissions. It implies a non null transmission duration for the end-to-end acknowledgement of the reception. During this period of time, the source is still transmitting useless packets, creating a specific overhead we define as the overflow. This paper brings the overflow problem to light and analyses its impact on the network performance. Our work can be viewed as the networking counterpart of the results presented by Pakzad et al. at ISIT 2005 applied to WSNs.