Steel orthotropic decks in bridges and ferries have shown to be prone to fatigue cracking. A typical fatigue crack observed in these structures initiates at the root of the weld between the deck plate and stringer and grows into the deck plate. Due to limited fatigue test data available and the deviating characteristics of this type of crack as compared to other types of fatigue cracks, the fatigue strength is uncertain. In this study, a linear elastic fracture mechanics model has been developed for this type of crack, providing insight into its fatigue performance. The model predicts a relatively high fatigue resistance which is close to the results of fatigue tests. The model further predicts a relatively long residual fatigue life after crack detection, providing large inspection intervals.