Due to the complexity of the Wire-arc Additive Manufacturing (WAAM) process, it is prone to the occurrence of defects in the product. One of the most common defects is porosity, which is detrimental to the mechanical and fatigue properties of the product. To guarantee the product quality, it is important to detect its occurrence. In this research, a strategy was developed to detect the occurrence of porosity and to identify its root cause. To develop a porosity detection strategy, a monitoring set-up was designed in which the WAAM process is monitored by a current sensor, condenser microphone, structural Acoustic Emission sensor, spectrometer and flow meter. Seventeen experiment samples were produced and the sensor data were stored using a data acquisition system. A sensor data analysis showed that current and acoustic signals could be correlated with the occurrence of porosity. From these sensor signals, 74 features were extracted. A validation of the samples, based on milling, grinding and image processing, was developed to obtain the pore density values for the corresponding feature set. A long short-term memory neural network was trained through supervised learning to classify feature data into acceptable or non-acceptable pore densities. A test accuracy of 82.52% was achieved. Apart from the porosity detection, a strategy for porosity root cause detection was developed in order to distinguish between process related porosity and contaminant related porosity. An accuracy of 90.67% was achieved, whereby the remaining inaccuracy is interpreted as a physical phenomenon and the results from the model are interpreted actually as more accurate than the prior assumption of the training labels.