Reliable Web effort estimation is one of the cornerstones of good Web project management. Hence the need to fully understand which factors affect a project’s outcome and their causal relationships. The aim of this paper is to provide a wider understanding towards the fundamental factors affecting Web effort estimation and their causal relationships via combining six different Web effort estimation causal maps from six independent local Web companies, representing the knowledge elicited from several domain experts. The methodology used to combine these maps extended previous work by adding a mapping scheme to handle complex domains (e.g. effort estimation), and the use of an aggregation process that preserves all the causal relations in the original maps. The resultant map contains 67 factors, and also commonalities amongst Web companies relating to factors and causal relations, thus providing the means to better understand which factors have a causal effect upon Web effort estimation.