If you are making a claim for damages in a construction case that involves construction defects or damage resulting from construction defects then you need a construction expert witness to offer evidence and opinions based on scientifically valid reasoning. Clients often want to act as their own expert stating opinions about the defects. While they may offer factual observations they can not draw conclusions from those opinions, it takes an expert witness to do so.
An expert witness is an expensive proposition, however, without them the case will have difficulty proving that the work performed was not performed in a good workmanlike manner and/or that damages are resulting from the construction defect that the expert observed. An expert should be retained early on to allow them to view the Project as left by the Contractor so the expert can document the defects and the potential resulting damage. Performing remediation or repairs unless they are performed because of an emergency prior to obtaining and having an expert review the Project is a mistake that can damage your change of recovery.
Make sure the expert you or your attorney selects is qualified to testify as to the issues that have arisen. Also make sure the expert has been previously certified as an expert in the court where they will be testifying. It is not a good thing when the other side seeks to disqualify the testimony of your expert at trial because he/she are not and cannot be qualified as an expert in the jurisdiction you are in. For example do not hire a general contractor expert licensed in Nevada to testify in a California court unless they can demonstrate they are knowledgeable about the California Building Code and have experience applying it to situations similar to yours.