in src/main/java/org/apache/maven/plugins/site/deploy/SiteStageMojo.java [114:133]
    protected File getExecutionRootBuildDirectory() {
        // Find the top level project in the reactor
        final MavenProject executionRootProject = mavenSession.getTopLevelProject();
        // Use the top level project's build directory if there is one, otherwise use this project's build directory
        final File buildDirectory;
        if (executionRootProject == null) {
            getLog().debug("No execution root project found in the reactor, using the current project.");
            buildDirectory = new File(project.getBuild().getDirectory());
        } else {
            getLog().debug("Using the execution root project found in the reactor: "
                    + executionRootProject.getArtifactId());
            buildDirectory = new File(executionRootProject.getBuild().getDirectory());
        }
        return buildDirectory;
    }