Setting default page is a tricky process, as umbraco always defaults to
the first page in document structure. Sometimes, the first page in your document
structure may not be the default document you. In such case you can use the below
approach.
In the template of the first document (or create new document type for
tis and use this for creating a root document), add java script redirection as
below .In my case it will redirect to index.html.