By default, Siteground's servers are configured to process the following index files:
index.html index.cgi index.shtml index.php index.htm default.htm default.html home.htm
In the above order, first the webserver first looks for the file index.html. If it is not present it looks for the next "index.cgi" and so on.
In case you'd like to use a different index file please create a file called .htaccess and specify in it:
DirectoryIndex myfile.html
This will cause the webserver to look only for myfile.html. If there is no such a file it will list the contents of the directory.