The problem is most likely caused by incorrect file attributes of the alternate directory root. Issue the following command and compare the output:0-
ls -ldZ /var/www/html/ /your/alt/dir/root
You should get something that looks like this for your original root directory:-
drwxr-xr-x apache apache system_u:object_r:httpd_sys_content_t /var/www/html/
If the alternate directory root, and all the files below it, aren't given the correct SELinux context (system_u:object_r:httpd_sys_content_t), then SELinux will block access to it. They really should be owned by apache as well.
This command will set the appropriate SELinux context for your new directory root:-
chcon -R --reference=/var/www/html /your/alt/dir/root
Let us know how you get on.