Apply www-data group to all new files created

I'm trying to grant access for group www-data automaticly to every new files/folders created on /var/www I already tryed theses commands:

sudo chown www-data:www-data <DIRNAME>
sudo chmod g+w <DIRNAME>

But everytime I create a new file or directory the group for the file are root and not www-data. So, am I doing it wrong ? Or is there any other way to make this work?

Thank you very much.