We have monthly Navigo passes for zones 1 and 2 and need to make several trips to CDG. My question is whether we need to pay full fare going to the airport from the Gare du Nord station, or whether we just pay a supplement to go along with our pass? Also, does it matter if the trip is on the weekend or not.
Thanks for any responses.

Due to a very recent change, monthly Navigo passes have unlimited zone access on the weekends. You should not need pay extra, just use your normal zone 1-2 Navigo pass to travel on the weekends to CDG (zone 5).

Otherwise, the central Paris to CDG fare is 9.50€ for which your pass will offer no discount.

Thank you; i thought i had previously read that about weekends but i could not find that information again.

Here is the reference:

<i>A partir du 1er septembre 2012, pendant les week-ends et jours fériés, les voyageurs détenteurs d’un forfait Navigo mois, Navigo annuel et Solidarité Transport Mois pourront utiliser l’ensemble des transports en commun d’Île-de-France, quelles que soient les zones de validité de leur forfait, avec leur abonnement habituel.
Le dispositif est valable sur l’ensemble du réseau métro, RER, bus, tramway, Transilien et Noctilien* (les nuits du vendredi au samedi et du samedi au dimanche), pour une liberté totale.</i>

http://www.ratp.fr/fr/ratp/r_72748/d...-jours-feries/

giro - there is another recent change to the RATP ticket policy and, as a Navigo pass holder, you should be able to purchase tickets outside of your normal zone coverage (zone 1-2) at a reduced rate. Ask at the service window.