Our ovens are designed to be used under an open sky. We do not recommend operating any of our ovens under a roof or covering as there is the potential for discoloration from the smoke on your ceiling surface.We've had customers who have installed a custom length chimney through a roof with great success. Unfortunately, we no longer offer chimney extensions or elbow joints.
These parts will have to be sourced locally by you. If your oven was purchased prior to May 2020, it has a 14cm diameter chimney pipe. You will need get a Metric to Imperial adapter for our 14cm chimney. Domestic chimney extensions are sized in the Imperial system (inches, not centimeters).
If your oven was purchased after May 2020, it has 15cm diameter chimney pipe. The 15cm diameter mates well with Imperial 6" diameter pipe and requires no Metric to Imperial conversion. Duravent is a popular manufacturer of stainless steel chimney extensions in North America. You may be able to search online for "Duravent 6" chimney pipe" to find a 3rd party retailer who sells an extension to suit your needs.
As far as the oven's functionality any extra chimney exhaust pipe added above the damper control doesn't affect the oven's performance. In fact, this one of the ways they test units indoors at the Fontana Forni factory in Italy.
The one other thing we'd strongly recommend is to check with your local building and fire codes for this sort of thing as these regulations vary from one location to another.