As far as I know, the only wallet that uses the term "root key" for the backup is Armory.

Armory Root Key: 18 four letter "words".
Example (from bitkee.com): eoaj gghu ruaf ghwe jnrh ftuu hweu aeun agkg tudt waja gunn oawg jkwh dhei hjdn itar naoj
Use Armory. See this post to export private keys.