Here are some pictures of a grooved roof mount. Took two tries because I used a poor sealer the first time. Has been fine since. You would be cutting one of the five roof stringers. To get the roof strength back I made a frame of 4/4" x 5" oak to tie the middle cut stringer into the ones on either side.
The hole. Front light wiring passes in this area and tapped off of it for the fan.
1st installation was "poor". Planned as a future project of reinstall fan with strengthening frame. Roof leak for wrong sealer moved the project up on the priority list.
Inside view of fan
Remounted fan with heavy oak frame to tie stringers together and used my traditional sealer (ACE rope caulk).
Note shaped oak pieces in roof grooves
To do project similar to method I used would require access to a table saw.