I am replacing the original roof vent in my Boler with a 14” one from here: https://vintagetrailersupply.com/14-...d-lid-vts-280/
Cutting the hole larger was not a problem and is very straight if I do say so myself. However, I need to know if I should make a frame and fibreglass it to the roof to account for the extra depth? If so how do I do that?
My suggestion is to just build a nice wooden “spacer” of the necessary thickness .. Secure it to the ceiling from the outside with a couple of screws.. Dry position vent and drill holes for vent screws through fibreglass roof.. Place butyl tape to cover the screws and in position for vent.. Position vent on top of butyl and screw it in place utilizing the predrilled holes and into the wooden spacer.. Suggestion, build and finish the interior spacer before you do the install because it will be really tricky to finish it nicely once installed..
One suggestion on the "spacer" if I may please. Consider using "vinyl plank" from any of the lumber stores. Seals with caulking much better than wood, goes well with fiberglass and no water issues! I've made windows with this stuff, works great!