Designing Kitchens and Bathrooms - Bathroom Ventilation Fans

My 2 bathrooms have ventilation fans that are vented into the attic; not to the roof or soffits. I am going to replace both fans with higher quality fans that are sized properly for the space. Current ones are too small. The 2 fans are very close to each other since the bathrooms share a wall. Can I vent both fans into a wye and then to the roof with 1 vent? Or do I need a seperate vent for each fan?

Welcome to the forums! It is not recommended to tie two fans together. However in a practical world, since they will probably not be used at the same time all the time, you could run 4" flex from each fan and wye it into an 8", but that is a rather large pipe to vent to the outside. I would recommend running separate 4" lines and allowing them to exit under the soffit with proper soffit vents. I hate cutting holes in perfectly good, non leaking roofs.

I am so glad you brought up this topic. Years ago I had a shower vent/light unit installed above my shower in my mountain cabin. I do not recall a new vent going through the roof. I currently have long last found a handyman (not the best) to reroof my pumphouse shed and cover my soffit and fascia. I have a little laundry list of other things that I can afford this season. I will have him check it out if the vent goes through the roof. I am now paranoid, and I do not think it does. I have dropped ceiling in a poorly insulated cabin in the mountains with ice damns and the big icicles. My goal is to get rid of the dropped ceiling and open flourescent lighting and install insulation, a new roof with soffit and ridge vent ventilation. It's just now that I am so curious where that handyman vented the new light/vent over the shower.