Bath Fan Installation Question

[depawl]

Hello. As part of a bathroom remodel getting ready to install a new vent fan and had pretty much decided on a Panasonic or higher end NuTone. The installation notes on these say:
"Fits 2 x 6 Construction" or "Fits 2 x 8 or larger joist".
Wondering exactly what this means, as the bottom chord of my attic trusses is a 2 x 4.
The installation instructions for these generally show them being mounted between trusses with hanger bars, so I don't see why the bottom chord height would be an issue.
Thanks.

 

[Stuff]

First need height for box and exhaust pipe in case there is a floor above. Then without hangers a 2x4 might not be tall enough to anchor the side of the box.

 

[Sponsor]

 

[hj]

If it were a "flat roof" the joist size would be a factor. For trusses, it is not.