On a truss website I saw where for increasing snow loads sometimes the top chord of trusses is increased in size from say a 2x4 to a 2x6. I have also seen the increasing the bottom chord approach used (but not increasing the top chord in size). Does anybody know under what circumstances you would want to increase one chord over the other?
Attic / bonus room trusses generally will have a bottom chord thats is 2x10 -ish , top chords can be made larger is say a dormer will be "riding " on top of the truss.
Snow load would be another reason to increase size.
(https://i21.photobucket.com/albums/b299/PEG688/dill21.jpg)
(https://i21.photobucket.com/albums/b299/PEG688/dill20.jpg)
(https://i21.photobucket.com/albums/b299/PEG688/dill29.jpg)
The spaces are where the dormers where placed.
(https://i21.photobucket.com/albums/b299/PEG688/diil23.jpg)
There are other reasons as well I'm sure.
Hefty trusses there, PEG.
Proper truss sizing for conditions would always be calced by an engineer and or his software program on a permitted project.
There are quite a few homemade truss plans out there which may have calced loading on them. Usually it is quite an involved calculation.
The type of roofing material also come into play. If you were to want Slate or Clay Tile on the roof you would upsize the top cord for the added weight. :)