I briefly looked through some old job pictures trying to find the framing details on this paticular job we did. Customer had a large deck on the south side of the house and never was able to use it due to extreme sun/heat during the summer and wanted a shadey spot. Unfortunately budget wise they could'nt swing my inital proposal of doing a roof style tied into existing to match/blend into the house to make it look seamless, after a budget was determined this was what we came up with...they got their shade, but obviously I was'nt happy about overall look since it looks added on and i find that disgusting, I strive for integrated.
Anyhow, you dont have a choice but to consider snow load...not just because your inspectors will make you do it or make you tear it down if you under build it, but mainly for yourself and family preservation...all it takes is one good snow season and down she comes. you CANNOT tie into rafter tails, lets put that out there right now. you have no choice but to tie directly into the home framing itself so there is strength involved, tails are not upto task to support one side of a rood, esspecially one the width your describing, that is suicide and a huge waste of time/money/effort as it wont pass code and be dangerious.
the build below was a 16'x46' deal. Since we could'nt tie into existing roof, but still needed pitch to allow rain/snow to fall off, we measured down below the existing soffit to allow for roof deck thickness, ice guard, steel roof ribs and a cushion space of an inch. We lagged the ledger board into the side of the house with 1/2"x6" lags 16" o.c. into studs 2 in each stud all the way across 46'. Since we're in the midwest we have to follow span guides BASED ON snow loads. to keep what headroom we could at the end of the deck going to back yard the most we could use were 2x8 rafters, and to span that far with the snow load framing of the 2x8's was done 12" o.c. to meet IIRC and local code. Roof was decked with 1/2" OSB, we then covered the entire roof with ice guard and laid down raised rib steel panels. They did'nt want to use a membrane and pitch is too low for shingles, so the color we ordered matched their trim color, but you can barely even see any of the roof due to hte low slope.
We placed 6x6 posts roughly 10' oc (we were allowed to span 16') to allow for as much open viewing of backyard as possible giving the framing and spans we had to contend with, we nothced in 11 1/4" LVL's doubled up all the way across the front nailed off on a 5 nail pattern stacked 16"oc both sides. then just built up the exterior side with multiple 1x6's, 1x4's to given the large flat panel some depth and break it up a bit before putting the gutters on
I apologize if these pics are huge, i just took them off my photobucket and dont know if i resized them...but essentially it sounds much like what your planning to do, and I just dont want to see you halfazz it like you were initally thinking only to have the city come by and make you tear it all down and red tag your home until it's torn down due to the impending danger of wanting to tie into rafter tails.