Toilet venting using wet and dry combination

[WN1]

Thanks in advance for giving advice. Long time lurker, but I haven't been able to see this issue addressed previously.

Short version: for a toilet, would a 2" wet vent (shower drain) connected to a 1.5" dry vent (shower vent) be okay?

1. Current tub has a 1.5" drainpipe, 1.5" dry vent, and connects into the main stack.
2. I'm thinking of moving the tub a bit and changing it to a shower with a 2" drainpipe. I know a 1.5" vent is acceptable for a 2" drainpipe and it would be easy to re-use the existing 1.5" dry vent. Changing the dry vent to 2" would be a lot of work and unnecessary for the shower, as I understand it.
3. Rather than connecting the new 2" shower drainpipe to the main stack (difficult due to access and other issues), I would like to connect it to the toilet drainpipe. Thus, the toilet will then be vented by a 2" wet portion (shower drain) and a 1.5" dry portion (shower vent).
4. Toilet drain currently has no vent and is approx 5 feet from main stack (allowable by local code). Shower drain would connect to toilet drain approx one foot from main stack.

Would the 1.5" dry vent portion be okay (in combination with the 2" wet portion) for the toilet? I know that 2" venting throughout would be optimal, but considering that the toilet isn't even separately vented now, having an additional vent would seem to be an improvement...

Here's a diagram. Assume my slopes and fittings are correct, but just don't show up in my diagram.

 

[Stuff]

Don't know Ontario code but some other codes require 2" for a toilet vent. The blue horizontal and the black horizontal "dry" vents are prohibited by most codes as well. Horizontal dry vents need to be at least 6" above flood level.

 

[Sponsor]

 

[WN1]

...The blue horizontal and the black horizontal "dry" vents are prohibited by most codes as well. Horizontal dry vents need to be at least 6" above flood level.

Sorry, my diagram wasn't clear with slopes. For the blue dry vent, the section below the 42" vertical section is actually at 45 degrees. The horizontal section is 6" above flood level (i.e. 42" height). Revised diagram:

 

[Cacher_Chick]

Maybe it is the drawing, but i concur thar where I am the vent take-off from the shower drain must be vertical. No part of the vent can be horizonal until it reaches the 42" height.
As to the other part of the question, If the toilet is really plumbed as you show, it is not properly vented, so one option is to wet vent through a proper shower vent, and the wet portion of vent must be 2".

 

[dandan401]

The shower is being back vented and the toilet is being wet vented off the shower. Just make sure that the tee-wye off the shower drain is positioned nominally horizontal on a 45° so that the vent is elevated from the fixture drain. Minimum size wetvent for a toilet is 2". Once the vent becomes dry it can become 1 1/2". This is legal in ontario so dont worry. Also ontario plumbing code just says vent connection must be above flood level rim. It dosnt specify a measurement , so even if you are 1/2" above you are ok.

 

[Plumber69]

Where's your sink. Wet vent the bathroom with a 2 inch drain/vent via sink