I've been looking for an AP Janna build in-case my AD Carry is failing, but I couldn't find any up-to-date ones online.
You can't switch from support Janna to AP Janna. Here is why:
As a support you should have full support masteries and runes. So you won't have enough damage as a mid carry AP that has his full AP masteries and runes.
Second, you should have maxed shield, and probably slow in second hand with one point in the Howling Gale. Since the Howling Gale is your only attack capability, you're already far behind another AP with 3 damage skills.
Third, you have probably bought Golden heart and Philosopher stone. Those don't build on AP stuff. So selling it is very inefficient.
For all that three reason, an AP Janna can only be useful if built AP from start, getting farm, with AP masteries and runes, and buying AP stuff. Even then, it's not really the best AP carry in the game. Sure she can have some impressive damage and the shield helps a lot, but she has only one attack skill supported by a CC-Slow a shield and a CC-Knocback-Heal. You cannot pop in and burst someone. Eventually she can be a good farmer, but this don't make the game.
If you still look for a build, I guess you could try :
- Forget Rod of Ages, since it takes 10 minutes to reach full capacity
- Get a Rabaddon Deathcap or Deathfire Grasp
- Get a Lich Bane so you can add some of your AP to auto-attacks
- Void staff or Abyssal Sceptre can also help.
But honestly, don't. You can't change role during game with Janna.
So you want a build that transitions from support who has maxed shield? The whole point of AP Janna is to win her lane in farm and keep pressure on the tower while also being nearly ungankable. Transitioning from support Janna to AP is ineffective. I would just build tanky maybe get a DFG and start roaming trying to feed your other lanes if your ad carry is failing.
If your ad carry is failing, then you're already behind and need to help your teams make bigger plays by investing in an early oracles, warding, and roaming to help your other lanes win. Anyways, pure AP isn't really viable for janna, so opt for maybe a WoTA or abyssal scepter as that can help your AP lanes.