At first, I had just one build, a laning support Janna. That's Build 1.

I was going to test it, but my team wanted me to roam that game, and I ended up improvising a pretty fun build for that, too. This is Build 2.

Ok, so Janna has all defensive skills, some of which do damage. Mostly she's there to CC, shield, buff, harass, and not die. With very little in the way of survival items, you still shouldn't die much because 1) your team loves you because you're supporting them properly and 2) you're popping them up and running every time they get close, and you took Flash. However, some hp5 is necessary just to avoid recalling excessively.

Mana management is a huge issue. Janna is a pure support caster and someone can get killed if you oom right when they need a shield. Your mp5 is their hp5.

AP is nice if you can get it. Mostly for the shields, but the tornado is no joke, either. If your team already has a jungler who's doing all the warding, then you can go ahead and spend more money building AP alongside everything else. In particular, Archangel's Staff is the first time this build truly has enough mana to shine, and you have to be more frugal with mana before then. However, you'll be snowballing AP from there on out. It may or may not be worth it - I provided a second build.

CDR is important, but pointless if you're just going to be OOM anyway. If you have to wait 30 seconds for enough mana to cast, then you've got +200% CDR, not -40%, no matter what your CDR stat claims.

Finally, GP10 is pretty important. Even if you lane, you won't be getting many kills and you won't want to take priority when it comes to killing minions. This is an expensive-ish build, so two GP10 items are taken.

So here it is as a list. As you can see by looking at the build, the progression is more or less how I described it.

Health management: Philosopher's Stone, Greater Seal of Replenishment.
Mana management: Philosopher's Stone, Archangel's Staff, Mana Potion. Meki Pendant is an option to start with.
AP: Kage's Lucky Pick (later sold), Archangel's Staff, Rabadon's Deathcap
GP10: Kage's Lucky Pick, Philosopher's Stone
CDR: 9% (masteries), 15% Ionian Boots of Lucidity, 15% Shurelya's Battlesong, optional 1% from rune tweaking

Janna is mana hungry. You might even want to grab a couple Mana Potions
Philosopher's Stone More mp5. GP10 because you don't go for kills much.
Kage's Lucky Pick Even more GP10. AP doesn't hurt.
Pretty necessary.
Finally enough mp5 to spam.
relatively late because you want the GP10 and because the CDR is wasted if you don't have the MP5 to take advantage of it anyway.
Better shields all the time

That's it for the core build.

A waste if you have an AD-heavy team. Otherwise, it's a solid support item. Make your shields better while helping them spellvamp.
Only Howling Gale and Zephyr proc this, but they're both solid - the first is a multi-target spell, and Zephyr becomes a potent single-target slow. I don't know if it's worth it, though, since those are the only two ways to proc the effect. As a sixth item, it's a bit of a throwaway, and games aren't usually that long anyway. You might as well ward if you want to burn cash that way.

A bit selfish for a support item, nevertheless you can Flash- Monsoon with relative safety if you get this. Probably should replace Will of the Ancients in this case. Catalyst the Protector lets you spam more (before you hit 18), and can be taken around the same time as Tear of the Goddess.
Armor and MRes also helps you get the most out of your shields. Fairly early is better, when the increase in armor/MR will be more noticeable.
A decent choice for the support on a budget, this gives you a panic button. A cheaper way of getting in a Flash- Monsoon and possibly living to tell about it.
Replacing Shurelya's Battlesong for CDR and MP5. Has a little mini-nuke, sort of anti-tank I guess. You'll be more squishy, and you'll lose the HP5 when you eventually sell Philosopher's Stone. This may be acceptable if you get Will of the Ancients for sustainability and you avoid focusing.

9% CDR from masteries. Runes could be tweaked for an extra 1% if necessary. This is more or less the entire reasoning for going 9/0/21.

Greater Mark of Magic Penetration because it's the only appealing option, really. Since this build is heavy on AP, it's a natural choice.
Greater seal of replenishment because the Philosopher's Stone is all you have otherwise.
Greater Glyph of Scaling Ability Power for better shields especially, and the damage is nice.
Greater Quintessence of Gold because this build is a bit pricey. Cooldowns, move speed, AP, etc. seem to be taken care of already for the most part.

Flash+ Monsoon can really have an impact on teamfights if you do it at just the right time and don't get yourself killed.

Clarity is tempting but I'd rather build around having to get it.

Clairvoyance is what your team probably wants you to get. I never used to get this, but my god, is the cooldown short!

Heal is an ok choice since you don't really want to have to use waste your ult on healing if you can at all help it.

Exhaust is mighty handy. It's a fun replacement for Clairvoyance in bot games, but your carries might already have it anyway. Still, I've never felt too bad because our team had too much Exhaust.

Fortify is underrated. If no one else takes it, it's decent.

Teleport is nice to have. It also lets you recall when you're oom with minimal harm to the team.

Cleanse might save you if you screw up.

Smite might be ok in Build 2 if you think you can steal the enemy's buff. I'd rather have Clairvoyance, though.

If you need Revive, then you either need practice, more map awareness, or your whole team's getting aced anyway and it doesn't matter much. If your team gets aced, you're not the hardest turret dive in the world anyway.

Ghost isn't necessary. You're already quick and collision-free because of the Zephyr passive, and you have and AoE knockup to get away. On top of that, you should be well-protected at all times. Flashing over a wall is more effective in any case.

Smite is bad if you're laning.

If you have a jungler already, laning support Janna is a viable duo-lane choice. I recommend Kayle.

As I said before, lane with someone. AD is better because of the AD bonus on Eye of the Storm. Kayle, for example, can sustain and punish early-to-mid game with Eye of the Storm, Righteous Fury, and some attack speed. Having shields, heals, AND the ability to activate 5 seconds of a double B.F. Sword that splashes at Level 9 is insanely good for just one lane. When you see Righteous Fury go up, give her Eye of the Storm. Then you can double-slow them with Reckoning and Zephyr, and if you're lucky, you might pop them up at some point, too.

This seems to be what pre-made teams would rather have. Your shields are 100-200 hp weaker, but you ward the **** out of everything. I was going to test out my first build, and then my team wanted me to roam, so I improvised more or less what you see above.

Note that I take Chalice of Harmony early, skip Philosopher's Stone, and make up the lost HP5 with masteries and runes. The GP10 this time comes from Kage's Lucky Pick. This build is really cheap, and there are only 5 items in it - the sixth slot continues to be occupied by wards, but if wards stop being necessary in the final teamfights, you can buy the item of your choice instead if you wish.

Ward ward ward ward ward, wards can cost an arbitrarily large amount of money, so get to it. When you're warding, your build is never done because you need more wards, always.

First of all, the build isn't appropriate if you have a jungler. Janna isn't really a jungler, as she shouldn't be trying to kill neutrals. But in a premade, she's a good roamer, but your team *has* to have your back, ready to leave lane to help you out when things happen. You can't have a 1v1 with Warwick.

However, she's great at getting away. She's fast and she has a pop-up and an emergency knockback, AND we'll be taking Flash. So here's what you do:

* Ward the **** out of everything
* If you think they're warding, get Oracle's Elixir while you're at it.
* Keep track of the enemy jungler (wards+ Clairvoyance)
* Help your team gank their jungler

That's right, in Soviet Russia, enemy jungler gets ganked by YOU. That's what's great about wards. A tornado from the bush is a great way to set them up to get ganked by one of your laners. Then Zephyr them when they need to run, and Eye of the Storm when they try to fight back. Ideally, you should be 3v1, but 2v1 is more than good enough if you catch them trying to snag a buff early.

A lot of the same remarks apply. Priorities are about the same, except AP is nerfed in favor of wards and feeding exp to the rest of your team.

Because it's cheap and it's all the MP5 you'll need. You can have wards, and you can have Archangel's Staff, but not both.
Kage's Lucky Pick For the GP10. We don't need Philosopher's Stone, so a little AP won't hurt.
Because you know where the enemy is and you're avoiding them at all times; you just need to spam spells.
Because getting this early makes your shields last that much longer. The earlier you get it, the larger the multiplicative increase of effective health your team experiences.

The last two are throwaways, really. But they're nice.

This helps you escape. You can even bait them a little. Give them the Sash & Flash! (Note: I might have to rename the build... that's funny) Anyway, it's better to be standing against a wall if you know you might have to do this.
Maybe more appealing than the sash, this caps your CDR, but costs more. The extra MP5 won't be noticed. The active is sort of nice, but your AP isn't huge enough to make it really count, but you might as well use it if your Sash & Flash is ready to go.

BUILD 1:

Pros:
* Better shields
* Noticeable damage
* Spell vamp
* More slow
* Fight presence
* Double GP10

Cons:
* Expensive
* MP5 starts out ok and doesn't pick up until mid-game
* Loss of map control (if no one jungles)
* Squishy

BUILD 2:

Pros:
* Crazy map control
* Good anti-jungler (with coordination)
* Team will love you
* Sash & Flash
* Your team will have two solo lanes

Cons:
* Crazy underfed
* Team dependent for roaming safety (not viable in a bad solo queue)
* Damage in team fights just about ignorable

This build lets you get some decent items while picking up a few wards every trip. I was finding that early on, fairly often I had enough money for a GP10 and a couple wards, or an Amplifying Tome and a couple wards. If you end up selling GP10 items later on, you can improvise what else to get. If you don't like Heart of Gold, you can get Kindlegem. You have to sell at least one GP10 item to get Rabadon's Deathcap + Sight Wards anyway.

It was sort of a long game, and all I got beyond Rabadon's Deathcap was a Negatron Cloak with no real idea what I was going to get next. Anyway, Deathcap is sort of fun if you can get it. If I expected to ward that much and still finish a bigger build, maybe I'd add more, but meh, just get a lot of wards. I might remove one of the other two builds, since I sort of liked how this one played, but we'll see.