I am working on a cooldown tank build for Janna, but I don't know if it's effective, any tips?
(PS: I don't build the items in this order necessarily, I build off of the smaller items first.)

1. Frozen Mallet
2. Mercury Treads
3. Frozen Heart
4. Thornmail
5. ??
6. ??

I never get to build items for the last two slots. XD

When you build her as a tank, she's also a support like when you're building her as AP. It's just that her damage output is pretty much lesser than that build. However, you get the toughness and survivability to spam your spell effects to save your friends.

Janna can't be the best tank but she can be a tough disabler. Frozen Mallet and Frozen Heart can be pretty useful for slowing enemies stuffs down. You can also add some more CDR items like Soul Shroud (HP for survivability and cooldown for your own team), add in Guardian Angel for double life or add in Force of Nature/Banshee's/Warmog's. Basically anything that give you a lot of health and reductions to damages. However, you should prioritize her CDR first. My support tank build I used to play was:

- Soul Shroud
- Mercury Treads/Ninja Tabi
- Warmog's Armor
- Guardian Angel/Frozen Heart
- Banshee's Veil/Force of Nature
- Spirit Visage or probably some potions/wards/elixir to survive in teamfights.

Thornmail is not a bad idea against burst DPS as well. Frozen Mallet can always replace the 5th or 6th slot in my build as you will help your friends to slow enemies down, but I don't really use it.

Frozen Mallet seems odd on a champion with a mediocre autoattack and no other physical items. Also your champ already has several methods of slowing and delaying the enemies, making it less worthwhile.

Also, you have lots of armor between Frozen Heart and Thornmail, but nothing in terms of Magic Resist. This will make you incredibly squishy to any spellcaster.

I recently started throwing in a Banshee's Veil, too. And Mercury Treads have +30 Magic resist.

I went up against a Janna today that went Frozen Mallet/Frozen Heart and she was a huge asset to the team. She kited people all day, and was able to chase and slow. It is a very effective build. She ended up 2-3-14 and a victory.

I could see Nashor's tooth replacing the Frozen Heart for less tank & more damage/dps.

Isn't it ironic that one of Janna's default recommended items in game is Frozen Heart?

I've thought about throwing in Nashor's Tooth before, but if you want better DPS, it might be better to just build Janna as DPS. I've noticed that splashing your build tends to weaken it on both sides. I don't think plus 30% attack speed will benefit Janna much without also increasing attack power and crit. strike.

Frozen Heart decreases enemy attack speed and grants you 99 armor, which in a way helps counter DPS itself, since more than less DPS champions rely on physical damage.

However, a bit of DPS couldn't hurt, and I might consider throwing in Wit's End. +40% attack speed, but also +30 magic resistance to help out the tank build.

I'm sorry but I won't listen to any build that starts off with DORAN'S *whatever*. What a waste of money when you think of it. You can get first blood with or without, the difference is too... Small... Be skilled and you won't need DORAN to help you.

"Tank" build:
Grab Aegis first no matter what opponents you have.

Then, if Physical dps > Frozen Heart + Ninja Tabi
If Magical dps > Banshee Veil + Mercury Treads

The game should be over soon, due to Janna's abilities to sustain and help pushes (shield + aoe heal).
If not, add in a Guardian Angel.