Roach/Infestor (vs. Zerg)
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Roach/Infestor is the standard mid-game strategy in Zerg vs. Zerg. Roaches are the most cost-effective mid-game unit against Zerg, beating Zerglings, Banelings, and Hydralisks cost-effectively.
In a Roach vs Roach game, Infestors become useful, since their Fungal Growth ability does large amounts of AOE damage to Roaches, as well as giving a positional advantage and preventing retreat.
The goal of this build is to build up a large number of upgraded Roaches together with 4-5 Infestors. It is flexible enough to be either defensive or offensive. you can attempt to deny your opponent's 3rd base, make 2 Evolution Chambers for fast upgrades, or go for a quick Infestation Pit and try and do a timing attack with your Infestors, or you can slowly build a large Roach/Infestor army on 3-4 bases.
Basic Build Order
A large part of ZvZ is successfully getting your first expansion up safely. There are many different ways to do this, and they depend on your opening and your opponent's opening.
This build is a mid-game strategy, and it starts once you have your second base up and running, and after starting Lair
The timings of the Roach Warren, Evolution Chamber, and Infestation Pit can vary wildly. But regardless of timings, your goal is still to build up Roaches and support them with Infestors.
You may opt to get the Roach Warren immediately after going Hatchery First, to protect from Banelings. Or you may open with Banelings yourself, and wait until after you have secured your expansion before making a Roach Warren.
For the timing of the Evolution Chamber, you can get it soon after placing your Roach Warren, or even before placing your Roach Warren, so that you can have +2 ranged attack completed in time to attack with a Roach/Infestor army.
For the timing of the Infestation Pit, you should get it whenever you feel safe enough to do. Generally "feel safe" means that you are able to hold off an attack should one come while you are waiting on your Infestors. At the very latest you should be getting your Infestation Pit soon after starting your 3rd base.
The most important thing that needs to be scouted for when doing this strategy is a Spire. Roaches are unable to attack air, so if your opponent is able to field Mutalisks before your Infestors are ready, then you will not have any anti-air defense except for your Queens, which will quickly die.
Make sure you morph an Overseer soon after your Lair finishes, and scout your opponents base for a Spire. Another clue that your opponent is going for Mutalisks is a lack of Roaches, or seeing a large amount of Spine Crawlers at the natural.
In order to engage effectively, you need to have a better concave then your opponent. This ensures that all of your Roaches are attacking as often as possible, and that your damage is spread out so that you do not have overkill shots and waste damage.
After getting a good concave, your next priority is to maximize the damage from your Fungal Growths. This is just a simple matter of picking a spot that hits the most Roaches possible. Since most players tend to put all their Roaches in 1 control group, forcing your opponent to re-position is a great way to make all of their Roaches bunch up, which lets you maximize your Fungal Growth damage.
If you notice your opponent is not getting Overseers, you can also get Tunneling Claws and use burrow movement to get good positioning on your opponent (and regenerate your Roaches hp).
If you are playing aggressively, you may want to try and attack before your opponent is able to get his Infestors out. If you are playing defensively, you may want to get your Infestors earlier than normal, to protect against a Roach attack.
If you scout a Spire, you need to get Spores and Queens around the time that the Spire completes, and immediately start your Infestation Pit if you haven't already. Focus all of your gas on Infestors until you have enough to deal with Mutalisks, then you can resume Roach production.
After getting 5-7 Infestors, you should then add in a few Hydralisks. If you rely only on Infestors, your opponent can take advantage of your lack of sustainable DPS against air units and fly in small groups of Mutalisks so that you waste large amounts of Infestor energy on taking out only small amounts of Mutalisks at a time.
This mid-game army composition and strategy work on any map, and does not rely on any specific map features.