Relevant Combat Mechanics for Healers

FFXIV's combat system has a number of quirks and intricacies that significantly inform the way we play. If you have been playing for a while, you are likely familiar with most or all of these, but these are the mechanics that are most relevant for healers to understand.

Movement a positioning are core parts of every encounter, and GCD uptime is the single most impactful factor for your healing and damage throughput. Most of our spells have cast times, so movement often comes at the expense of uptime unless we find ways to work around the movement. The primary way we maintain uptime while repositioning is called "slidecasting".

There is a small window at the end of every cast (about 0.5s, subject to latency) during which you can begin moving, and your cast will still complete. That is the slidecast window. Regardless of the length of the cast, that window is always about 0.5s. Healers' filler damage spells have a 1.5s base cast time and a 2.5s base recast time. With the 1s of downtime between casts and the 0.5s slidecast window, we actually only have to stand still for about 1s out of every 2.5s to maintain full GCD uptime.

Slidecasting enables a tremendous amount of movement, especially on healers, since our basic damage spell cast times are so short. Some GCD heals have 2s cast times, so you have to stand still just a little longer to get those casts off, but you can still slidecast out of them. Slidecasting is considerably slower than simply running, so it is not always sufficient on its own if you need to cover a larger distance and/or you have less time to get there. However, using Sprint while slidecasting still lets you cover more ground. Combined with an instant-cast spell like a DoT refresh or a Swiftcast spell, you can cover quite a large distance without losing any uptime.

Virtually all actions in FFXIV can be categorized as GCDs or oGCDs — actions that activate the Global Cooldown or actions that are off the Global Cooldown. Standard spells and weaponskills are GCDs. Their base recast time is 2.5s, reduced by skill speed/spell speed/attack speed. Actions that are labeled as abilities are generally oGCDs. They do not activate the Global Cooldown, and their recast is fixed and unaffected by skill speed/spell speed/attack speed. (Some spells and weaponskills have fixed cooldowns, but they still activate the Global Cooldown and are considered GCDs.)

Our GCDs list a Cast Time and a Recast Time. The Cast Time is the duration of the cast bar, and for standard GCDs, the Recast Time is the total time until we are able to activate our next GCD. Our basic damage spells have a 1.5s Cast Time and a 2.5s Recast Time. That leaves 1s of empty space between finishing one cast and beginning the next cast. That empty space is called a "weave window". That window is long enough for us to use a single oGCD action without delaying our next GCD. The animation lock for most oGCDs is around 0.7s, depending on your latency.

At most latencies, you can comfortably weave a single oGCD after each 1.5s cast and two oGCDs after an instant-cast spell. Weaving two oGCDs in a row is called "double weaving," and it is standard practice to do so after an instant-cast spell when necessary. Weaving more than one oGCD after a 1.5s cast or more than two oGCDs after a standard instant-cast would take more than the duration of the weave window, delaying your next GCD. This is called "clipping" your GCD, and it is effectively a loss of uptime that results in a loss of damage or healing output; when you are clipping your GCD, you are using up time that could be spent on GCDs instead of using up the weave windows that cannot be used for anything else.

Sometimes, clipping your GCD is necessary to get out all the healing or mitigation you need, but it should be avoided whenever possible.

Both incoming and outgoing attacks "snapshot" damage modifiers such as damage buffs, mitigation, and even shields. The point at which the damage modifiers are snapshot for the damage calculation is when the action is considered to have been used, not when the attack lands. For outgoing damage, that point is the start of the slidecast window. Once the slidecast window begins and you are eligible to start moving without cancelling the cast, the spell is considered to have already been used. For instant-cast spells and abilities, it is generally near the start of the animation.

The damage (or healing) being applied is subject to animation delays, but the calculation may snapshot the damage modifiers much earlier than the end of the animation, depending on the action. As an example, healers often reapply their DoT in the opener, right before raid buffs wear off. Depending on your GCD speed, it is possible to cast your DoT right before buffs expire, but have the DoT visually apply to the boss after buffs have expired. The DoT would still snapshot all the raid buffs being present when the spell was cast, not when the DoT was actually applied.

This also applies to incoming damage. Some boss attacks may snapshot earlier than you expect, based on the cast bar, so mitigation may need to be in place a second or two before the end of the cast. However, it also means that you can sometimes abuse that snapshot to double-dip mitigation abilities used on the previous mechanic that will last just long enough to snapshot the next cast from the boss. Snapshotting mitigation like this is also extremely important for short-duration or channeled mitigation abilities like Collective Unconscious and Passage of Arms.

In the case of damage-over-time and healing-over-time effects, the entire duration of the DoT or HoT is based on the initial snapshot from when the action was first used. If you mitigate the initial hit of an incoming DoT (excluding persistent DoT effects that have no displayed duration), the damage of every tick of the DoT will be reduced, even after the mitigation expires. This is why mitigating raid-wide DoT attacks is extremely important, as their total damage is extremely high. Similarly, if you have a healing buff, you can cast a HoT right before the buff expires and every tick will be buffed.

Damage-over-time and healing-over-time effects operate on a 3s "tick" rate. A DoT with a 30s duration and 70 potency will tick a total of 10 times, for a total of 700 potency. The ticks are always 3s apart, but they may occur at different points relative to the DoT timer. For example, that DoT may tick instantly at 30s, then again at 27s, with the final tick being a 3s. It is equally likely for its first tick to be at 28s on the timer and the last tick to be at 1s, but there will always be 10 ticks.

The imprecise nature of the exact timing of ticks means that incoming raid-wide DoTs can catch you by surprise if you do not notice that the tick is late and you use your final heal on the same GCD as usual, but there is a final tick right after that. Or, the tick may be extremely early, and failing to notice and use a heal a little early might result in people dying because they received 4 ticks between heals instead of 3. Similarly, you cannot always rely on everyone receiving the same number of HoT ticks in a limited time frame from pull to pull.

You do not need to meticulously count DoT/HoT ticks, but it is helpful to be aware that DoT/HoT tick timing can fluctuate by more than the length of a full GCD, so you may sometimes need to adjust your healing.