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The cantrip Shape Water can freeze a 5' by 5' by 5' cube of water. The water unfreezes in 1 hour.

I've often read or hear that this can be used to make an ice bridge across water, by continuously freezing 5' of water.

However, the spell has stipulation:

If you cast this spell multiple times, you can have no more than two of its non-instantaneous effects active at a time.

To me this implies that the spell can at most make a 10' long bridge of ice.

Am I missing something, or is the cantrip Shape Water unable to build ice bridge longer than 10'?

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  • \$\begingroup\$ Are you assuming the water is at rest? \$\endgroup\$
    – Mark Wells
    Commented Feb 1, 2020 at 2:17
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    \$\begingroup\$ @MarkWells no, but that could be an interesting special case. \$\endgroup\$
    – Guillaume F.
    Commented Feb 1, 2020 at 2:38

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You can freeze at most 2 areas of water at a time.

The cantrips Control Flames, Mold Earth and Shape Water indicates that effects are instantaneous by

  1. Including the adjective "instantaneously" in the description of the effect:

You instantaneously expand the flame [...]

If you target an area of loose earth, you can instantaneously excavate it [...]

You instantaneously move or otherwise change the flow of the water [...]

  1. Instantaneous effects are listed before non-instantaneous effects for each cantrip.

  2. Non-instantaneous effects have a duration of 1 hour.

The freezing water effect, in isolation, could be either instantaneous or non-instantaneous. However, if we look at the broader context of the Control Flames, Mold Earth and Shape Water cantrips, we can see that

  1. Its description does not contain the word "instantaneously".
  2. It is listed last, after a non-instantaneous cantrip.
  3. Its effect is reverted after 1 hour.

Hence we can conclude that the freezing water effect is intended to be non-instantaneous, and as such only two such effects can be active at a time.

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To me this implies that the spell can at most make a 10' long bridge of ice.

Agreed, but you can still use the cantrip to cross a longer distance of water slowly, with multiple casts per trip.

Freeze a 5' cube, step onto it (along with however many creatures you want to try to squeeze on).

Freeze the next 5' cube, step onto it. Repeat.

As you freeze the cube in front, the cube behind will lose its magically-frozen status and revert to liquid instantly, or start to thaw normally depending on how your DM wants the spell to work. If instantly, the entire group making this crossing has to squeeze onto one space. (The cantrip has 30ft range, so a caster can do this for a group not including themself.)

Of course, once you cast it a 3rd time, it's no longer frozen to any land behind you, so unless the water is shallow, it's going to be tippy. At least you always have a 5x5x10' "log" of ice, so you have more stability in one direction.

But then it's not really a bridge, or even a set of stepping stones, since it's not locked in place by being frozen to anything solid.

So probably more sensible use it as a 5x10' raft, using the instantaneous move/flow effect to move the raft. (Or a section of the surrounding water to push on it, but you can pick 5x5x5' at the middle of the raft, for example, and the rest of the raft will come with)

You instantaneously move or otherwise change the flow of the water as you direct, up to 5 feet in any direction. This movement doesn't have enough force to cause damage.

This effect is instantaneous, so doesn't unfreeze either block, so you can sail your 5x10 raft at a speed of 5' casting once per round.

Would that much ice be buoyant enough to carry 2 or 3 humanoids? Maybe not. You might need to Shape Water to create air pockets in your 5x5x5 cubes before you freeze it, like a simple lattice pattern of water inside, with solid sides.

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    \$\begingroup\$ FWIW a 5' cube would almost certainly be able to hold the party (assuming they can balance) as there would need to be over 6400 pounds of displacement before it submerged. \$\endgroup\$
    – aslum
    Commented Jul 12, 2022 at 19:38
  • \$\begingroup\$ @aslum: That's way too high, close to the total weight of the cube. Maybe you got an extra zero in there? Since you're working in inconvenient imperial units, wolframalpha has conversions and data: 0.03312 lb/in^3 is what it gives as the density of ice. (916.9 kg/m^3). So a 5' cube is 125 cubic feet or 216000 cubic inches, with a weight of 7154 lb. The weight of liquid water of the same volume is 1000/916.9 times that, or 7802 lb. So the buoyancy of a 5x5x5 cube fully submerged is 648 lb. \$\endgroup\$
    – Peter Cordes
    Commented Jul 13, 2022 at 2:54
  • \$\begingroup\$ You're absolutely right, I grabbed the wrong half of the equation (amount submerged not amount above). Of course only 1 medium creature can be in a square (and not prone) at a time... \$\endgroup\$
    – aslum
    Commented Jul 13, 2022 at 12:47
  • \$\begingroup\$ @aslum: Yup. But good to know that it's buoyant enough (even in pure water, never mind salt water) for most PCs, probably only the burliest of Goliaths or Firbolgs or other almost-Large races would come close to that, even carrying a heavy pack. And a 5x5x10 could easily take the 2 to 3 PCs that you would fit on it without squeezing too much, without getting their feet wet. \$\endgroup\$
    – Peter Cordes
    Commented Jul 13, 2022 at 13:10
  • \$\begingroup\$ Re: occupying a square: that's during combat movement, when creatures aren't hampering their ability to fight. A sensible DM would let two humans hug each other within a 5x5 space. Two people balancing on an ice cube is probably more difficult than one, but they could maybe stepping-stone together, with arms around each other's shoulders, \$\endgroup\$
    – Peter Cordes
    Commented Jul 13, 2022 at 13:12
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Freezing water is instantaneous

The main argument is that the effect:

You freeze the water, provided that there are no creatures in it.

Is instantaneous. You instantly freeze the water, then it unfreezes over time naturally. You magically froze the water. "The water unfreezes in 1 hour." isn't a magical effect, it's advice on how long the ice takes to melt. There is no lingering effect to be canceled, casting the spell again won't turn the ice back into water.

You can make an ice bridge up to 60 * 10 * 5 ft long (60 minutes per hour, 10 turns per minute, 5 ft per turn = 3000 ft

If your GM says that ice melting is magical then...

There are three alternate options:

  • Have multiple people use Shape Water to make a longer bridge
  • If your GM rules that the ice unfreezes instantly: keep freezing the 5ft ahead and allow the last 5ft to unfreeze. This is less of a bridge and more of an iceberg raft.
  • If your GM rules that the ice begins to freeze instantly: You still have a long time until the ice is completely thawed.

Option 1 is easier but you need many people with shape water. Option 2 can be done alone, but your DM will probably throw some dex checks at you to stay balanced (I know I would)

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    \$\begingroup\$ I would argue that "The water unfreezes in 1 hour." is not just advice on how long it takes to melt. If you were in a hot springs location where the water is well above melting point, it will still unfreeze in an hour, not less, despite being in hotter conditions. It is magically frozen and the duration is 1 hour. \$\endgroup\$
    – RallozarX
    Commented Feb 1, 2020 at 2:58
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    \$\begingroup\$ @RallozarX I agree that ice should melt faster/slower based on the situation, obviously many rules in D&D vary based on the situation, and that is for the DM to determine. It depends on your interpretation of "unfreeze" too, but regardless that seems to be the main argument for making a long ice bridge. Even if the effect ends, you are still left with a 5ft cube of ice which would take a long time to thaw in most situations. \$\endgroup\$
    – user-781943
    Commented Feb 1, 2020 at 3:15
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    \$\begingroup\$ I had considered before this that when it "unfreezes" it just returns immediately to a liquid state, but you make a good point about that. It might just be when it starts to melt. However, I do still believe it cannot begin that process within the hour because it is being frozen magically for the duration, and it will only begin to able the laws of thermodynamics after that hour is up. Also, I'd like to point out that the spell specifies the duration of its effects as "Instantaneous[inst] or 1 hour", where it says in all the other points either inst or1 hour. If this was inst it would say \$\endgroup\$
    – RallozarX
    Commented Feb 1, 2020 at 7:14

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