When does magic work through walls?

[Noumenon]

I know that spreads work around walls, though not through them, but bursts and emanations don't. And walls block line of effect when you initially cast a spell, but your grease and summons don't go away if you go behind a wall later.

• Can Mage's Faithful Hound see you through a closed door? It would seem that Alarm can. Also, Alarm's "bell" effect specifically works through closed doors. What if you open the door, cast the Hound, then close it? Does the Hound keep functioning in its whole area, the way a Grease would?
• Is Ice Storm blocked by walls? After it's working, if you make a 40-ft high wall blocking my line of effect to the other side of it, does it stop working?
• Does Detect Magic only work through walls because it says it does, or do a lot of spells work that way?

 

[Vegepygmy]

Can Mage's Faithful Hound see you through a closed door? It would seem that Alarm can.

What makes you think that? (And I see nothing in the faithful hound spell description that even hints at it seeing through closed doors.)

Also, Alarm's "bell" effect specifically works through closed doors.

The sound the spell creates can be heard through closed doors; the emanation of the alarm spell itself does not so extend.

What if you open the door, cast the Hound, then close it? Does the Hound keep functioning in its whole area, the way a Grease would?

Yes. Faithful hound is an Effect spell, and once created, effects no longer require line of effect to the caster.

Is Ice Storm blocked by walls?

Yes. Ice storm is an Area spell. (See "Line of Effect," PHB page 176.)

After it's working, if you make a 40-ft high wall blocking my line of effect to the other side of it, does it stop working?

It's still there, but it can't harm any creature that it doesn't have line of effect to.

Does Detect Magic only work through walls because it says it does, or do a lot of spells work that way?

Specific exception to the general rule.

 

[was]

when they're those ricepaper, decorative oriental-style walls

 

[Noumenon]

What makes you think that? (And I see nothing in the faithful hound spell description that even hints at it seeing through closed doors.)

The impetus for this question was a faithful hound in Castle Whiterock, which the designer put inside the door so it would bite the first person through, instead of outside where it could see what was going on. The text implied the hound would function as normal.

With Alarm, I guess it was all the stuff it can do up to a mile away, making it seem like it ignored walls. And there are spells like Dominate Person that let you affect things throughout an entire plane. So I'm seeing that line of effect doesn't apply to everything a spell does. It applies mainly to 1) targeting and 2) bursts, cones, cylinders, and emanations. But with the second group it's line of effect from their origin that matters, not from you, which was another thing that was confusing me about spells working when you didn't have line of effect to them.

Yes. Faithful hound is an Effect spell, and once created, effects no longer require line of effect to the caster.

This seems right. It's a little weird because of the line "The hound sees invisible and ethereal creatures," but it's a strict reading of the rules, and you could say it detects the creature through the door by scent.

Yes. Ice storm is an Area spell. (See "Line of Effect," PHB page 176.)

It's still there, but it can't harm any creature that it doesn't have line of effect to.

After pondering countering an Ice Storm by covering its origin point with a hemisphere from Wall of Ice, I'm not sure you're right about that. I found this about cylinders specifically:

When casting a cylinder-shaped spell, you select the spell’s point of origin. This point is the center of a horizontal circle, and the spell shoots down from the circle, filling a cylinder. A cylinder-shaped spell ignores any obstructions within its area.

Or as page 176 seems to put it, the entire circle counts as the cylinder's origin point. But seeing as how there are only two cylinder spells in the entire SRD (Ice Storm and Flame Strike), I think I accidentally stumbled on a special case. I get the general idea.

 

[Greenfield]

Lots of spells continue to work regardless of distance or intervening walls.

Most Detect spells will sense through a wall, unless it's stone or metal.

Very few will let you cast through a wall.

Which of these three categories are you asking about?

 

[Noumenon]

I think some part of my confusion was on whether area effects work through walls. The other part was the difference between spells that work through walls, and spells that can be cast through walls.

I had been confused about a lot of spells, like misdirection, where I was like "so do you and the second object have to stay within Close range of the first or the spell stops working?" (this was about the difference between "legal to cast" and effects, where anything goes.) For stuff like scrying, I was like "How can you create the sensor without line of effect to it?" etc, etc. (Now I think scrying's "which may be at any distance" line is a special exemption to line of effect.)