Summoning a spider onto a wall or ceiling

Question

The conjuration rules state:

A creature or object brought into being or transported to your location by a conjuration spell cannot appear inside another creature or object, nor can it appear floating in an empty space. It must arrive in an open location on a surface capable of supporting it.

Is a wall or ceiling a surface capable of supporting a giant spider?

example 1: An enemy is floating 5ft above the group and I want to give the party rogue a flanking partner so I summon a spider onto a wall above and behind the floating creature.

example 2: we are fighting opponents I want to be entangled so I summon a spider out of reach of the opponents onto the dungeon ceiling.

Is it possible?

Answer 1

This GM would allow a summoned monstrous spider to appear on a ceiling or wall

This GM would allow a caster to conjure using the spell summon monster a celestial, entropic, fiendish, or resolute monstrous spider (and similar creatures) onto a wall or ceiling that was both free of obstructions and could support the creature. Despite this, this GM would not extend such ceiling summonings to all creatures with a climb speed—like the planar ape or planar venomous snake—, and this GM would evaluate individually any wall or ceiling onto which a creature with a climb speed would be summoned, confirming the surface fell within the creature's capacity to scale by taking 10 on its Climb skill checks. While this may seem restrictive, it's generous compared to the printed rules.

In Pathfinder there's not really that much difference between having a climb speed and just having a really high Climb skill modifier. The skill Climb on Climb Speed explains the differences between a creature using the Climb skill and a creature having a climb speed:

A creature with a climb speed has a +8 racial bonus on all Climb checks. The creature must make a Climb check to climb any wall or slope with a DC higher than 0, but it can always choose to take 10, even if rushed or threatened while climbing. If a creature with a climb speed chooses an accelerated climb (see above), it moves at double its climb speed (or at its land speed, whichever is slower) and makes a single Climb check at a –5 penalty. Such a creature retains its Dexterity bonus to Armor Class (if any) while climbing, and opponents get no special bonus to their attacks against it. It cannot, however, use the run action while climbing.

(As Pathfinder tends to scatter rather than consolidate information, that there's not more double-secret information about having a Climb speed can be confirmed by the sections on Movement on Climb and on Universal Monster Rules on Climb.)

That's it. Those are all the differences between just using the Climb skill to climb and having a climb speed. And, weirdly, these rules are unchanged whether the creature is, for example, an ape, a monstrous spider, or a venomous snake. By default, then, a ceiling shouldn't be a viable summoning destination for a monstrous spider because it's impossible for any creature using only its climb speed to roam freely across ceilings: The Climb skill says, "With a successful Climb check, you can advance up, down, or across a slope, wall, or other steep incline (or even across a ceiling, provided it has handholds) at one-quarter your normal speed" (emphasis mine). Yes, the rules mandate even a monstrous spider typically need handholds to traverse a ceiling! Having a climb speed is not—in this case, paradoxically—like being subject to a continuous spider climb effect. (Seriously, that the spell spider climb says that "[t]he subject can climb and travel on vertical surfaces or even traverse ceilings as well as a spider does," yet a Pathfinder monstrous spider can't traverse ceilings, is hilarious.)

Thus another GM could justify a No summoning spiders onto ceilings! by citing these rules and by pointing out how different from even our world's largest spiders are these fantasy monstrous spiders. (Really, despite my entomological knowledge stemming mainly from comics and B-movies, even I know that the game's vermin are closer to a-wizard-did-it living killing machines than the real-world beasts they just so happen to resemble.) Further, such a GM may argue, a typical monstrous spider traverses its lair's ceilings because it had previously covered the ceiling with sheets of webbing via its extraordinary ability web, the special ability allowing the monstrous spider to "move across its own web at its climb speed." This would likely give monstrous spiders a reputation for traversing ceilings despite not being able to do so willy-nilly.

But, to this GM, both expectations and verisimilitude demand that monstrous spiders (and other vermin like the giant centipede) be able to traverse ceilings without, for example, first webbing the crap out of the ceiling or having carpenter ants install handholds. Disallowing such movement (and, likewise, summoning)—while possible according to the rules—, in this GM's opinion, seems to subtract more fun than it adds.

Answer 2

No reason not to.

If spider has climb speed, and usually it does, then it means walking on such things is natural, and surfaces like stone walls or ceilings can support it. Just make sure it is in spell range.

If wall is thin, cheaply build, DM may decide it cannot support weight of your spider. Similarly, if ceiling was already damaged in battle, it wouldn't be unreasonable for him to say no. These are purely situational and should be decided case by case.

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Note that most of the Pathfinder and D&D fight descriptions are based on 2D square grid (5 foot). For example, there is "threatened area", not "threatened volume". 3D interactions, while not forbidden, usually need a lot of interpretation by DM, so the best course of action is to ask him how would flanking work in such situations.

Personally I use 5 foot cubes when needed, assuming humanoids can fit in one cube or stretch to occupy two. That way, what you described is perfectly valid. But that's not a rule, just my fix at lack of rules.