Designing A Trap

Mechanical Traps: Simply select the elements you want the trap to have and add up the adjustments to the trap's Challenge Rating that those elements require (see Table 1) to arrive at the trap's final CR. The DC of the Craft (trapmaking) checks a character must make to construct the trap are derived from its size and complexity (see Table 2).

Magic Traps: As with mechanical traps, you don't have to do anything other than decide what elements you want and then determine the CR of the resulting trap (see Table 1). If a player character wants to design and construct a magic trap, he must use the Craft (artificing) skill. In addition, he must be able to cast the spell(s) that the trap requires - or, failing that, he must be able to hire an NPC to cast the spell(s) for him.

Challenge Rating of a Trap

To calculate a trap's Challenge Rating, add all the CR modifiers (see the tables below) to the base CR for the trap type.

Mechanical Trap: The base CR for a mechanical trap is 0. If the final CR is 0 or lower, add features until you get a CR of 1 or higher.

Magic Trap: For a spell trap or magic device trap, the base CR is 1. The highest-level spell used modifies the CR (see Table 1).

Average Damage: If a trap (either mechanical or magic) does hit point damage, calculate the average damage for a successful hit and round that value to the nearest multiple of 7. Use this value to adjust the Challenge Rating of the trap, as indicated on the tables below. Damage from poisons and pit spikes does not count toward this value, but damage from a high strength rating and extra damage from multiple attacks does.

For a magic trap, only one modifier applies to the CR - either the level of the highest-level spell used in the trap, or the average damage figure, whichever is larger.

Multiple Traps: If a trap is really two or more connected traps that affect approximately the same area, determine the CR of each one separately.

Multiple Dependent Traps: If one trap depends on the success of the other (that is, you can avoid the second trap altogether by not falling victim to the first), they must be treated as separate traps.

Multiple Independent Traps: If two or more traps act independently (that is, none depends on the success of another to activate), take the higher CR and add half the value of the lower CR to it.

Mechanical Trap Cost

The base cost of a mechanical trap is 500 gp. Apply all the modifiers from Table 1 for the various features you've added to the trap to get the modified base cost, then determine the trap's size and complexity (see below). The final cost is equal to the modified base cost x the size/complexity modifier + extra costs. The minimum cost for a mechanical trap depends on its size and complexity, as noted on Table 2 – if the final cost is below the listed minimum cost, increase it to that number.

After you've multiplied the modified base cost by the size/complexity modifier, add the price of any alchemical items or poison you incorporated into the trap. If the trap uses one of these elements and has an automatic reset, multiply the poison or alchemical item cost by 20 to provide an adequate supply of doses.

Table 1: Trap Pricing and CR Modifiers

Feature Cost Modifier1 CR modifier
Trigger Type
Location
Manual -50 gp
Proximity +500 gp
Touch
Touch (attached) -50 gp
Timed +500 gp
Reset Type
No reset -250 gp
Repair -100 gp
Manual
Automatic +250 gp (or 0 if trap has timed trigger)
Bypass Type
Lock +50 gp (Disable Device DC 30)
Hidden switch +100 gp (Search DC 25)2
Hidden lock +150 gp (Search DC 25,
Disable Device DC 30)2
Reflex Save DC (Pit or Other Save-Dependent Trap)
19 or lower -50 gp/point below 20 -1/5 points below 20
20
21 or higher +100 gp/point above 20 +1/5 points over 20
Attack Bonus (Melee or Ranged Attack Trap)
+9 or lower -50 gp/point below 10 -1/5 points below +10
+10
+11 or higher +100 gp/point above 10 +1/5 points over +10
Damage Bonus
High Strength rating
(ranged attack trap)
+50 gp × bonus (max +4)
High Strength bonus
(melee attack trap)
+50 gp × bonus (max +8)
Damage/Effect
Average damage +1/7 points3
Miscellaneous Features
Alchemical item Level of spell mimicked
Liquid +5
Multiple targets +1 (or 0 if never miss)
Never miss +500 gp
Onset delay 1 round +3
Onset delay 2 rounds +2
Onset delay 3 rounds +1
Onset delay 4+ rounds –1
Pit +50 gp/10 ft. of depth +1/20 ft. of depth
Pit spikes +50 gp +1
Poison Cost of poison4 CR of poison used4
Touch attack +1
Search DC
19 or lower -50 gp/point below 20 -1/5 points below 20
20
21 or higher +100 gp/point above 20 +1/5 points over 20
Disable Device DC
15 or lower -50 gp/point below 20 -1/5 points below 20
20
21 or higher +100 gp/point above 20 +1/5 points over 20
Magic Traps See Table 5 +1/7 points of avg. damage
OR + spell level

1 See Table 2.

2 Increase the cost by +100 gp per points of the Search DC over 25, and +100 gp per 5 points of the Disable Device DC over 30.

3 If a trap (either mechanical or magic) does hit point damage, calculate the average damage for a successful hit and round that value to the nearest multiple of 7. Use this value to adjust the trap's Challenge Rating, as indicated on the tables below. Damage from poisons and pit spikes does not count toward this value, but damage from a high strength rating and extra damage from multiple attacks does.

4 Modify cost by x20 if the trap features an automatic reset. See below for poison CR adjustments and here for poison costs.

CR adjustments

Incidental Damage: The aim of some traps (like the falling portcullis) is not to inflict damage, but unlucky souls can still get caught in the trap and take damage anyway. In this case, halve the CR modifier (round down).

Size and Complexity

A trap's size and complexity apply modifiers to its end cost (and, by default, its crafting time) – a simple trap is a lot easier and cheaper to make than something that's huge and has a lot of moving parts. Table 2 displays modifiers for all combinations of size and complexity; this number is multiplied by the initial cost to get the trap's end cost.

Size: The trap's size. This is generally based on the trap's size and/or its area of effect, and does not directly correlate with creature sizes. A tripwire, for instance, would be Small; a door trap would be Medium; a moving wall would be Large; and a compacting room would be Huge. Pit traps are a special case – since the opening is the "area of effect", normal 10-foot-square pits are treated as Medium; "wide-mouth" pits are Large. Depth, however, increases the trap's complexity – it's a lot harder to dig and line a 70-foot pit than a 30-foot pit.

A rule of thumb is this: If the trap is smaller than the average person, or has a very small area of effect (one person), it's Small; if it's around the same size as a person, or affects 1-2 targets, it's Medium; if it's larger than a person, or affects most of the party (2-4 people), it's Large; if it's really big, or affects an entire party and then some (5+ people), it's Huge.

Complexity: This is a rating of how complicated the trap is, and how hard it is to make.

Simple traps: These are traps with no moving parts, which are generally fairly common and easy to make, and require little to no maintenance. Tripwires, open pits, items placed on the ground (marbles, caltrops, etc.), most traps with a manual or repair reset, or pit traps up to 20 feet deep are simple.

Moderate traps: These are items with a few moving parts, or that have a slightly larger area of effect. A spray of acid, a fusillade of missiles, a pit that closes and locks after someone falls in, or pit traps from 30-50 feet deep are moderate.

Difficult traps: These are traps with several moving parts, are hard to make, or that have several different functions. A hall of swinging blades, combination (dependent) traps, most traps with a timed trigger or onset delay, or pit traps from 60-80 feet deep are difficult.

Complex traps: These are traps with a high degree of craftsmanship or a large number of moving parts, or are generally complex in nature. A crushing room that resets itself, a statue that senses the presence of living beings nearby and shoots crossbow bolts at them, or pits traps 90+ feet deep, are complex.

Table 2: Trap Size and Complexity Modifiers to Cost

Complexity
Size Simple Moderate Difficult Complex
Small x1/2 x1 x2 x3
Medium x1 x2 x4 x6
Large x1.5 x3 x6 x9
Huge x2 x4 x8 x12

The minimum cost for a mechanical trap depends on its size and complexity, as noted on Table 3.



Table 3: Minimum Costs for Traps
Complexity
Size Simple Moderate Difficult Complex
Small 50 gp 100 gp 200 gp 400 gp
Medium 100 gp 200 gp 400 gp 800 gp
Large 150 gp 300 gp 600 gp 1,200 gp
Huge 200 gp 400 gp 800 gp 1,600 gp

As mentioned above, crafting times are based on the trap's size and complexity. The DM is free to rule that a trap takes more or less time than is listed here; these are mostly guidelines.



Table 4: Crafting Times for Traps
Complexity
Size Simple Moderate Difficult Complex
Small 1 day1 2 days 4 days 6 days
Medium 2 days 4 days 6 days 10 days
Large 4 days 6 days 10 days 15 days
Huge 6 days 10 days 15 days 25 days

1 Small, simple traps often take far less than 1 day to make – contact poison, caltrops, marbles, or a tripwire are examples of this kind of trap – so the DM is free to adjudicate a shorter time if the situation warrants.



Multiple Traps

If a trap is really two or more connected traps, determine the crafting time of each separately, then add those values together and add 1 more day per complexity catergory (1 for simple, 2 for moderate, etc.). This holds for both multiple dependent and multiple independent traps.

Magic Device Trap Cost

As with mechanical traps, a magic device trap has a base cost of 1,000 gp. Magic device traps are treated as wondrous items; the Craft DC is based on the end cost (see the Craft (artificing) skill).

Table 5: Cost Modifiers for Magic Device Traps

Feature GP Cost Modifier CR Modifier
Alarm spell used in trigger
Highest-level spell used (one-shot) +50 gp x caster level x spell level Special1
Each spell used (automatic reset) +500 gp x caster level x spell level Special1
Lesser spell used (one-shot) +50 gp x caster level x spell level
Lesser spell used (automatic reset) +500 gp x caster level x spell level
Extra Costs (added to base GP Cost)
Material Components Cost of all material components (x100 gp if automatic reset)

1 See the note above regarding traps that deal damage. For a magic trap, only one modifier applies to the CR - either the level of the highest-level spell used in the trap, or the average damage figure, whichever is larger.



Craft DCs For Mechanical Traps

Once you know the Challenge Rating of a trap, determine the Craft (trapmaking) DC by referring to the table below, which provides guidelines based on the trap's size and complexity.

Table 6: Craft (trapmaking) DCs

Complexity
Size Simple Moderate Difficult Complex
Small 10 15 20 25
Medium 15 20 25 30
Large 20 25 30 35
Huge 25 30 35 40

Additional
Components
Modifier to Craft
(Trapmaking) DC
Proximity trigger +5
Automatic reset +5


Poisons

To calculate a poison's CR, start at CR 0 and add modifiers from the list below as appropriate.

  • +1 if the poison is contact or inhaled.
  • +1/3 points of the save DC over 10 (round down).
  • +1/7 points of average damage (round to the nearest multiple of 7, as with normal traps).
  • -1 if Initial or Secondary damage is 0.
  • +1 if the poison causes Con damage.
  • +1 if the poison causes damage to two stats.
  • +1 if the poison causes ability drain (any stat).
  • +1 if the poison reduces any ability score to 0.
  • +1 if the poison causes unconsciousness or paralysis.

Table 7 shows the CR modifiers for most common poisons.



Table 7: Poison CRs
Poison CR Poison CR
Black adder venom +2 Large scorpion venom +3
Black lotus extract +8 Malyss root paste +3
Bloodroot +1 Medium spider venom +2
Blue whinnis +3 Nitharit +4
Burnt othur fumes +6 Purple worm venom +6
Deathblade +6 Sassone leaf residue +4
Dragon bile +9 Shadow essence +4
Drow poison +1 Small centipede venom +1
Giant wasp venom +3 Terinav root +5
Greenblood oil +2 Ungol dust +4
Insanity mist +3 Wyvern venom +5


Reflex Saves vs. Attack Rolls

In some cases, it simply makes more sense to require a Reflex save to avoid a trap, instead of the trap making an attack roll. Some examples of this include falling objects, slamming doors, rolling objects, and rope/razorwire strung across a hallway. Some might argue that with the evasion and improved evasion, this gives rogues too much of an advantage, but it would make more sense for rogues, who are naturally quick on their feet and who have experience dealing with traps, to be able to avoid them more easily.

Converting from one to the other is simple: For traps with an attack roll, add 10 to the attack roll to make it a Reflex save, or subtract 10 from a trap's Reflex save to get the attack bonus.

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