Designing a Trap

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Gustave Courbet (1819-1877) Title: Fox Caught in a Trap Date 1860
Gustave Courbet (1819-1877) Title: Fox Caught in a Trap Date 1860

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: CR Modifiers for Mechanical Traps) to arrive at the trap·s final CR. From the CR you can derive the DC of the Craft (trapmaking) checks a character must make to construct the trap.

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: CR Modifiers for Magic Traps). If a player character wants to design and construct a magic trap, he must have the Craft Wondrous Item feat. In addition, he must be able to cast the spell or spells that the trap requires·or, failing that, he must be able to hire an NPC to cast the spells for him.

Challenge Rating of a Trap

To calculate the Challenge Rating of a trap, 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 your 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: CR Modifiers for Magic Traps).

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), use their CRs to determine their combined Encounter Level as though they were monsters. The resulting Encounter Level is the CR for the combined traps.

Modifiers for Mechanical Traps

Feature CR Modifier
Search DC
15 or lower -1
25-29 +1
30 or higher +2
Disable  Device DC
15 or lower -1
25-29 +1
30 or higher +2
Reflex Save DC (Pit or Other Save-Dependent Trap)
15 or lower -1
16-24
25-29 +1
30 or higher +2
Attack  Bonus (Melee or Ranged Attack Trap)
+0 or lower -2
+1 to +5 -1
+6 to +14
+15 to +19 +1
+20 to +24 +2
Damage/Effect
Average damage +1/7 points*
Miscellaneous Features      
Alchemical device Level  of spell mimicked
Liquid +5
Multiple target +1 (or  0 if never miss)
Onset delay 1 round +3
Onset delay 2 rounds +2
Onset delay 3 rounds +1
Onset delay 4+ rounds -1
Poison CR of poison (see below) Poison CR of poison (see
below)
Black adder venom +1 Large scorpion venom +3
Black lotus extract +8 Malyss root paste +3
Bloodroot +1 Medium spider venom +2
Blue whinnis +1 Nitharit +4
Burnt othur fumes +6 Purple worm poison +4
Deathblade +5   Sassone leaf residue +3
Dragon bile +6   Shadow essence +3
Giant wasp poison +3   Small centipede poison +1
Greenblood oil +1   Terinav root +5
Insanity mist +4   Ungol dust +3
Wyvern poison +5    
Pit spikes +1
Touch attack +1
* Rounded to the nearest multiple of 7 (round up for an average that lies exactly between two numbers).

CR Modifiers  for Magic Traps
Feature CR Modifier
Highest-level spell + Spell level OR +1  per 7 points of average damage per round*
*See the note following Table: CR Modifiers for Mechanical Traps.

Mechanical Trap Cost

The base cost of a mechanical trap is 1,000 gp. Apply all the modifiers from Table: Cost Modifiers for Mechanical Traps for the various features you’ve added to the trap to get the modified base cost.

The final cost is equal to (modified base cost x Challenge Rating) + extra costs. The minimum cost for a mechanical trap is (CR x 100) gp.

After you’ve multiplied the modified base cost by the Challenge Rating, 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.

Multiple Traps: If a trap is really two or more connected traps, determine the final cost of each separately, then add those values together. This holds for both multiple dependent and multiple independent traps (see the previous section).

Cost Modifiers for  Mechanical Traps

Feature Cost  Modifier
Trigger Type  
Location
Proximity +1,000 gp
Touch
Touch (attached) –100 gp
Timed +1,000 gp
Reset Type  
No reset –500 gp
Repair –200 gp
Manual
Automatic +500 gp (or 0 if trap has timed trigger)
Bypass Type  
Lock +100 gp (Open Lock DC 30)
Hidden switch +200 gp (Search DC 25)
Hidden lock +300 gp (Open Lock DC 30, Search DC 25)
Search DC  
19 or lower –100 gp x (20 – DC)
20
21 or higher +200 gp x (DC – 20)
Disable Device DC  
19 or lower –100 gp x (20 – DC)
20
21 or higher +200 gp x (DC – 20)
Reflex
Save DC (Pit or Other Save-Dependent Trap)
19 or lower –100 gp x (20 – DC)
20
21 or higher +300 gp x (DC – 20)
Attack Bonus (Melee or Ranged Attack Trap)
+9 or lower –100 gp x (10 – bonus)
+10
+11 or higher +200 gp x (bonus – 10)
Damage Bonus  
High strength rating (ranged attack trap) +100 gp x bonus (max +4)
High Strength bonus (melee attack trap) +100 gp x bonus (max +8)
Miscellaneous Features  
Never miss +1,000 gp
Poison Cost of poison*
Alchemical item Cost of item*
* Multiply cost by 20 if trap features automatic reset.

Magic Device Trap Cost

Building a magic device trap involves the expenditure of experience points as well as gold pieces, and requires the services of a spellcaster.

Table: Cost Modifiers for Magic Device Traps summarizes the cost information for magic device traps. If the trap uses more than one spell (for instance, a sound or visual trigger spell in addition to the main spell effect), the builder must pay for them all (except alarm, which is free unless it must be cast by an NPC; see below).

The costs derived from Table: Cost Modifiers for Magic Device Traps assume that the builder is casting the necessary spells himself (or perhaps some other PC is providing the spells for free). If an NPC spellcaster must be hired to cast them those costs must be factored in as well.

A magic device trap takes one day to construct per 500 gp of its cost.

Cost Modifiers for
Magic Device Traps
 
Feature Cost Modifier  
Alarm spell used in
trigger
One-Shot Trap  
Each spell used in
trap
+50 gp x caster level
x spell level,

+4 XP x caster level x spell level

Material components + Cost of all material
components
XP components + Total of XP components
x 5 gp
Automatic Reset
Trap
 
Each spell used in
trap
+500 gp x caster level
x spell level,

+40 XP x caster level x spell level

Material components + Cost of all material
components x 100 gp
XP components + Total of XP components
x 500 gp

Spell Trap Cost

A spell trap has a cost only if the builder must hire an NPC spellcaster to cast it.

Craft DCs for Mechanical Traps

Once you know the Challenge Rating of a trap determine the Craft (trapmaking) DC by referring to the table and the modifiers given below.

Trap CR Base Craft (Trapmaking) DC
1-3 20
4-6 25
7-10 30

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

Making the Checks: To determine how much progress a character makes on building a trap each week, that character makes a Craft (trapmaking) check. See the Craft skill description for details on Craft checks and the circumstances that can affect them.