Step 5: Craft the Trap

Now that you’re finished with the design, it’s time to build the trap. Depending upon the components, this may require purchasing raw materials, using the Craft (trapmaking) skill, casting spells, or some combination of these steps.

Mechanical Traps

Building a mechanical trap is a three-step process. You must first calculate the DC for the Craft (trapmaking) check, then purchase the raw materials, and finally make a Craft (trapmaking) check every week until the construction is finished.

The Craft Check DC: The base DC for the Craft (trapmaking) check depends on the CR of the trap, as given in Table 2–5.

Table 2–19: Craft (Trapmaking) DCs

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

Add any modifiers from the second part of the table to the base value obtained in the first part. The result is the Craft (trapmaking) DC.

Buying Raw Materials: Raw materials (including weapons, poison, and incidental items) typically cost a total of one-third of the trap’s market price. At the DM’s discretion, however, unusual traps may require raw materials that aren’t available where the trap is being constructed. This forces the builder to either undertake a journey to obtain them or pay a higher cost. For example, giant scorpion venom may not be readily available to a character who’s fortifying a polar ice castle.

Making the Checks: To figure out how much progress you make on the trap each week, make a Craft (trapmaking) check. If it is successful, multiply the check result by the DC for the check. The result is how many gp worth of work you accomplished that week. When your total gp completed equals or exceeds the market price of the trap, it’s finished. If you fail the check, you make no progress that week, and if you fail the check by a margin of 5 or more, you ruin half the raw materials and have to buy them again.

Check Modifiers: You need artisan’s tools to build a proper trap. Using improvised tools imposes a –2 circumstance penalty on the Craft (trapmaking) check, but masterwork artisan’s tools provide a +2 circumstance bonus. In addition, dwarves get a +2 racial bonus on Craft (trapmaking) checks for building traps that involve stone or metal.

Assistance: If the trap requires construction work, it helps to have another set of hands available, even if they’re unskilled. Unless the trap is so small that only one person can effectively work on it at a time, the help of one or more assistants speeds the work along. As long as you have the optimal number of assistants (DM’s decision as to how many that is) helping you, you accomplish double the gp equivalent of work each week that you could have alone.

Example Trap: Baltoi’s Rolling Boulder

Here’s an example of how the trapmaking rules work.

Step 1 (Concept): Baltoi the dwarven rogue wants to construct a trap to prevent incursions into her underground lair. She has in mind a huge boulder that rolls down the entry corridor, crushing the intruders.

Step 2 (Trigger, Reset, and Bypass): Baltoi has had bad luck with flying intruders, so she wants a proximity trigger—a very sensitive apparatus that starts the rock rolling upon the slightest disturbance in the air. A manual reset sounds fine to her, even though it’ll take all her guards to roll the boulder back into position. For a bypass, Baltoi opts for a well-hidden switch (Search DC 30 to locate). The proximity trigger adds +1,000 gp to the base cost and the bypass switch adds another +400 gp. The manual reset doesn’t change the cost. None of these components change the CR. Base cost modifier so far: +1,400 gp. CR modifier so far: +0.

Step 3 (Numbers): Trying to save some money, Baltoi doesn’t spend too much effort hiding the big hole in the ceiling through which the boulder drops. She reduces the Search DC to 16, thus shaving 400 gp from the base cost. As a point of pride in dwarven stonecraft, she leaves the Disable Device DC at 20.

The rolling boulder is a melee attack, and Baltoi wants to make sure it connects. Therefore, she decides to increase its attack bonus to +15, which adds +1,000 gp to the base cost and increases the CR by +1.

Baltoi picks a boulder big enough to do 6d6 points of damage. Its average damage is 21 points (+3 CR for high average damage), and it’s wide enough to hit two intruders standing abreast (+1 CR for multiple targets).

Step 4 (Cost): The base cost is 1,000 gp. Adding the base cost modifier of +2,000 gp gives Baltoi a modified base cost of 3,000 gp. Multiplying that by the final CR value (5) gives the market price of the trap: 15,000 gp.

Step 5 (Craft): Table 2–5 gives the base DC for a CR 5 trap as 25. Adding the +5 modifier for the proximity trigger gives a final Craft (trapmaking) DC of 30. Baltoi buys 5,000 gp worth of raw materials (one-third of the trap’s market price) and gets to work.

Baltoi normally has a Craft (trapmaking) bonus of +19. For the purpose of this trap, she gets an additional +2 for being a dwarf working with stone and another +2 for using masterwork tools, giving her a total Craft (trapmaking) bonus of +23. Her first roll is a 15. Adding 23 gives her a check result of 38—success! Multiplying 38 by the DC for the check (30) results in 1,140 gp worth of work completed the first week. The DM says that five assistants would be useful building such a trap, and she easily has that many helpers to call upon in her complex. Her assistants double the amount of work completed, giving her a first-week total of 2,280 gp.

Building such a trap is no easy task, it seems! Assuming average rolls, it’ll take Baltoi another seven weeks to complete the trap. Maybe the proximity trigger was a poor choice. . . .