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Trapsmith: Devious Pit Design

Trapsmith: Devious Pit Design

Every good trapsmith knows about the three C’s: Cheap, Crude and Cruel. The correct application of the three C’s while designing a trap gets you the desired result when the PCs have to decide whether they want to risk pushing on for “just one more room.”

And nothing is as cheap as a hole in the ground, so let us examine two common designs for pits, and how to improve them. [More…]

Camouflaged Pit Trap
CR 1; mechanical; location trigger; manual reset; DC 20 Reflex save avoids; 10 ft. deep (1d6, fall); Search DC 24; Disable Device DC 20. Market Price: 1,800 gp.

Deeper Pit Trap
CR 1; mechanical; location trigger; manual reset; hidden switch bypass (Search DC 25); DC 15 Reflex save avoids; 20 ft. deep (2d6, fall); multiple targets (first target in each of two adjacent 5-ft. squares); Search DC 20; Disable Device DC 23. Market Price: 1,300 gp.

For a hole in the ground, both traps are quite costly. Durability seems to be a factor in the design: their internal workings are sturdy, so they fail the crude check. Finally, neither trap passes the cruel check: the lethality of these traps from the DMG is quite low.

Improved Camouflaged Pit Trap
CR 1; mechanical; location trigger; no reset; DC 24 Reflex save avoids; 20 ft. deep (2d6, fall); pit spikes (Atk +10 melee, 1d4 spikes per target for 1d4+2 each); Search DC 24; Disable Device DC 10. Market Price: 1,500 gp.

After it is sprung, the improved camouflaged pit trap becomes an obstacle for future intruders. That is, the pit spikes at the bottom can still claim victims if a Jump or Climb check is failed, or if someone is pushed in during combat with a simple Bull Rush or gust of wind.

Even Deeper Pit Trap
CR 1; mechanical; location trigger; repair; hidden switch bypass (Search DC 25); DC 15 Reflex save avoids; 40 ft. deep (4d6, fall); multiple targets (first target in each of two adjacent 5-ft. squares); Search DC 21; Disable Device DC 15. Market Price: 200 gp.

Sacrificing quality to increase lethality. Sometimes, hard choices are easy to make.

DM Tips
Average damage output for these improved traps is one to two levels above the traps challenge rating—be careful.

Some quick design notes on the improved camouflaged pit trap show how the CR was retained at 1.

No reset: we are saving 500 gp.

DC 24 Reflex save avoids: we increase the DC to enhance lethality, but we keep it under the CR boosting level of 25.

20 feet fall:
2d6 damage averages to 7, which gives us a CR of 1.

Pit spikes: These increase the CR by 1.

Search DC 24: keeping it under 25 just like the original.

Disable Device DC 10: lowering the DC makes the trap cheaper and gives us a -1 modifier on the CR.

The result is a CR 1 trap, from base 1 + 1 + -1.

Design notes on the Even Deeper Pit Trap

Repair: by spending 40 gp you can reset this trap—quite a good deal—and we are saving 200 gp.

Hidden switch bypass:
the bypass means we can place the trap in a high traffic area, so we need to keep it.

DC 15 Reflex save avoids: We need the -1 CR modifier. It weakens the lethality of the trap, but we must keep it.

The 40 foot fall gives us 4d6 damage and averages to 14 damage. This gives us a CR of 2.

Multiple targets: another +1 CR.

Search DC 21: now only characters with the trapfinding class feature can find this trap, and it only costs 200 gp.

Disable Device DC 15
: we need another -1 CR modifier, and dropping Disable Device to DC 15 gives us that and it also decreases the cost.

Overall, then we are still at a CR 1, from a -1 + 2 + 1 + -1. Voila, a 40 foot pit that can appear in low-level scenarios!

Like our series on trap tweaking? Like us to ramp up the CR? Let us know in comments!

6 thoughts on “Trapsmith: Devious Pit Design”

  1. NICE! I really like the inclusion of the design notes.

    Any YEAH! How did (the original) 20 foot hole in the ground cost 1300 gp to make?

    It has always bugged me how costly traps are.

    I mean, why not hire a team of 5 commoners for a week (at a whopping 10 gp each to make them rich) an you could easily have a 20′ hole for 50gp. Those guys would consider you a SAINT!

    Great series.

  2. Dungeons – Subsidence and Cave-ins

    DETERMINING THE STATE OF A CAVE

    Subsidence is defined by the cross-sectional width of a cave divided by its height below surface.

    100% Subsidence: Width of Cave/Cover depth=1.1-1.4*
    10% Subsidence: Width of Cave/Cover Depth=0.1-0.5*

    *This range depends on the rock type. The high thresholds are more likely granite, midway is sandstone and limestone, while the low end of the scale is dirt/gravel.

    That means that a cave with a 50′ cross section sitting below 20′ of rock has a value of 2.5 (well in excess of 100% Subsidence): Such a cave began to transfer the effects of subsidence to the surface back at 10% so the ground will be sagging and unless this chamber is propped up with columns it will collapse like a sinkhole.
    Likewise a 10′ wide tunnel below 20′ of rock has a value of 0.5 (at the upper limit of 10% subsidence). Here the stress of failure has reached the surface – Fine cracks that will probably not be visible at the surface due to topsoil.

    REDUCING SUBSIDENCE TO 10%

    Columns are rarely introduced after the effect. Standard Practice requires a column to be cut from the existing rock strata and left behind undisturbed by mining and excavation.

    Column Width= 0.12 x cover depth
    Goaf (space between columns) width = 0.6 x cover depth.

    What does it mean? Our 50′ cross-section cave above can be reduced to a subsidence of 10% by including at time of construction columns to support the ceiling.

    Column Width: 0.12 x 20′ = 2.4′
    Column Spacing: 0.6 x 20′ = 12′

    A chamber supported by 2.4′ wide columns spaced 12′ apart. Inflict some damage to those columns (beyond the damage inflicted by the existing subsidence) and the entire Dungeon will have to be tested (%percentage roll for each chamber and corridor) as to whether it has collapsed with the mountain coming down on the Adventurers.

    SUBSIDENCE AT THE SURFACE

    When Subsidence exceeds 10% it begins to show at the surface. To determine the Surface Area Affected by Subsidence, you must take a 35 degree angle from the bottom of the workings edge. This means that even Rock that doesn’t overlap a collapsed/failing Cave will be showing failure. The Actual depth of collapse at surface is 80% of actual cave height into which the rock may collapse.

    Subsidence= 0.8 x cave height
    Cave-edge collapse radius = SIN 35 degrees x depth of cave bottom

    So our 50′ cross section cave with its 15′ high chamber will if and when it collapses create a 12′ deep sinkhole with signs of subsidence out to (SIN 35 degrees x 35′) = 20′ beyond the area of the cave (effectively the 50′ cave left a hole 90′ across – slopping down into a 12′ deep, 50′ wide pit).

    SURFACE SIGNS OF SUBSIDENCE

    As subsidence progresses toward 100% the effects of subsidence are transferred to the surface with increasing severity.

    10%<————————100%
    Cracks* – Sagging – Depression Bowl
    Cracks* – Fissures – Chimneys
    Cracks* – Block separation – Terracing
    Cracks* – Sagging – Sinkholes
    *Cracks are not always visible at the surface due to topsoil
    NATURAL SUBSIDENCE OVER TIME AND MAGICAL TUNNELLING
    As a mine is increased through mining it becomes increasingly unstable due to the natural failure over time. The rate of failure over time is determined by a curve associated with the rate at which mining occurred.
    A tunnel cut slowly by Dwarves might progress toward a natural failure over years or even centuries. Whereas the use of Disintegrate to dig your way through is considered an instantaneous advance which means the natural progression toward failure is instantaneous.
    The use of Disintegrate to tunnel instantly pushes the rock structure to 100% before adjusting to its new subsidence value (ex-removal of a 10′ x 10′ x 10′ cube below 10′ of rock by disintegrate requires a check against the cave’s new 10% subsidence because of the instantaneous stress change).
    Our 50’ cross-section cave might progress to 100% subsidence over eight years but the instant someone uses a Disintegrate spell to tunnel through a wall, the section may collapse as a block or separate from the old cave as a stress boundary fissure or chimney.

    ——————————————–

    Applying Cave Subsidence to Pits
    It means that any unreinforced Pit wall will fail out to 35 degrees from bottom of any pit with 11′ of width per 10′ of depth. Killing anyone stupid enough to dig one below 5′ depth x physical height.

  3. I’ve always assumed that the costly parts in the pit traps were the parts that hid the trap and kept it hard to disarm.

    How is a hole so hard to find only Rogues can do it? Even weirder, once a hole is found how and why is it so hard to disarm it?

    I’ve always imagined it as the trap door moment from cartoons. If a rouge disarms it, he wedges it so it can’t open and somehow becomes just another part of floor (since disarming a pit trap means the pit trap doesn’t happen. huh?)

  4. I love these trapsmith articles. My players are 8th lv. and they have learned that even when a trap takes them by surprise they will still survive. They don’t fear traps at all anymore. I would love to see some poison needle traps that would put the fear into them again.

  5. Well, I did improve the two CR 1 poison traps and must admit that my #2 design should remain effective even at that level or use a nastier poison :-)

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