As far back as the Elizabethan period in theatre history, production designers have been achieving special effects with the use of hidden trap doors on the stage. Trap doors can produce a delightful surprise for the audience, making an actor magically appear or disappear from "out of nowhere." But the engineering required to make trap doors safe can be challenging. Much planning and many precautions must be taken to prevent injuries.
Determine the height of the "drop." For an actor to "disappear" on stage there must be between 10 and 15 cm (4 and 6 inches) of clearance from the trap door height to the "landing pad." For an actor to "appear" out of a trap door, the height factor differs. A floor-to-platform height of 1.2 m (48 inches) can accommodate most adults if they will only be crawling out of the trap and then standing up. In order to make an actor rise out of a trap in a standing position, the platform must be the height of the actor in standing position plus an additional 30 to 45 cm (12 to 18) inches for clearance. Magical "risings" out of trap doors require the use of some type of "elevator."
Provide the safest "drop" or "lift" distance for the trap door by elevating the stage using platforms. Cutting holes in the actual stage floor is normally prohibited. Trap doors can be built over an orchestra pit, however.
Determine the width of the trap hole. Measure any actors who will be using the trap. Add 60 cm (24 inches) of additional clearance to this width for safety. This measurement will determine the size of the platform unit containing the trap door system; make adjustments in stage designs accordingly.
Cut a square hole to proper size in the trap unit platform. Create a soft, deep "landing pad" under the hole. Test the "drop" using a "stand in" as the test subject; provide a football helmet for safety. Instruct the tester to drop into the hole, feet first, arms at their sides. Observe the results and make adjustments as needed. For "elevator" traps, test the lift mechanism and adjust until safe.
Create a trap door. Cut a door from 1.8 cm (3/4 inch) plywood, 6 mm (1/4 inch) smaller than the trap door opening. Hinge this door to the underside of the platform, opening down. Test the fit of this door when closed, avoiding wide gaps. If the actor will be rising out of the trap on a lift, make sure the trap doors can be fully swung down and out of the way and then secured in the open position.
Create a trap door locking mechanism. Install two or three 5 x 10 cm (2 x 4 inch) boards so they can be slid into a fixed track under the trap door to keep it closed and provide support for walking on the platform. Provide room to slide these 5 x 10 cm (2 x 4 inch) "lock bars" out of position when opening the trap. Test these mechanics thoroughly and make adjustments. Train technicians to operate this system flawlessly.
Use blue "running lights" under the trap for safety; do not depend on mere torches. Lock the trap after every use.
Ultimately, the safety of cast and crew is more important than the benefits of any stage effect.