Stage & Theater Considerations
In my experience stages fall into two broad categories. Traditional Theater Stage (proscenium, arena, thrust) that includes stages that are used for plays, musicals, and dance; and Platforms that include stages that are devoted to only speaking, town meetings, video and presenting.
Stages can be designed for a specific purpose, e.g. drama, musical theater, dance, or musical performance; or they can be multi-purpose. Once you pass the limits in the building code for a Platform, there are several 'triggers' for specific requirements. Take a look at Chapter 410 of the International Building Code.
The type, size, flexibility and sophistication of the venue (whether Stage or Platform) is dependent on the function and end user: Amateur/Professional, Primary/Secondary School, University/Professional Theater, Broadway Touring House or Regional Theater. All of these considerations will affect the design criteria much more.
A theater design specialist is worth their weight in gold to guide you through both the big picture and the many, many details that are required. I can recommend Todd Berling of Harvey Marshall Berling Associates (www.hmb-a.com), who contributed to this article.
The traditional stage can also work for the uses that a Platform is intended for; but the reverse is not true. Traditional stages invariably have a sprung wood floor that is stained black to avoid detraction from the performance. Black also interferes less with lighting. For less professional venues, a lighted edge strip let into the wood floor provides a visual clue of the location of the front edge of the stage even in darkness.
A good rule of thumb is that bigger is better with the traditional type of stage. Just about half of the width would be beyond the proscenium to the left and right. The depth of the stage is almost as important, and should have at least 25' of usable space.
Acoustics are of course a huge part of all of these facilities. Reverberation Time (RT60) is determined and the facility tailored to meet the requirements of the performance style. Concert halls might have a target Reverb time of 2.5 seconds; a drama theater .8 seconds to ensure speech intelligibility; a multipurpose room usually leans towards the "drier side" with a 1.0-1.2 RT60 target - since speech is still a critical component.
Stage height is specific to the decisions made about purpose and venue, including technical staff availability and qualifications. Again the IBC has a few "triggers" that add cost to the project if you intend to have a full fly loft or just dead hung curtains and lighting.
The traditional type of stage has many possible uses, but it can cost between $250 and $500/SF for construction (circa 2005), depending on sophistication and location. Specialty systems for lighting, rigging, sound, video are additional to construction cost.
The Platform, which is focused on video, speaking, and presenting is a much simpler affair. The simpler focus of the stage extends to materials, color, size, clear height; and cost may be a small increase over typical construction cost. While some music performances are accommodated by this type, a recital or a small group of musicians is the limit of its suitability.
Architekwiki has more about theaters. Do a search here.