Since I am an HT builder in the Midwest, and lot of my jobs are in basements, I get this a lot.
Your absolute FIRST concern is not to violate building codes or compromise safety. I have been doing LOS (line of sight) charts for multi-row theaters for many years, and I thank you guys for the application. Sometimes low ceiling height requires that I cannot build the riser to the optimal height.
Different communities have different requirements for minimum ceiling height for an inhabited space, and many times it is tied to a maximum amount of the floor area/ceiling area. Your code may read something like this:
"In habitable rooms, ceiling height not less than 7' over 50% of room area.
Remaining area 5' minimum. Minimum height under duct and beams not less than 6'-4'."
This one is based on the BOCA code I believe.
It is a little vague, and many areas have tons of amendments.
I definitely wouldn't count on a favorable inspection using the 5' minimum for 49% of a room... and that wouldn't be safe anyway.
That portion was intended more for the sloped ceilings of attic spaces and other areas where the ceiling slopes with the roof line.
If you go by the 6'-4" clearance, this means your 7'-6" basement gets a 12-13" max riser by default. Keep in mind that pad and carpet will add some height. 6'-4" is pretty low, you may want to experiment standing on a stepladder or with a temporary platform to see if you would be comfortable that close to the ceiling. You don't want to re-do a riser...
Many times this will not be the ideal riser height by the calculator, but safety must come first.
Consult an architect and/or your local building inspector before you push the envelope.
In these cases, compromise and creativity are required.
If your guests are constantly hitting their heads when they step up on the riser, or stand up from the seats, they will be seeing stars, not your properly laid out stadium seating theater.Some ways to improve LOS in low ceiling applications:1) Choose taller seats in the rear row(s)
Continental is one company that makes a "tall" chair to match a popular model that is like 5-6" higher than the matching regular height model that we would use in the front.http://www.continentalseating.com/gable.html
This way the seated height is higher, but the riser height and standing headroom are still acceptable.2) Make the front row the "money" seats, and choose non-reclining chairs for the riser that can be placed closer to the front row.
Irwin and a few others make nice non-recline chairs that are exact replicas of commercial theater chairs in the industrial upholstery or finer finishes. If you match the upholstery these chairs do not look out of place, and kids love them.3) Raise the height of the screen a few more inches
If the rear rom are your money seats, you will want to LOS to be as close to perfect as possible. You don't want the front rows to be craning their necks up either, nobody wants a stiff neck after a movie. Be careful how much you modify this dimension.
Depending on ceiling height, basement theaters will get usually get a 5-8" step up riser or a 10-15" two step riser, and even with the creative planning for the seating and screen height, the LOS is sometimes slightly compromised.
IMO, this is still the appropriate way to plan.
The building code requires that a step in a home is no greater than 8" per step, and the run (tread depth) is no shorter than 9".
Above 8", you have to install a step on each side and that eats floorspace and/or riser space. A 10-12" run on a tread height that is half the riser height is the rule for us, no higher than 16" of course, or a third step is req'd.
Also, 4 or more steps require a handrail in any area, and in some areas it is 3 or more steps.
DIY theater guys need to heed the building codes if you ever want to sell your homes down the road. Otherwise some of your theater budget for your new home will be spent making changes to the theater you are leaving before you can sell the house.