Posted April 04 2011 - 07:41 AM
I think I can help, since I have built three risers now for three separate home theaters. I'll give you a pretty simple plan that I hope you can follow.
You need to decide how High your riser should be. I'd recommend 8-10". For the purposes of this, let's say it's going to be 8". You also need to decide on general dimensions. If your HT is 11 feet wide, your couch is say 3 feet deep (you want to account for both leg room and also enough walking depth for people to get through in case they have to come/go during a movie). So, let's say you want 11 feet x 7 feet for your riser that is about 8" tall.
To keep it simple, I'm not adjusting for the thickness of the top. This should add about 1" or a little more to height if you carpet it.
For a very stable / solid riser, build a box out of 4 pieces of 2x8 (two sides 11' long, two sides 7' minus the width of two 2x8's). Screw these together with 3.5" decking screws (you can drill pilot holes if you want, but I don't). Make sure this is as square as possible.
From there, cut more 2x8's to serve as ribs. Make sure all lumber you select is free from bows (side to side or flat). Your "ribs" go front-to-back and are the same length as your side pieces (7' minus width of the 2 2x8 front/back pieces). Put in a rib about every 20-28". Attach to outer box (front and back pieces) using 3.5" deck screws the same way you attached the side pieces of your "box."
When this thing is finished, it's going to be heavy, so make sure you're building it not far from where you'll be using it.
You can optionally strengthen this structure by installing corner brackets or L brackets inside the box where ribs and sides attach, but that is optional.
From there, cut a top to fit. To cover 11 x 7, you'll need 3 pieces of 3/4" plywood (4x8), which you will cut to fit. If you want a "lip" on the edge, leave 1" overhang (these are good for mounting rope lights underneath the ledge for down-lighting). Screw the top down onto the ribs and sides using 2" deck screws. 3/4" is probably overkill, but it's what I use to make it feel very stable.
IMPORTANT: When cutting and fitting your top pieces, make sure an edge lines up in the center of a rib. This allows two top pieces to join right over a rib so if someone steps on the edge of one of these pieces, it won't flex down between ribs. Always line your top edges up with RIBS so the edge rests on a rib at center.
Once your riser is complete, you can put a couch or two on it, and guests can enjoy it. When you have the money, go to a discount carpet place and have carpeters come out and carpet the thing for you. I have done this myself and it always looks better when a professional carpeter has done that part of the job.
My latest riser has a 10" front riser and a 20" back row riser with a stair @ 10" high. Looked great! For the 20" riser, I used 2x6 ribs and used deck hanging hardware to position the ribs. The sides were 2 2x10's stacked and connected with decking hardware. This worked out very well and was extremely stable.
Hope this helps,