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Help: Appropriate framing direction for a riser in Home Theater room

I'm building a 12" high riser for second row seating in a HT room in the basement. The area is around 6ft by 12ft. Currently I plan to have 4x2 treated lumber and vapor barrier for the first layer of wood (laying flat), then will sit 8x2 (laying tall) untreated, and need an extra final layer of 4x2 laying tall before adding the plywood surface.

My question is, does the 4x2 have to sit on top of the 8x2 in parallel or can I run the 4x2 at 90 degrees to it ?

I want to do it like this guy did (but he has different dimension lumber): there anything wrong with that? Every other install I see, that uses multiple layers of lumber, they tend to run the lumber/joists all in the same direction.

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The 2x4s going perpendicular to the 2x8s is probably more stable. What matters more is the spacing of the joists and the thickness of the subfloor you are using. With appropriate thickness subfloor, a 2x4 spanning 16” is fine if you’re using standard joist spacing.

Edit: why not just use 2x12s?

Original Poster1 point · 6 months ago

Well, yes 2x12 would have been better, but Lowes doesn't sell them in 8ft lengths (Max I can get into the minivan), and I was too lazy to ask them to cut down longer pieces to fit.

The only downside I saw was it makes it a little more tricky to stuff it with insulation (something HT needs as part of the sound treatment).

You are building out this project, and presumably it's important to you, instead of getting exactly what you needed you didn't because you couldn't be bothered to have the guy at HD cut the wood?

I bet your basement floor is constructed to dry to the interior, if so don't use a vapor barrier on the inside. An article on basements and moisture control.

Original Poster1 point · 6 months ago

My barrier will be harmless either way - strips of thin barrier foam under each piece of treated lumber, not huge sheets that could cause a moisture problem (having said that, the carpet underlay I pulled up had a vapor-impervious layer on it, and the entire basement was done that way by the original builders)

Oh man, I hate asking them to cut stuff too. I got one of those battery powered circular saws for Christmas so I think next time I’m going to park in the back of the lot and cut them up myself before loading them into my car!

I was 7 when my dad built our home theater. I can ask him tomorrow how he did it. But these pictures might give you some inspiration:

1 point · 6 months ago · edited 6 months ago

Why so much wood? I would build four 12ft x 12" stud walls and then line with 3/4 particle board flooring on top and 1/8 bracing ply on the sides. Why do you need treated? Sitting in a puddle? You don't even need 2x4 for the stud walls. If you have something like 1.5x3 y there, user that. Studs every 18"-24".

Original Poster1 point · 6 months ago

Treated lumber for wood that is in contact with concrete basement floor is a code requirement for North Carolina. Insect & rot concerns I guess.

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