I have sistered on 2 x 8's to my existing attic floor. It originally was framed with 2 x 4's and then 2 x 6's were sistered along to them. All the 2 x's are resting on the top plate. It feels sturdy enough to lay subflooring onto, but it is a twenty foot span. Will this be adequate?

I would say yes with a 3/4 plywood.But what supports the center of that span?

Is there a wall below this span?A post perhaps?

Back to you.

There is no center supports for the span, this is above a garage. It does have a little flex, but I a little is not an issue. I am more concerned about putting a compressed bamboo flooring on it. Also the subflooring is 7/8'' tongue in grove chip board.

I take it you haven't pulled a building permit for this project?

What is the purpose of the floor being installed,is it for storage,or an apartment?

This is necessary to give you the proper advice.

KBC

No permit. It's a small reading room above my garage. Barely any livable space in room to start (the roof is pitched). This will never be a "legal" space (ie no heat, undersized door, etc), just a small bit of extra dead space being uitilized.

Check.

This being the case,the sub-flooring might be just fine.I would be concerned about the deflection in the middle.

Maybe some more ideas will crop up tonight and/or others will have an idea or two.

I have paved the way for them to answer you!

KBC

My worst case scenario involves to simspon brackets, a 2 x 8 running perpendiculiar underneathe (in the garage) and a lot of sinkers. Although the room may look like it was designed for a hobbit, I'd like to keep the fun house motif isolated to one room ;)