shelving
I want to build a bookcase as part of a large built-in, and due to the geometry of the situation, it would be desirable to have one column of shelves be 66" wide. The problem is that even using hardwood boards, a shelf 66" will sag under the weight of heavy books.
So, the natural solution is to use a a steel flat or angle to reinforce the front edge of the shelf and then hide it somehow so it cannot be seen.
How can I do this?
"Not square" can mean two different things: not a right-angle, or rounded.
If the walls are not at right angles, you can use a T-bevel to measure the angle between the walls. If you don't have a T-bevel, you can hold a piece of light cardboard against one wall and fold it so the crease is against the other, which you can use as a template for cutting the shelves.
If the corners are rounded, I'd use the cardboard again: estimate the radius of the corner, cut a quarter-circle of that radius, and check how well it fits into the corner. If necessary, adjust the estimate and repeat. Then use the quarter-circle as the template.
It sounds like you have a floating shelf. Because of the shear forces, these don't hold very much as they just end up pulling away from the wall.
If you want to pile heavy books on it, you are going to need to support it properly. The words "decorative" and "strength" don't usually go well together. I would think you need something solid and not a rope. With rope you have to worry about tensile strength, stretch and how to anchor it (will the anchor now hold the weight?).
The best thing to do is install a bracket below the shelf to support it. How many of these you need will depend on how long your shelf is and what it's made out of. You could always start with two (one on either end), and if the shelf is sagging, install another one in the center. There are "decorative" brackets out there but the basic principal is the same. Make sure you anchor these into a stud and not just the drywall or else you'll end up just pulling the drywall off your wall.
Best Answer
You can put a 1/2"-thick wooden board over the metal piece to camouflage it. And if the shelf is really that long, you'll need something strong for the metal piece. A structural engineer could probably tell you the minimum dimensions, but I would guess a 1.5" angle, or square tube, would do it. And that means you're limiting access to the shelf below that.
If this was my shelf, I'd focus on finding shelf supports that take up as little space as possible. Something like this: