Maybe a torsion box shelf.

 

I might use solid 3/4" stock for the front edge, 1/4" plywood of matching species for the top & bottom, and 1/2" plywood strips for the back and spacers. For an overall thickness of 2 1/2", rip the 12'+ 1x4 to that width, then rabbet the edges so the 1/4" (nominal) ply sits flush, and then rip the plywood fillers to the width remaining between the rabbets. 1/4" hardwood plywood is not expensive and often good looking so the butt joints would not be obvious. Assemble with glue and brads on a flat surface and be careful not to sand through the veneer. Make a template from scrap to get the fit right,& angle the ends back a bit to make installation easier

 

I’m picturing this shelf being against the wall, not away from it, is that right? If so, a wall cleat fastened to the wall should go far to support a 6”-8” wide shelf. If you don’t mind the look of a couple of wall brackets under it you’d have no problem supporting it. If you don’t want to see brackets you could use steel “L” brackets turned upside down and recessed into the top surface of even a 3/4” thick shelf (or better yet sandwiched between two 3/4” layers for a 1 1/2” shelf). The bracket legs would then be screwed to the wall above the shelf and would be hidden by the items on the shelf.

Jack

 

Jack's idea is a good one, that same method (L brackets upside down
on top of the shelf, and a wall cleat) is what the original owner of our house did. In the photo this shelf is over a window nook in the kitchen. rom the left the shelves are 3' long, 3' back into the nook, 7' across the nook, 3' out, then 5' to the right (not in photo). For my wife's teapot collection it's perfect. The L brackets they used aren't all that large either...these shelves are 12" deep.

PS, the house was built in 1998, so the shelves have been there quite a while. Also, the PO had ceramic Santa Claus figures displayed on them, much heavier than the teapots and other junk you see.

 

In post #4 JackP has an interesting idea.
BUT … I recommend Aluminum angle (or channel or square tube.
I have a stock of Aluminum angle that is 1 1/2” and 8 feet long. VERY light and very strong.

 

I use lag screws into the studs for things like floating shelves and mantle pieces that aren't to be taken down for many years. The hard part is getting the screws in straight and then cutting off the bolt heads without bouncing the hacksaw across my knuckles. I found that using a 4x4 with a slot cut in the end keeps the screws from bouncing around. Drill corresponding holes in the shelf and then you simply place the shelf over the screws. I glue the shelf to the wall with liquid nails.

 

“So for these suggestions of putting L brackets on the top of the shelf, how big of a bracket are we talking about, and is the screw then holding all of the weight? I'm confused about how the bracket is supporting it from the top?”

There’d be several possible ways of doing this. I think if I were doing it I would build the shelf from two layers and sandwich one leg of each bracket between them. You said you’d like the shelf to be natural wood with a light finish. That, combined with the fact that it’s more than 8’ long would suggest using a solid wood board (rather than plywood which would require an end joint) for the bottom layer of the shelf “sandwich”. Get a single 3/4” thick board which would be long enough to span the distance and wide enough to give you both the shelf width you’d like and also a strip to use for an edging piece to cover the “sandwich” line. You could get a separate piece for the edging strip if you like but if you rip the edging piece off the same board, the grain and color of the edging strip will match the shelf material as well as possible. Rip the board to shelf width minus 3/4”, then when you add the 3/4” edging strip back on, the shelf will be the right width.

I’d use 1/4” thick steel brackets with one leg almost as wide as the desired shelf and one shorter leg to stick up and screw to the wall studs above the shelf. Like this— Shelf brackets

I would find the studs in the wall, then hold the shelf board up and mark the stud locations on it to ensure that the brackets will line up with the studs. I’d then screw the brackets onto the top side of the bottom board. Then cut a 3/4” plywood top layer exactly the same width as the bottom board. The top layer plywood can have end joints in it because it won’t show. Cut 1/4” deep recesses in the bottom of the plywood to match the brackets and 1/4” notches in the back edges of the top board for the brackets’ vertical legs to stick up through. Then glue the top layer to the bottom. Glue the edging strip on to cover the front edge and it will look like a solid 1 1/2” thick shelf. Here’s a little sketch—

Then set the ends of the shelf on your cabinets, make sure it’s straight (not sagging at all) and screw the brackets to the wall studs. The cabinets will support the shelf ends and the brackets will support the middle.

Jack

 

Thank you for the sketch and detailed reply!

 

With a torsion box, no fasteners are needed at all- just sits on the adjacent cabinets.

 

For sure, lots of different ways to build this shelf. A 6” wide shelf that’s 2” or 3” thick would look just right in some situations. If the OP‘s “trying not to have a chunky fireplace mantle look” some wall support would just be one way to help keep the thickness down.

Jack

 

I’m guessing you have a sheet rocked wall (?). About 30 years ago I installed a similar shelf, actually a double shelf that extended about 20 feet. I used metal brackets that I made from 1/8”x 1-1/2” flat iron. I removed the sheet rock and dadoed the 2x6 studs so the brackets would be flush. I then used red oak 1x10’s, sandwiched the brackets between them and staggered the 10’ long pieces. I cut dado’s on the top of the bottom pieces to allow for the brackets, attached with 4 short, countersunk screws. The dado was cut about an inch short of the edge of the shelf. I replaced the sheetrock and finished it before putting on the shelves. I glued the pieces together after the bottom boards were attached. This was stained, not painted, and looked incredible. I received many inquiries as to how the shelves seemed to magically hang, unsupported, on the wall. I lived in that house another 14 years with the shelves loaded with my tech reference books and never had a sag. I put the brackets with the long end facing down in order to take advantage of the higher compression resistance over the elasticity of the weight opening up the brackets. If I remember correctly, the brackets were 8” x 12” with the 12” length attached firmly with 4” countersunk deck screws, every 3”, into the studs.

A lot of work but if you can find a metal shop to bend the brackets or find them to purchase, doubtful, it would make it easier and, even painted, would look incredible.

Just some food for thought. Enjoy your project whatever you do!