I am building a fireplace with bookcases too. My wall is 13' also. Here's what I'm building.

You could do the surround in marble or slate. We did marble around ours. It can be any size you want. I wouldn't do painted drywall.

You can pull off just about anything if you are leaving the natural finish of the cherry. I personally prefer a multi-color slate with lots of copper tones, but I have seen everything from deep blacks to blondes and stark whites. A lot of it is just personal preference.

Good luck.

bry, the cherry is what I'm using. I don't think the OP mentioned a wood they were using? I posted mine to show the OP that there are options for a 13' wall.

If this is a direct-vent gas fireplace, each manufacturer and model will have its own requirements for installation including distances of combustible materials from the opening and whether or not a hearth is required. Select the fireplace model first and study the installation instructions carefully.

@cpartist- what material are you using for your surround? We are maximizing the size of the adjacent bookshelves/cabinetry since that's where our TV is going. We have 49" from left to right for the surround + fireplace box. I like the look of ledgestone but think there isn't enough space for it.

@bry911- thanks! I believe our cabinetry will be maple painted white.

With only 49" allotted to the fireplace, you'll end up with a microscopic firebox. Might as well skip the whole thing and just do a wall of built in cabinetry.

@sophie wheeler- we are going with the Bentley 39 by Marquis. Final dimensions with the frame are 36.5" wide x 32" high. That leaves approx 6-6.5" of surround on either side

I would strongly suggest taping it out on a wall. it is very skimpy. or go contemporary with a long narrow box

I like the concept of the "wide shelf all the way across" like CPArtist has shown above. Yes, it's mainly a Craftsman thing, but I think it can work in other applications as well.

If you're going to use tile on a fireplace, I agree with going with something really great -- and handmade to your specifications sounds ideal. This is a focal point, not a place to scrimp!

I think "taping it out" is a wonderful idea. You'll need a big, wide space, but go into a hallway if you have to. Or even do it outside. So many of us are visual people, so SEEING is best.

If you're trying to fit a TV on one side or the other, here's a question for you: Must your built-ins be symmetrical, or would you be okay with one side -- the TV side -- being larger than the other side? This question has no right or wrong answer; it's your opinion.

And one last thought: If you do end up with a SMALL fireplace (and I didn't do the math, so I don't know), you might consider going with a small wood stove (or a gas stove that looks like a wood stove) instead of a fireplace. Do some googling. It can be a good look, either inside a masonry fireplace or placed in front of a stone or tile wall.

O don't think the proportions of 6" flanking a 40" fireplace will look right in any material except maybe slab. And that's a maybe.

You'll want 25-33% surround on each side for width to look right. 12-15" on each side.

Actually mrspete has a good suggestion of doing a wood stove instead and placing it in front of a stone wall.

BTW: I will be placing my tv above the fireplace. That's why my center section is larger than the two sides.