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Building a Bumpout For Gas Fireplace

cookieman's picture

Am installing a gas/direct vent fireplace in my LR (1st floor over full poured concrete basement) and need to build a 12" bumpout so only half the fireplace projects into the room. How should this be framed, insulated and enclosed....exterior is vinyl siding? Really want to avoid dealing with concrete footing.......since bumpout will be 12-24" above ground and about 12" deep, think that should be OK. Checked this site and Fine HomeBuilding mags I have for How-To-Do articles but came up empty. Please help asap. Thanks.     

(post #99648, reply #1 of 9)

One that I saw was framed with a recessed tv nook above. Very convenient place to put it and a good use of "free" space. Not what you asked, but something to consider as you plan the whole thing.

(post #99648, reply #2 of 9)

Can't someone out there please help me with this?

(post #99648, reply #3 of 9)

I've installed several this way and it always worked for me...first I determine the overall dimension of the unit across the front, then I add two wall thicknesses, and two sheet rock thicknesses, add a little wiggle room, and use this as the rough opening...

I cut this opening thru the wall, and header it off, now the sneakey part...I sheath the bump out to the new trimmer studs, and build the floor inside of this opening...then build the walls on top of the floor, insulate, sheet rock, etc...


the overall effect is that the entire unit is supported by the sheeting, that is supported entirely by the trim studs....if this interests you at all, let me know privately, and I'll be able to make a sketch for you, or offer any othe rhelp I can....

I wish I didn't know now, what I didn't know then

(post #99648, reply #5 of 9)

So, you've access to the u/s of the floor? See which way the joists run, and if you can scab on about 4' or 5' of similar-sized joists to run thru the rim-joist to the o/s, thereby creating a cantilever for the req'd depth of stove . Glue and nail well .

Ah, you've only access from the house. Since your stove is  only halfway into the wall it kind of balances, doesn't it? Make a rough opening 1-1/2" larger than req'd with doubled studs each side, then use 3/4" plywood to create the sides of the bump-out. This will cause everything to 'hang' off the studs. Frame out around the o/s, to code. Put a nice little roof on it. If you want to be sure - a pair of knee-braces under to help support the load. Don't forget that code stuff for base and roof as well as walls - don't need the heat going o/s, eh.

All the best...

To those who know - this may be obvious. To those who don't - I hope I've helped.



(post #99648, reply #4 of 9)

Are your floor joists exposed in the basement so that you can cut the box out and slip in joists and cantilever them outside?

You'll also have to cut out your outside wall and put a header in. Is there a second floor above this outside wall?

Also, is there a basement window underneath where this fireplace will be above?

Joe Carola

Edited 10/29/2005 9:34 pm ET by Framer

Joe Carola

(post #99648, reply #6 of 9)

If the bumpout is not even with the existing first floor (sounds like you are considering putting its floor maybe a foot higher?) you could remove a piece of the rim joist on the second floor and sister "extension" joists to the existing joists (if they run the correct way) and sort of hang the bumpout from them. Or do the other poster's idea of hanging it from the sheathing on the sides--or a combination of both. If the joists run parallel to the bumpout, you could build a "ladder" (like you would frame an overhang on a gable end of a roof) and nail that to the rim joist. It would be stronger though if you could tie it back into the last joist about 16" in from the rim joist.

If you did this at the top and bottom, it would be a lot stronger than you need. And even if the bumpout floor is higher than the first floor, you could still extend it from the existing first floor joist and build a new floor up from those, or you could make the bottom a series of right triangles with the longest legs as the supports angling down to the rim joist and the members that act as the floor tied into the wall studs. Putting OSB or plywood on top, bottom and sides, glued and nailed would make it really strong (could even put in one or two traingular plywood gussets in the middle glued and nailed to the frame members. Make roof the same way.

Probably way more than you wanted to know!

(post #99648, reply #7 of 9)

I would be a good way to build it on top which has been mentioned. You can build it as if it was a 12" window projection. Which I also do with 3/4" plywood going in flush to the inside of the jack stud. I even cut the slope of the roof out.

If you want a raised hearth it would be even easier. You can build a platform by setting 2x6's or 2x8's or whatever height you want sticking past the house 12" and then whatever your inside depth rough opening is plus the length of the hearth you want.

If the inside depth is 25" and you want a 16" hearth your platform will be 41". The rough opening width you would add for the thickness of the walls for each side. On the inside of the house you can make the walls wider so that you can have space on each side of the fireplace for a mantle or tile. This way the outside box isn't to wide.

If you didn't want it raised it can still be done but that would put you closer to the grade level which I don't think you want.

I drew a raised hearth situation for you. I hope it's clear.

Joe Carola
Joe Carola

(post #99648, reply #8 of 9)

Everyone.......Thanks for your advise.

Joe.........thank you for the drawing.......sorry didn't respond to questions in your (Joe) earlier message but was out most of day. Fireplace will be flush with LR floor but outside ground under that portion of house slopes so bottom of bumpout will be 18"-24" above ground.

Since fireplace will be 10-12" into LR (and its 22" deep overall) almost all its weight will be supported by the LR floor/joists and the basement foundation wall, so seems to me method of attaching 3/4" plywood sides to trim 2"x6" and attaching bumpout framing (2"x4" with 2" + 1" foam insulation board & sheetrock)  to that will work for my application.

Again thanks to everyone for your responses. 


(post #99648, reply #9 of 9)

No problem, Cookieman.

Just make sure that the pipe has enough clearance around the header being that it's sitting half in and half out> Do you have a cross section with all the dimensions of the fireplace yet?

Here's the drawing with it flush to the floor.

Joe Carola
Joe Carola
Bump_out_fireplace_flush_with_floor.JPG32.99 KB