The correct way would be to either put in enough joist to support that big of a live load or more likely you should probably just have a beam going crossways below the fireplace with some posts or even a 2x6 wall underneath the fireplace. Ofcourse this isn't ideal if it's a finished basement, that might be right in the middle of a living room, but me personally I would get some wood between that fireplace and the cement floor. Your right to be worried, it's going to be cheaper to fix it now

Edit: you'll probably need to cut out the cement floor and put in some real footings too honestly, god knows what they have for a floor and I bet it doesn't have rebar. Even spread across a long 2x6 wall would be pushing your luck with say a 2000lb+ fireplace. I highly doubt your going to get a joist to hold that with any span over say 10ft

I'de go with option #2 or #3.

I'de go with option #2 or #3.

Install two temporary adjustable Lally columns near the middle of the beam to level it while you work. Drop a plumb bob from the center of the beam to the floor and mark it, then make that the center of a 12” square to cut out of the floor. Dig down to whatever the frost line is where you live (say, 4 feet). You can calculate how much concrete mix you should need, but I always buy more than I think the job will take, concrete mix has no expiration date. Pour about 12” into the hole, using the sides as a form (if you want to install steel reinforcement, bend it so it extends up 2 feet above this). Once that has set drop an 8” Sonotube onto it and fill it with concrete to the level of the floor. 72 hours later, level the joist with adjustable columns. Go slowly and check for cracks in the masonry above. Measure the distance from the beam to the new footing, taking base and top plates into consideration and cut a cement filled permenant Lally column to fit. I cut the pipe with a grinder and break it with a hammer. Raise the joist 1/4” or so to fit the new column, plumb it from 12 o’clock and 9 o’clock and lower the beam onto it. When you remove the temporary posts the beam should be level and solid. If the crack is bad, I would sister an LVL to it with 3/8” galvanized through bolts and washers. Next step, build a fire, crack a beer, and put on the game.

Here’s some pics. The first one shows under the fireplace from the side. The right hand is the front of the fireplace and you can see an area of concrete or mortar which is the fireplace floor. The red arrow is pointing at the crack.

https://imgur.com/a/Wa3akM8

The second is a close up of the crack.

https://i.imgur.com/5czNq1G.jpg

Jesus, I would get some poll jacks under that asap. Dont try to lift it, just hold it in place. The floor probably won't handle the entire load. I think your going to need some professional help with this one I think. I mean if you really love that fire place I would be tempted to just anchor it in place where it is with 3 6x6s and some nice big wide fat footings. You may also want to consider just getting rid of the fireplace. They cause so many issues, next you'll be redoing the roof because its leaking. But that's not cheap either. Trying to lift those things sketchy, but it's also super sketchy now and it would be all around better if it was entirely vertical again. Hard to say what's best

I wish you were my neighbor and/or dad.

He could be your dad.

You never know.

Is it necessary to dig to the frost line if the building is heated? I ask because I have little floor jacks under my kitchen that I presume the renovators put in to help support the tile and granite that was installed. This is on a crawl so it gets cold below the house but it doesn't freeze or else my pipes wouldn't be very happy

Did your home inspector not catch it??

Bags of concrete absolutely have an expiration date. If you live anywhere humid they will absorb moisture and will set up in the bag. Never plan on storing bags of concrete or mortar long term

You don’t need to dig below any frost line. Your basement already does that. You’d cut out the floor below your post because you can’t trust the concrete floor. Who knows how thick it is and how well it was done

that's a gonner . seems unsafe

I second /u/Spinaccio's approach for putting a post in. Or posts - I'm having some trouble envisioning how exactly the fireplace is supported by that (where is the ash pit?)

Another option to explore with an engineer... In our house (1931), I've got full access to the exterior basement wall "top plate" (concrete) and the interior wall top plate. If I had this problem under my fireplace, I would just raise the floor a bit temporarily with a jack and slide in at least two brand new joists. I don't know that you would want to sister these to the old joists, though.

But as for your dip... really depends. I've noticed a lot of uneven spots in my floors, but no damage to the joists underneath. What I suspect is happening is 100 years of uneven wear and settling and compression in the softer wood subfloor. If it moves, you could also have the classic subfloor board that ends just shy of a joist.

[removed]

I’m guessing that it’s the hearth in front of the flue that’s the problem. The chimney itself would never sit on a wooden floor.

Good advice. Did the same thing with sagging joists in my 1850 house. Still rock solid after forty years.

Sounds good to me. I'm also reminded of advise i received on my saggy old house - it took 100 years to get like that. Best to lift it slowly to level. And expect some cracking noises.

A home inspector’s only responsibility is to cash your check. Never trust them to find anything.

Steel telepost under the floor to crank it back to place.

Steel plate across the Crack.

My crawl space is just a dirt floor and only the foundation wall footings are below frost line. The dirt in the middle isn't dug as deep.

Quickcrete is like $8 a bag. If your really worried about it going bad id just wrap it in a plastic bag.

I'de go with option #2 or #3.

###Please read this entire message in length before messaging the moderators or you will not receive a response.
 
Hi willxthexthrill,

Your post has been automatically flagged for removal because it appears you are asking:

• for brainstorming ideas.

• how to "get started" on a project.

• what is the best method or approach to a project.

• whether or not you should do a project.

• if anyone has used a product, or for a product recommendation.

While we do allow questions in /r/DIY, they must relate to a specific aspect of a project, include the research you've done to find an answer yourself and explain why that research didn't answer your question.

If you have any of these questions, some places to get help are:

• The Getting Started thread at the top of the subreddit.

• Our Discord server in the appropriate channel.

• You may also try other subreddits - /r/homeimprovement, /r/woodworking, /r/electronics or /r/findareddit to help you find a subreddit applicable to your question.

Please read our help request Full Sub Rules before resubmitting.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

Yeah, I was thinking maybe it was a large hearth that had wings to the side. I don't know that the hearthstone on the floor would cause that sag though.