Hello guys, lucky I found here a lot handy guys sharing experience about hardwood flooring. I'm facing challenges to install hardwood floor around the fireplace. There are some irregular concrete areas around fireplace, with rough surface and higher level than the wood subfloor, as shown in the pic https://photos.app.goo.gl/FXeCHZKg1US8jRiN9.
Now I don't know how to deal with the this concrete irregular areas.

One option is to only install hardwood on the wood subfloor and leave these concrete areas as is, but this may look not nice. My basic idea is to seperate the concrete parts and the subfloor parts and use reducers to connect them. Have you guys experienced these kind of installation? Any help and suggestion would be much appreciated!

Is there 16 inches between the opening of the fireplace screen and the edge of the hearth? If the hearth is not wide enough at the opening and you want to keep the brass screen, you will not be able to safely install a wood floor. The building code requires 16 inches, I believe. This may be the reason for more concrete next to the sub-floor directly in front of the fireplace opening.
For floor installation, you will either need to shim the sub-floor with another layer of underlayment or grind the concrete and chip it down so glue could hold where no nails are possible.

Thank you for the help. I just checked the distance between fireplace screen and the edge, and it is less than 16 inch so you might be right that’s why the builder kept some concrete areas around.

You suggested to glue down the hardwood floor on the concrete areas, which I agree and plan to do so. But did you mean to separate the concrete and subfloor areas with reducer to connect them, or flooring like a whole area. My concern here if I floor these areas together without gaps the concrete parts will make squeaks when I step on the wood subfloor areas since they are connected. Please advise, Thank you.

Double check the distance for combustibles from fireplace opening. From what I just read it is 6 Inches. You could verify with your town building inspector.
If 6 inches is correct then I would grind/chip/fill and level the concrete. Next question.....what is your plan for the flooring abutting the hearth e.g. flow under or picture frame?

Thank you. I just checked my local building code, the clearance distance is 150mm. After grinding and leveling, would you suggest to install hardwood floor on top of the concrete and wood subfloor together or separately? My concern is the concrete parts will make squeaks if they are installed jointly. And any suggestion for the tools to grind the concrete. If I can install them together I would prefer undercut otherwise I will just glue flooring along the edge since they are separated. Thank you.

Angle grinder with a masonry wheel and a masonry chisel. It will be dusty so wear a mask and try and use a vacuum. How do you want the finished floor to look? Check the internet to see what the floor will look like if it flows under the edge of the brick work vs picture framing the brick work......this is personal preference. If you like the flow under look you can rent an undercut grinding tool......too expensive to buy. If you want one homogenous floor you can nail to the wood and glue to the concrete.....or if you have the time you could use masonry screws in the concrete area. If the floor is firmly attached it should not squeak.

I got it. I will check the local rental stores for the tools to grind and undercut. Thank you very much.

Just to clarify. You can find a relatively inexpensive angle grinders at Harbor Freight......might be cheaper to buy than rent.
Undercut saws are expensive......better to rent than buy.....some Home Depot stores rent tools and should have one.
Undercutting with an angle grinder is doable but tricky and you risk making a poor uneven undercut.

Thank you for the clarification. I have a multifunction oscillating saw (Mastercraft 2.2A Multi-Crafter Kit), can I use it to do the undercut if I change the blade for the concrete? Another question, to grind the concrete floor, may I use the angle grinder? I thought it was only for undercut, but based on your clarification, seems like undercut saw is particularly for undercut. Thanks.

Angle grinder to grind down the concrete floor....use masonry blades. There is a special blade, that is used with an angle grinder, which is sized to grind out the mortar when brickwork joints are repaired (repointed).....this blade will also cut brick with ease (like butter).....you could undercut the brick with this but it is tricky because you will be holding it at an angle and a straight even cut will be difficult unless you have a special attachment or a very steady hand.
The oscillating multitool is useable for grinding or undercutting but is an extremely sloooooooow process if you have a job the size of yours.

Thank you. Another question, for the glue between concrete and hardwood, do you have any suggestion? I checked PL400, but it is not as healthy as Lepage wood glue. Since this will be used inside the room, so the healthier the better. Otherwise I may choose speical screws. Thank you.

Is the room built on a slab or is there a crawl space or basement below it?

There is a finished basement below it, and that room has a corner shaped similar to the upstair one where fireplace located, as shown herehttps://photos.app.goo.gl/1mPfDVezBGTRjaer9

Thank you.

What type of hardwood flooring will you be installing? Solid or engineered? Species? Width?

Solid hardwood, 3/4",3 1/4", thank you.