I am installing 3/4 x 4 prefinished Brazilian Cherry floor, which needs minimal 1/2 expansion gap. I like the finish of picture framed hearth shown in the pictures below. My question is: where to have the expansion gap?

Just leave the expansion gap around the perimeter of the room. The fireplace will not be an issue. We do them like this all the time.

Thanks, RhodesHardwood.
That's a good news. I still have more concerns. The exterior walls, which the fireplace is on and it is facing directly, are about 35 feet apart. Please see the floorplan below, the rectangle represents the fireplace. Do you think it is wise to lay 4" Bralizian Cherry horizontally (joists run vertically) over this 35 feet span without any tranistion molding? If I choose to picture frame the hearth without expansion gap, will it lead to inadequate room for the woodfloor to expand during summer time, since partially it only get 3/4" expansion at one end? Humidity varies from 30% to 90% where I live. And now it is winter, the wood shrinks to its mininum. It is suggested to nail every 8-10". Should I put in more nails like every 6" to restrain the expansion?
Am I overthinking?
Please Advise. Thanks a lot!

If you are installing 3/4 x 4" prefinished Brazilian Cherry floor best results can be accomplished by nailing and gluing down your hardwood flooring. You need to buy some hardwood adhesive since you are installing a 4" hardwood flooring.

You really need to know the m/c of the flooring and the m/c of the subfloor to give you a more educated answer. Adding more nails wont stop that floor from moving might slow it down some but it wont stop it completely. The r/h you keep hearing about is normally talking about interior levlels, outdoor only comes into play in a uncontrolled crawl space.

also floors will expand more toward the side of least resisitance witch is the tounge side. Its hard for the boards to move back due to the 45% angle of the fastners they move quite a bit more freely the other direction so the best way if you have a concern is to start in the middle/ spline the starter row and work both directions effectively diminishing the long run in half.

Thanks for your reply. One guy recommended "nail and glue". However, when I got the floor from BR111, it only says nailing in the instruction. I am a novice so I tried to follow the manual 100%. But I believe "nail and glue" would work better in this case. I have already finished the second floor with nailing only. Here I have a question for you. I used #15 felt paper as underlayment. If I use glue for my first floor, where will it go, between the felt and woodfloor or not using felt?
I will use "nail and glue" when I do the staircase. Thanks in advance.

Hi Kevin,
Thanks for your response. What does m/c stand for in this field?

Hi Kevin,
I thought about starting in the middle to cut the length in half. However, I don't trust my measurement that I can install 10 feet and get exactly full board width when reaching the hearth. My current plan is to start the first row from the edge of hearth and use spline to reverse the direction for the area between the first row and the closest wall. In this way the groove side touching the hearth. I hope floor won't expand much towards groove side. Then the plan is viable. However, since I am doing such a long run, I am fraid 3/4" expansion is not enough. BR111 actually only recommends 1/2" for expansion gap. I have been using 3/4" to be safe since I am installing in the winter.
If you think it is still too risky. I can use T-molding to frame the hearth and leave expansion gap. But I really like the look of picture-framed hearth.
Hope you can help.

MC = Moisture Content.

It is tested using a pin-type wood moisture meter.

I see. I don't have moisture tester. Old carpet has been removed more than a month ago. And woodfloor was brought in to acclimate since then. Heating is on all the time. So, is it still necessary to check the moisture difference between subfloor and woodfloor?
By the way, I have a walkout basement under first floor, I think the subfloor should be quite dry.
How does M/C matter in picture-framing a hearth?
Thanks!

Oh, a walk out basement. This should be fun. I'm pretty sure but I could be wrong but that guy standing at the hearth did not install that floor!

You're probably in good shape with the MC, but if you install without checking, and the floor starts cupping later down the road, you'll regret it.
If the wood floor is installed on a subfloor with a high MC, then it's definitely going to expand, how much depends on the difference between the two woods mc. If the subfloor mc is low, then your flooring will shrink and leave gaps.
Without checking, you're guessing. Not recommended.

I'm thinking about acclimation with the heater running and drying the interior way down.
Come summertime, it may want to expand a lot more than expected.

acclimating with the heat can be pretty dangerous endeavor if you dont understand what you are trying to accomplish. The wood comes out of the mill 6-9 percent and depending on where you live and the construction of your home that could be perfect, or it could be way too low or way too high. A floor that is already dried to say 7% then you bring it on the job for 3-4 weeks with the heat running. The heat will continue to dry the floor out even more dropping the m/c even more down to maybe 5% thus shrinking the floor just slightly. So you happy, its sat there for x number of weeks and install the floor tight together everything looks good and life goes on. Next summer you notice your floor is a little wavy and the edges are higher than the middle? whats wrong? I let the floor acclimate like the instructions say? What most manufactures dont put in the instructions is that you should acclimate to a year round average/ median temp and humidity not the current conditions.