In the ground floor of my house I have three rooms which were connected with hardwoods across all three. The foyer had 3" oak which was installed with the house in '94. I had Acacia put into the two rooms which connect on opposite sides of the foyer. When it was installed they were put adjoining in the door ways, no thresholds. The doors are opposite one another so it flows acacia to oak to acacia. The board directions are such that the oak is perpendicular to the other rooms.

I'm in the process of ripping out the oak and putting down new flooring, which is 5" hickory. I planned to do a similar install where it is continuous flooring between the three rooms. I'd prefer not to do any thresholds. Can I do a similar sort of install? Should I plan to leave a hairline gap between the rooms to allow for some movement?

I wish I could post a picture on here to better explain what I' m looking at.

As a follow up... the original oak was installed so it butted up against the front door frame. At the bottom of the door is a strip of oak. I've undercut all of the walls and trim around the rest of the area to give me expansion room. I'm not sure undercutting this oak piece under the door would be too easy. Can I do like the original install and have that one portion of the floor be touching the door frame (no expansion room) or should I plan a small gap and need to add a covering like quarter round?

Hickory does expand and contract more over different seasons than oak. It would not hurt to leave a hairline gap between the two different wood.
Wood flooring does not need an expansion gap at the ends of the boards. It does not expand and contract lengthwise. You can lay the flooring right up to the door trim.
Quarter round should not be used for floor trim. Use base shoe to fill a gap under the base board if it is not scribe fit to the floor.

Thank you very much for the feedback. It is very useful!

A follow up on the room transitions... The acacia I will be mating to has the groove side showing on both transitions. So I can easily tie the tongue on one side. On the other side (where I need to cut boards to length) would it be worth using a table saw to make a tongue to slip into the groove or make a groove in the new piece and use spline to merge them together? Alternatively I could cut them flat and just put them next to each other. That is what the previous installer did.

Any way to post a picture or a diagram? Undercut door trim and slide flooring under if you can. An oscillating multi tool works well. You can an inexpensive one at HF for about $15. The HF blades aren't very good and good blades are pricey.....get Bosch blades. Butting boards without a tongue and groove will create a 'weak joint' and could be a squeak point. Whether to create a tongue or groove on a board depends on what the board that will abut this board will require. I'm a little confused about what the situation is. You can do either the tongue or groove on a table saw......test with scrap to dial in your fit. You will have to make or buy spline. Do you have a router table? There are tongue and groove flooring bits available or you can get a fairly inexpensive slot cutter to make a groove.

As I am re-reading, perhaps I misunderstood which direction you were saying doesn't need a gap... I'll try to figure out how to get a picture or outline.

https://youtu.be/9vDZ9RJiosk

Here is a quick pan of the area.
The acacia on both sides is installed left to right (as you look at it). The tongue side of the board edges are facing the area that does not have flooring now. The small piece of oak that you see I just screwed down as a threshold so we didn't damage the boards before putting in the new wood.

On the right is an exterior door.

On the left is a stair well going up and a short hall way.

The joists are going front to back as you look at the space (between the two rooms). I was planning to install the new floor front to back as well. In this case the long side of the board would be running parallel to the exterior door. I'm installing 5" hickory solid and I was planning to do combination staples and adhesive (because of the width of the boards).

I hope this makes it clearer...

Yes, so the new flooring will be run perpendicular to the flooring in the 2 rooms off the front hall. In that case you will be doing what is referred to as a 'net fit' between the 2 rooms. Depending on the direction you will be laying the floor i.e. starting at the front door or starting in the short hall add a spline/slip tongue to the room doorway board that will be receiving the groove ends of the perpendicular boards. I find it easiest to cut floor boards to size and fit them somewhere in the middle of the floor rather than where they abut the perpendicular room doorway board. It's easier to get very accurate measurements in the middle of the floor. The 'net fit' boards i.e. those that will go in the field of the floor will have to be cut to size and have either a tongue or a groove added to the end of the board. On a table saw you would have to hold the board on end to cut a groove or hold it flat to cut a tongue. You could install it without a new tongue or groove on the end but this could be a squeak point. To do this technique, once you get to the 2 rooms lay the new flooring from both directions. Pick lengths so that will have a decent length board to infill to complete the row joints remain staggered. To measure the infill board, take another board and lay it against the current row and align its end with the end of the board in the current row. Take a long infill board and abut its end to the board in the next row so that it spans the infill gap in the current row. Take a straight edge and lay it on the edge of the end of the current row board on the other end of the infill gap. The straight edge should be perpendicular to the current row boards and extend across the long infill board. Mark the cut point on the long infill board.....this will give you the exact length the infill boar needs to be. If you need a groove on the end just groove it and you are all set. If you need a tongue you can either make it longer and cut a tongue or add a groove and glue in a spline to create the tongue. This 'net fitting' will take some extra time as you will sneak up on the exact length. When you install 'net fit' boards push them straight in...you can use a longer piece of flooring to move them straight.

JIMMIEM - Thank you for the suggestion. I'll plan to do it that way. It will be time consuming, but hopefully better in the long run.

My plan was to start at the exterior door and start laying out from there. It sounds like I'm going to have to find a way to undercut the strip of oak at the bottom. I'm not sure how thick it is. Alternatively I could use base shoe it sounds like.

Which strip of oak are you referring to? Also, don't forget to do your measuring and baseline layout before you start installing the floor so that you don't wind up having to rip down floor boards to fit around obstacles, e.g. the stairs. Also, get things square so that the new flooring will meet the flooring in the doorway at 90 degrees.

There is a strip of oak integrated into the exterior door. It appears to be part of the threshold. The wood is under the door and the metal plate is on the outside. It's a little more than an inch high by maybe an inch and a half in width. It has three screws in the top. I haven't tried pulling the screws yet but from first glance I'm not sure the strip is going to come out easily. It might be glued/attached to other items.

Good point about squaring things up.

I had previously measured to make sure that I was ok with length to the other end of the hallway. I'll check the stairs as well.

Are the three screws located right under the door itself and recessed? If so they are there to adjust the threshold. Is the height of the oak strip at the same height as the new flooring?

Yes they are directly under the door.

The heights are different. The strip seems like it might be too pieces, a larger base then a thinner top. The larger piece is ~1.25" thick. When you add the smaller the total height is ~1.5". I'm going to be using 0.75" solid flooring.

No problem butting the flooring right to the threshold. It will save undercutting the threshold.

So the threshold is 3/4" higher than the height of the finished floor? If so, and if you want to leave an expansion gap then you can cover the gap with a piece of the finished flooring wood and put a 15 degree bevel on the edge so that it ramps down onto the finished floor.

Pete - I thought I needed gaps on all sides, especially the long edge. Is that not the case? If I don't put one in for the door what should I expect to encounter, buckling?