2 sets of joists, 2 floors! How to replace top hardwood?

We tore up our carpet and padding, and in looking under the big cold air return grate, I noticed that we have two floors; not just 2 hardwood floors, I'm talking hardwood, then the wood subfloor, then joists, then under THAT is another "subfloor," or what is the ceiling in the basement, then more joists under that.
My vote is to sand and refinish the existing floor, but we may be getting brand new hardwood, which I believe means I need to tear up the old hardwood floor. I just wanted to see if anyone knows why I have these two complete floors, and if there's anything I need to be concerned about or be cautious of when potentially ripping up the old floor.
I have PICTURES and questions that relate to them. If the URL's below aren't links you can click on, just copy/paste the URL into your browser.
http://www.grillof.com/_images/AirReturn.jpg
The above is a picture looking into the cold air return. As you can see, the Sub Floor is coming in from the wall diagonally, running across the seams of the top hardwood floor. HOWEVER, look at the following picture in the SAME cold air return:
http://www.grillof.com/_images/StraightSub.jpg
In this picture, you see that the same sub floor that is diagonal pieces coming in from the wall is actually STRAIGHT pieces after only a couple inches in from the wall. The surface you see under the subflkoor is just some plank that's covering something, I don't know what yet. Look at the right of the pic; those pieces run diagonally, you can see from the seams between pieces. Now look at the top of the air return hole; those pieces run parallel with the top hardwood pieces! Does this indicate that the subfloor USED to be diagonal, but then someone cut it all up and replaced it with straight subfloor, leaving the diagonal pieces jutting out a little from the sides?
Now here's the view from the basement steps, looking back up towards the top; it shows the level of the floor upstairs, and then the joists that you can see in the basement (you can see from the distance between upstairs' floor and the ceiling joists in the basement that the basement joists are not the same ones seen through the cold air return upstairs):
http://www.grillof.com/_images/FloorLevels.jpg
Now, in the last pic, I'm looking up at the ceiling in the basement. You can see the cold air return duct on the left, the diagonal pieces that make the ceiling (NOT the subfloor of the upstairs), and the joists that can be seen from the basement:
http://www.grillof.com/_images/BasementCeiling.jpg
So, does anyone know why I have two sets of joists and subfloor, one on top of the other? I've been to look at 3 old houses around town, and they just have a floor, subfloor, and joists, and the joists are the same ones you see in the basement. In my house, I have a floor, subfloor, joists, then more floor, then more joists. The house was built in 1929, and is plaster walls/ceilings, if that helps anything.
So, to sum up:
1) Why do I see diagonal subfloor pieces that stop and are replaced with straight-running subfloor pieces upstairs? And does this add serious complications to the idea of tearing up the top hardwood floor?
2) Why do I have a "double" floor, like a set of two complete floor assemblies (subfloor and joists on top of subfloor and joists)?
3) Does anything described and seen in the pics mean that I'm in for a world of hurt in terms of tearing up hardwood and putting in new?
Thanks, everyone, for reading, for your input and thoughts, advice, cautions, etc.
Complete Newb
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Sorry, the pictures were not working, and I fixed them; here's the original post, w/ corrected pics:
We tore up our carpet and padding, and in looking under the big cold air return grate, I noticed that we have two floors; not just 2 hardwood floors, I'm talking hardwood, then the wood subfloor, then joists, then under THAT is another "subfloor," or what is the ceiling in the basement, then more joists under that.
My vote is to sand and refinish the existing floor, but we may be getting brand new hardwood, which I believe means I need to tear up the old hardwood floor. I just wanted to see if anyone knows why I have these two complete floors, and if there's anything I need to be concerned about or be cautious of when potentially ripping up the old floor.
I have PICTURES and questions that relate to them. If the URL's below aren't links you can click on, just copy/paste the URL into your browser.
http://www.grilloff.com/_images/AirReturn.jpg
The above is a picture looking into the cold air return. As you can see, the Sub Floor is coming in from the wall diagonally, running across the seams of the top hardwood floor. HOWEVER, look at the following picture in the SAME cold air return:
http://www.grilloff.com/_images/StraightSub.jpg
In this picture, you see that the same sub floor that is diagonal pieces coming in from the wall is actually STRAIGHT pieces after only a couple inches in from the wall. The surface you see under the subflkoor is just some plank that's covering something, I don't know what yet. Look at the right of the pic; those pieces run diagonally, you can see from the seams between pieces. Now look at the top of the air return hole; those pieces run parallel with the top hardwood pieces! Does this indicate that the subfloor USED to be diagonal, but then someone cut it all up and replaced it with straight subfloor, leaving the diagonal pieces jutting out a little from the sides?
Now here's the view from the basement steps, looking back up towards the top; it shows the level of the floor upstairs, and then the joists that you can see in the basement (you can see from the distance between upstairs' floor and the ceiling joists in the basement that the basement joists are not the same ones seen through the cold air return upstairs):
http://www.grilloff.com/_images/FloorLevels.jpg
Now, in the last pic, I'm looking up at the ceiling in the basement. You can see the cold air return duct on the left, the diagonal pieces that make the ceiling (NOT the subfloor of the upstairs), and the joists that can be seen from the basement:
http://www.grilloff.com/_images/BasementCeiling.jpg
So, does anyone know why I have two sets of joists and subfloor, one on top of the other? I've been to look at 3 old houses around town, and they just have a floor, subfloor, and joists, and the joists are the same ones you see in the basement. In my house, I have a floor, subfloor, joists, then more floor, then more joists. The house was built in 1929, and is plaster walls/ceilings, if that helps anything.
So, to sum up:
1) Why do I see diagonal subfloor pieces that stop and are replaced with straight-running subfloor pieces upstairs? And does this add serious complications to the idea of tearing up the top hardwood floor?
2) Why do I have a "double" floor, like a set of two complete floor assemblies (subfloor and joists on top of subfloor and joists)?
3) Does anything described and seen in the pics mean that I'm in for a world of hurt in terms of tearing up hardwood and putting in new?
Thanks, everyone, for reading, for your input and thoughts, advice, cautions, etc.
Complete Newb
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CompleteNewb wrote: ...

I'd vote for option A and why B?
...

No way to tell w/o being able to see the main portions of the structure, namely the foundation, additions, if any, etc., etc., ...
All we have are some basically cosmetic views from the outside but nothing from which to judge why it was deemed desirable to do what was done. Certainly, it would be only reasonable to assume there was a reason other than "just because".
One thought that is little more than conjecture given the age is that it might have been thought desirable to lower a 10-ft or higher first floor ceiling to cut down the heating load when the central heat was installed and it was done by raising the floor instead of lowering the ceilings.
As for the flooring, hard to imagine there would be anything major that would prevent pulling up what is there given the obvious solid construction from what is shown.
That said, again, can't see why one would choose to do that instead of refinishing what is there...
--


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(snip) My best guess, without an actual site survey, is that that way back when, the house had foundation problems and settled, leaving 'lumpy' or sloping floors. They faked it by overlaying another floor system, shimming as needed to present a level finish floor. Have you checked the bottom of the joists in the basement for level? Might not be apparent within a few feet, may need to use a laser or water level to check from wall to wall, wall to centerline beam, sags in the middle of long spans, etc. Another clue may be to pull the casing off some of the upper floor doors, and see if the header has been cut down, or the door opening otherwise altered. Usually pretty obvious if the doors are six inches shorter than normal. On an old house, hopefully the settling has stabilized, and it won't move any more, but just for grins, you may wanna have an engineer do an overall checkup before you sink any serious money in fixing up the place. I take it you didn't get an inspection before buying? Something like this should have jumped right out to a competent inspector.
And yes, the others are right- first preference is to refinish the existing wood floors. Probably be cheaper, and definitely have more character and appropriateness for the house, than anything you can buy now.
aem sends....
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I made a real bonehead mistake, and after some crawling around and prying things apart, I see I was wrong about the 2 floor assemblies; the ceiling in the basement IS the subfloor of the upstairs.
Sorry about that, everybody, but thanks for looking at the post and being willing to help.
Complete (and I mean COMPLETE) Newb
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CompleteNewb wrote:

It takes a big man to admit his mistakes. (And we all make mistakes sometimes...)
But, PUHLEEZE replace those torn vinyl or rubber stair treads before someone catches a toe or heel under one and takes a tumble...
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
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What's the dog's name?
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The dog is Francis, and she's a complete medical case. Every single movement and expression is absolutely adorable, though, so as practical as I am I still just love her to death.
Thanks again everybody
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On Tue, 10 Jul 2007 21:16:01 -0400, "CompleteNewb"

I bought a 50 year old house with floors about like that. I rented a big floor sander and sanded out all the bad stuff and then refinished it with a hard poly finish. It was pretty and everyone liked it.
I hated it. I wanted carpets.
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