Joist sistering, or replacement

I have a house that was built in 1936, and before anyone asks no I do not have carpet over hardwood floors. I was hoping someone could enlighten me on what my realistic expectations as a homeowner who isn't afraid to knock out walls and ceilings himself.

So the house was pretty simple in shape, then was added on to. The face has gone through so many face lifts internally that it resembles something more from the 50's than the 30's.

Here is the yuck part, the joists for the original part of the house are all (and this is a rough measurement since the lumber doesn't conform to the usual dimensional lumber standards) about 2x6. I want to say they are spaced about 16-18" apart, but I am at work and can't measure at the moment.

No beams show rot, but the subfloor looks like 3/4 thick planks running across the floor. If I were to guess, there is probably a layer of osb hammered down to the top of it to help fight deflection.

Now, as for my thoughts and wants. I'd like to either sister up the joists with something considerably larger, or find a way to put in new ones. Once that is done, I would go ahead and replace the sub floor with something more suitable for preventing floor deflection.

A great deal of this is motivated by the cracked tiles in my bathroom and the inability to fit 6-1/2" ducts between the main support and the joists (it's a scary octopus).

Am I just asking for a nightmare? Is any of this in the realm of a DIY person? I do plan on consulting a professional, but I would rather have a realistic set of expectations so I don't waste their time.

 

First of all what do you have for a foundation walls? I assume it is a stone and morter foundation. Is there a support wall or beam running through the center of the basement? If there isn't either of these and the existing joists extend over the foundation walls you can sister the joists with 2x8 or 2x10 and cut out the corner at the end of each joist to extend over the wall to match the existing joist. Then you should use 2 10d (minimum) nails every 16 inches all the way through the joist. Normally on these older homes the subflooring was 1x6 board installed diagonally across the joist. If this is the case, once you sister the joist with larger ones the deflection should go away. The cause for the current deflection is because of the 2x6 joist not being large enough to carry the weight. You shouldn't need to add any more subflooring to the mix or worry about the tile any longer.