Subfloor not flat

[mike4244]

I have been under checking it out, no issues in rot or block giving way or falling apart, what I see is
2x10 12'long on 16 inch centers for floor joist, what I think happened when the house was built 30 years ago, somebody was putting joist crown up and then every so often put one crown down, that created my high spots and my dips. I am going to do some serious sanding on the floor in the high areas which are not that many or that large of a area , then use some shingles laid out under the new flooring to build up a little where needed and see how this works.
God Bless and have a great day

Thin 2" wide strips installed with subfloor adhesives in the low spots. Hand nail a few 1"brad nails to hold the strips in place, leave the pins proud for removing later. 
Next day remove the brad nails. Make a carrier for a router that spans two or more strips. Screw a narrow board or even an angle to carry the router carrier. Set the depth on router for height needed. The boards or angles will be higher than the new strips. When the correct height is set you can traverse the router and you will have a flat in plane surface for your new flooring. I have done this twice before. First time I used yellow glue , worked fine. Second time glue was not practical because I could not get the plywood sheathing clean enough. I tried one small area, came back about 5 hours later and attempted to rout the strips. Knocked them loose, went and bought subfloor adhesive and applied it to same area. Next morning I easily leveled the strips. Did the rest of the floors same way with no problems. The subfloor went uncovered for many years as this was a cabin.Lots of dirt ingrained in the sheathing but the subfloor adhesive did the job.

mike

[Cooler]

             Shingles wouldn't be my choice of shim but anyway...  Assuming everything it basically sound and just sagging over time due to crown etc then yeah you can fill those areas with thin plywood or thin rips of wood then glue and screw it down to make up the majority of the volume up then floor leveler over it. There is a primer to use on wood before pouring floor leveler. 

       Our house was built in 1960 with recycled lumber from a really old house. Very heavy, hard and lots of growth rings and cut on a circular saw mill and most of it not straight or flat... ( I think it's fir) The floor joists are mostly 2x8 on 7' centers on poured concrete pillars and concrete perimiter wall (the only thing they got right on the house). There are a few spots where they put crowns upside down or the board was a little too narrow and resulted in a dip...  So yeah I have done allot of shimming and filling myself. I hate that old lumber but it sure is extremely strong stuff compared to the balsa wood we have today.

The builder of the homes in my area built the house I live in for himself.  The joists in the basement are all 3" x 8".  I don't know the species, but it is way harder than the Douglas fir lumber I am familiar with.  Nails bend, screw heads snap off.  Tough stuff, now a dark brown/red color from age.  But no floor sagging.

[Robert Adams]

The builder of the homes in my area built the house I live in for himself.  The joists in the basement are all 3" x 8".  I don't know the species, but it is way harder than the Douglas fir lumber I am familiar with.  Nails bend, screw heads snap off.  Tough stuff, now a dark brown/red color from age.  But no floor sagging.

   Yeah same here. Here is a pic of some of the lumber I removed during wall moving for the master bedroom and bath. I will not be reusing those studs in the house for walls again. They are already on their second life and not straight enough for a third and a royal pain to work with. I'm not sure what they are but they are very heavy and driving screws in with my milwaukee 18v impact driver is a challenge for it. Not to mention the nails used in the late 50s were very poor quality as the heads pop off with relative ease. 

          I haven't decided what to do with the left over lumber yet. Maybe a storm shelter... If I could get all the nails out plane them down and glue them up it could make a heck of a strong bench top.

        Here is a pic of the lumber and one end grain piece. Some of them have more rings than that one as well.