We are trying to put hardwood flooring in our living room, 450 sq ft. We removed current flooring to level out subfloor. The subfloor is composed of wooden planks with small gaps in between them. The house is about 50 years old and the subfloor has high and low spots. The deepest point is about 1 1/4" from the highest point which we will be leveling the floor from.
What is the best material to use to level the floor?
The lowest point is towards an edge that's about 15 ft in length.
Your advice is greatly appreciated.
The best 'material' is actually more of a method. You have uneven floor joists and/or support beams that need to be leveled out. This will require some jacking with a large hydraulic jack and shimming - possibly adding in some extra support columns and beams to carry the load on the places that need to be jacked up.
Once you get the floor framing level then you can skin over the existing planked subfloor with a layer of plywood. From there you can install your new floor.
You can take out minor dips in the floor by filling in the low spots with layers of roofing, felt or vinyl - or by using a floor leveling compound. Then cover over the repaired subfloor planks with plywood. While this method does work it's only for minor changes and cannot handle a 1-1/4" dip.
Based on your description of the subfloor, I suspect that the house is a little more than 50 years old. During that time it is likely that places have "settled." Jaybee is right about jacking up the support beams. When you do so you might put cracks in the wall, which may be plaster rather than drywall. You can minimize cracking by jacking up slowly, over a period of days.
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