Over time, it's normal for old floors to sag a bit. If your floors become crooked, saggy or wavy rapidly, it may be a sign of a serious structural issue and you should have it evaluated by a professional immediately. But if it happened over the course of decades, it's normal and to be expected.
How To Level Your Subfloor
Leveling a subfloor using 2x6s to sister the original joists can be a challenging task, but with the right tools and materials, it can be done.
The goal is to find the highest part of your floor (usually along an outside wall, a supporting wall or over a beam), and match the lowest parts (where it sags) to the highest point.
Here are the steps to follow:
Measure the sag in your subfloor and determine the amount of lift you need to level it.
Cut your 2x6 lumber to the length of the joists that you are going to sister.
Apply construction adhesive to the surface of the existing joist that you will be sistering to.
Place the 2x6 lumber abreast of the existing joist, making sure to check the level. Use a framing nailer to secure the lumber to the existing joist with 3-inch framing nails. Be sure to nail every 12 inches along the length of the board.
Continue the process along the length of the joist until you have sistered all of the joists that need to be leveled.
Once you have finished sistering all of the joists, use a level to check the subfloor for levelness. If you notice any high spots, sand them down until the subfloor is level.
Once the subfloor is level, install a new layer of subflooring over the 2x6s. Be sure to use screws instead of nails to secure the subflooring to the joists for a more secure and stable attachment.
By following these steps, you can level a subfloor using 2x6s to sister the original joists that have sagged. It is important to take your time and work carefully to ensure that the subfloor is level and secure.
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