Evidence of a Sagging Floor

By Jeff Trovato, Engineering Assistant | January 30, 2021

A common concern we receive from owners of older homes is a sagging/sinking floor. Sagging floors can occur for several reasons and might be scary for homeowners, but a qualified engineer knows exactly what signs to look for to determine how to fix it.

All structures settle over time (especially in the first few years) and may eventually become noticeable or even dangerous if not properly built in the first place. The good news is that since most residential homes are made from wood, floors very rarely catastrophically fail. Instead, they continue to sag and bend and deform before breaking, if ever.

In general, there are many signs of sagging floors. While conducting a recent inspection, I noticed two different types of wood compression:

  1. Compression perpendicular to the wood grain
  2. Compression parallel to the wood grain

Perpendicular compression occurs when forces press against a member at a 90° degree relative to its length and grain. Parallel compression occurs when forces press against a member along its length (with the grain).

A wooden member has greater compression resistance against forces parallel to its length than it does for perpendicular forces—the wood is “stronger” along the grain. In Photo 1, the floor joist is experiencing perpendicular compression and the vertical post is experiencing parallel compression. The floor joist is noticeably deformed, and the vertical post is not deformed.

Noticeable parallel compression is less common because wood is stronger in this orientation. Photo 2 is obviously a unique situation because the “post” is a log. Photo 2 shows the failure of a vertical post due to parallel compression.

The floor being supported in Photos 1 and 2 has sagged about 1 inch in the middle. This can be avoided by consulting an engineer who can calculate the necessary size of floor framing components. In general, a thicker beam has greater compression resistance.

In the example from Photo 1, we can properly support the floor by reinforcing the floor joist with a board on either side of the existing joist, by adding a plate between the post and the joist to distribute the forces over more surface area, and by fastening both ends of the post to prevent leaning and twisting.

The log from Photo 2 may be an economical solution, but it is not safe to use uncertified wood members in the structure of your home. Instead, a standard post and concrete footing should be sized to support the floor in that location.

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