Homefix: Dwight Barnett answers home-improvement questions. This week's topic is on a sagging floor.

Share story

Q: We are looking to buy a home built in the early 1950s, and I’m concerned about the sagging floors in the extra-large living room. The sales agent said the home has character, whatever that means, and sagging floors are normal for an older home. I had a friend who does some construction go under the house to look at the floors, and he said everything seemed fine. I just feel that I’m walking downhill every time I walk across those floors. Is this normal, or this a major a problem? Can something be done to raise the floors?

A: Just off the top of my head, I can think of four things that can cause a floor to sag:

1. Improperly sized floor joists. Normally, a floor joist is a 2-inch-wide-by-10-inch-thick piece of yellow pine or Douglas fir. Several other species of wood can be used, but these two are the most common.

2. The span of the floor joists may be too long. The span is the distance measured in inches from one supporting point to the next supporting point with no supports in between.

3. Decay or infestation damage can weaken the floor joists.

4. Improper loading. Loading would be the amount of weight applied to the floors when the home was occupied. For instance, a slate-bed pool table or a waterbed might be more weight than the floor was designed to support. Also, all lumber used for floor joists has what is known as an allowable amount of deflection or bending.

Wood is made up of fibrous materials that have an elastic characteristic, meaning the fibers will stretch when under tension. Testing of just how much “stretching”the wood will undergo is performed at the U.S. Forest Products Laboratory in Madison, Wis., and the findings are used in major code and engineering manuals to determine the size and type of wood to use for any given project. In layman’s terms, this means typical 2-by-10 floor joists, spanning 16 feet, can be expected to sag about one-eighth to one-quarter of an inch. Not much; however, as the span increases, so does the amount of deflection.

In any case, the floor system can be repaired. Damaged joists can be replaced. Joists that are too long or those that have sagged can be supported by simply adding an extra 4-by-4 wood beam with screw jacks resting on concrete footings located every 6 feet along the beam. The jacks are at first set tight against the joists and then raised approximately 1/8 inch every month to remove the sag. If, during the jacking, you notice cracks in the floor tiles or wall coverings, or if doors no longer close properly, then stop the jacking and leave the floor as is. The added beam will take all the bounce and movement out of the older floors, and you can add extra weight without fear of additional sagging.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at d.Barnett@insightbb.com. Sorry, no personal replies.