Ever since the article, Sagging Floors - Serious flaw or charming old house character, appeared on OldHouseWeb, I regularly receive questions from readers about their own sloping floor issues. I can’t reply to all the e-mails I get and of course, I can’t offer specific advice without seeing all issues first hand. I can however, offer some insight. I’ll spare you my usual comparison of old buildings to aging body parts.
Long ago, when most of the buildings that I inspect were built, there were no formulas, tables, charts or calculations used to determine the size of beams or joists. It was based on experience, learned from their masters, who learned from their masters. In other words, the process of selecting and preparing lumber for specific structural applications was from hundreds of years of cumulative experience. From what I see regularly, it often seems the joists and beams are larger than necessary for the span and intended load.
Now you might be thinking - “If the old lumber is so strong, why does every floor in every old house sag in the middle like the old couch Uncle Morris sleeps on?”
A joist or beam needs to be strong enough to support a load. Strength, in this case, means that under a load, it will not bend to the point where the wood fibers are stressed beyond their elastic limit. In other words, it sags to a point but doesn’t crack, split or collapse. This bending is referred to as deflection. A joist or beam may be strong enough to support a load, but not stiff enough to avoid deflection.
By the time architects and engineers got involved with the design of small buildings and regular houses, there were formulas to determine how much joists and beams would deflect under loads. They also made the call on just how much deflection should be allowed under the load, so things like ceiling cracks wouldn’t be likely and bookcases won’t lean too far away from a wall.
So if your old floor structure has survived almost a century or more without fire damage, termite attack or a contractor with a reciprocating saw, and only has some sagging to the middle, remember that strength and stiffness are two different criterion.