Q: Our house is over 100 years old, and I store my collection of mystery books upstairs. My husband says their weight will bring the ceiling down on us. Is that true? How many books can the floor hold?

— Judie, Morrison, IL

A: Tom Silva replies: Well, it's true that if you put too much weight on any floor, it will collapse, but before such a catastrophe happened, you'd see alarming cracks and sags in the ceiling.

Answering your second question would be simple if your house had been built in the last 30 years or so. A contractor could just look at tables in a reference guide called Floor Joist Spans for Common Lumber Species to determine how much weight a floor could bear without deflecting beyond the limits allowed by code. But your old joists aren't the same dimension and rated strength that are factored into the tables. Another complication is that most old houses have been remodeled a number of times, increasing the chance that the walls and floor framing have been weakened.

If you really want to find out how much weight your floor can safely hold, contact a structural engineer who is willing to make a house call. He or she will assess your framing and, based on half a dozen variables, calculate your floor's safe load limit.

In the meantime, you can minimize the impact of your books by placing them along walls rather than in the middle of the floor, and by distributing them over a wide area. Also, keep in mind that weight spread across the joists is easier on a floor than weight that's parallel to the joists.
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