I understand that this may very well have to do with age, etc., but the floor in a few old houses that I have seen (from the forties, maybe?) is bowed upward or otherwise warped in large contrast to houses built in the seventies to the present.
Was floor warpage, to an extent, tolerated fifty or more years ago?
No.
There are several possible causes for a warped floor (seismic activity, foundation settling, dryrot, others), but they were not built that way, nor was the condition considered acceptable (at least among those who had a choice re. their floors.
If you are considering pruchasing a house with noticable warpage, first find out why it’s warped - a failed joist is not something you want to learn of the hard way.
Not age per se, Joe K, but usually movement of structural members over time. In rare cases, seismic activity may cause something to heave upward, but in my experience the movement generally obeys the law of gravity – down. Barring the extremely unusual, a floor that appears to be “bowed upward” isn’t – something else has moved down, giving it that appearance. For example, an outside bearing wall may have settled for some reason, but the support under the center of the floor hasn’t, which will lend that “bowed upward” appearance.
Bad floors can come from a lot of reason.
Around here it is mostly water damage from flooding or long-standing roof leaks.
Sometimes of course it may be bad materials- lumber not dried enough in manufacture…
Other times it’s in bad planning of the site. If a foundation is laid on loose fill then it can erode every winter freeze and thaw, lowering the sides but not the center of the floor.
The warping/bowing tendency of timber can be quite reliably predicted by examining the grain and endgrain; it could be that the carpenters constructing the floor set the timbers in such a way that they would bow in opposition to the load forces (in the hope that everything would cancel out).
A word of advice:
Do not ever, and I repeat: EVER – buy a house with warped/bowed floors. They cannot be fixed. Jacking up the floors just covers up the problem. Sometimes you can rip the floors up, but you take the chance of the walls collapsing. Just say no, man.