Floors - how do they work?

August 26, 2004 8:37 AM

How are floors held up in a typical house? Such as, for instance, a Dutch colonial? How much weight can they support?
posted by adampsyche to Home & Garden (9 answers total)

I remember doing some research about this when I wanted to figure out if a dorm room could hold a hot tub. In the US, it depends hugely on how old the building is.

New-er-ish houses use these long joists that are like wooden i-beams made of special plywood. They have an acronym-type name like TSX or something. Occasionally, a long span in a house will necessitate installation of an actual steel ibeam. If you don't have a finished basement, usually you can go down there and look at what holds up the first floor.

The general case though in the US is that the minimum floor loading is spec'd in the building code. There are two stats they quote: 'live load' and 'dead load'. Dead load is static, live load has something to do with if the weight can move around. If you know your house was built in say, rhode island in 1996, you can check the building code for what it should support. In general, you have to pay to get a copy of the building code, but many US states use the same building code (like many eastern seaboard states use the NJ one) so even if your .state.us page tells you you have to order the book, if it's a common code sometimes you can find it somewhere else online.

I don't remember what the numbers were, but I remember that it was 'a whole lot' for new construction. A hot tub, which is really heavy, posed no problem. Also, older buildings sometimes weren't built to any code or were just under-spec'd in general. This is true of a lot of loft buildings and barns (they say the floors are 'overspanned', a lot of times if you have a springy feeling floor in an old building this is why).
posted by jeb at 9:08 AM on August 26, 2004

Response by poster: It's a house that was built in the 1910s, I think. Maybe a a few decades later.

I have an I beam that spans the basement, but what about the second floor? Just the walls?
posted by adampsyche at 9:16 AM on August 26, 2004

Yes, just the walls and then only the load bearing walls. The exterior walls are load bearing. Some interior walls may not be.

What is is that you want to put on the floor? Bookshelves can be a problem and should usually be placed up against a load bearing wall.
posted by caddis at 9:18 AM on August 26, 2004

Response by poster: It's nothing really, it's just that I have a hard time in my head reconciling the fact that a house can support the things that inside, such as beds, etc. when there isn't anything to support underneath, and rely on the walls.

And I do realize how phobic I sound. Fun!
posted by adampsyche at 9:32 AM on August 26, 2004

This is going to depend on the idiosyncrasies of your house. Not other houses that are like yours, but your house and only your house.

Note: if your house was built in 1910, then the building codes are useless, as they would have been erratically applied, if at all.

...but if you're just interested in the theory of it all, read this book: Why Buildings Stand Up
posted by aramaic at 9:46 AM on August 26, 2004

HowStuffWorks has a bit, with good pictures.
posted by Galvatron at 9:51 AM on August 26, 2004

Most floors are designed to hold at least 40 pounds per square foot; many can hold more, even if the design limit suggests otherwise. Much depends on what one considers to be acceptable deflection.
posted by Dick Paris at 10:00 AM on August 26, 2004

Response by poster: Hey, thanks for the links! Goolge wasn't very helpful earlier.
posted by adampsyche at 10:10 AM on August 26, 2004

If you are living in a Dutch Colonial from 1910 your house is probably of post and beam construction. If true the walls them selves do little but keep the wind out (and provide shear resistance in some designs).

A series of vertical posts, commonly 4 rows but could be 2 or 6-8, hold up beams that the floor and ceiling/roof attaches to. The advantage of the design is heavy loading capabilites with large open spaces. It is materially inefficent compared to stick framing methods in the same way that body on frame automobiles use more steel than unitbodies.

If you are considing a hot tub or library on your second floor it would be a good idea to consult with a knowledgable contractor or engineer wether your house is 10 or 100 years old. Even a recently built house covered by codes could have substandard construction or previous owner modification.
posted by Mitheral at 1:43 PM on August 26, 2004