Home & Garden Columns

About the House: Why My Floors Are Sloped

By Matt Cantor
Friday March 07, 2008

I live in a slide zone. As I understand it, the land my house is bobbing about on is a colloid of tumbled rock and Cuisinarted soil, the remains of an avalanche, hundreds of years now past. Since this material isn’t “consolidated” or compressed by time into a hard cake, it tends to amble downhill as gravity would have it. (I’m turning 50 and, as my friend Joann would say, my local gravity is also increasing so I know how the house feels). 

Although most homes are not located in slide zones, there are still forces that move soils around on many a lot and yours is very likely included. 

Gravity plays a vital role in all of these situations, soils types in many and water in most as well. Although I’ll briefly discuss the dynamics of this movement in the following paragraphs, what I mostly want to do is to talk about the import of the resulting deviations.  

As houses “settle” (a troublesome term because it says so much less than it should) they tend to lose their regularity (their squareness, their plumb, their level). They’re also doing all sorts of other funny things that aren’t obvious as well including sinuating (forming rolling S curves), bending and pulling apart. 

While a few of the houses I’ve seen have done some part of this weirdness to a dramatic level, most have not. Most floors I see are uneven but the great majority are not so uneven that I end up being concerned. Traditionally, a drop of one inch over twenty feet was considered unacceptable but if you try to use that standard in the Berkeley or Oakland hills, you’d find an enormous number of houses that won’t pass muster.  

The question is, why would this matter? With doorways or window frames, the function of the opening can be gradually impaired and a door may not close easily at some point. This is surely important but what usually happens is that alterations are made in the door or the lock receiving plates (AKA “strike” plates). With floors, it’s harder to make the argument. While I am not a fan of a big gap along the baseboards of a living room or a hearth that is higher (or lower) than the floor, these things rarely, in and of themselves, cause other significant problems. 

Most of our houses are built on perimeter concrete footings. These are very small relative to the size of the building and quite weak when compared with the destructive force of moving earth. While today’s foundations are much heartier than those of the past, they are still, mostly, not built to resist a great deal of earth movement without some malignation. We rely, instead, on the assumption that the earth will either not move or that it will move so little that we can tolerate it. 

So we end up with deformation in the foundation as the earth moves (which varies a LOT from lot to lot) and whatever movement we have in the foundation pushes the structure above it around in both hard and easily predicted ways. In short, as the earth heaves up one part of the foundation, you’re living room floor goes up too. The deformations in the floors are simply a reflection of those movements that the foundation experiences. This is the real argument for larger and stronger foundation and most specifically for the mat or raft foundation. I like the name raft foundation because it takes us right back to my original image of us bobbing about (albeit slowly) on our slow little sea of soil. The raft is thick enough and cohesive enough so that, regardless of earth movement below it, all components above it remain pinned to a stable plane.  

Now, that plane may tilt somewhat but unlike our perimeter footing, the structure will not be pushed and pulled at from many different points so that its deformation is complex, resulting in lots of parallelograms. It will simply tilt as a whole one way or another. Further, with such a large floating plane, the tendency will be for the whole to remain fairly level as forces pushing here and there cancel out one another. 

Now, again, this isn’t an argument against perimeter foundations or in favor of raft foundations (well, maybe it is, O.K.). What I really want to say is this: 

Variation in the level or square or a building don’t matter that much and they don’t necessarily predict the really important events such as collapse in an earthquake. These things have much more to do with the way in which the building is tied together. 

A building with really uneven floors and crooked doorways that has been properly braced and bolted to its old coral reef of a foundation will very likely survive a large earthquake with manageable damage (everything will have some damage, right?) while the neat, square unbraced house next door will be a mess. That’s the message. A little out of level is not unsafe, is not a predictor of major damage and is not bad for your teeth. 

To expand the argument just a bit, a house with a dangerous electrical system may look neat, square and have a fresh coat of paint. A house with a furnace that’s leaking carbon monoxide may have lots of great IKEA lamps. Things don’t necessary connect except when they do, right? Much as I hate to say it, you can’t tell a book by its cover. 

Of course, if a house is so far out of plumb that it’s in danger of falling over, that’s another thing. I do see one of those every once in a while but it’s pretty darned rare. I DO, on the other hand, see dangerous electric conditions in lots of houses, many of which have just been painted REAL nice. 

One last thought. When looking at deviations from square, plumb and level, be sure to consider the age of the house. When I see a quarter inch crack on a house that is three years old, I just about jump out of my skin but when I see the same crack on an eighty year old house, I just go on scratching my beard and reciting Kafka aloud. Cracks and deformations are the physical artifacts of movement. That’s why they’re meaningful. They are the rings on the tree. You have to divide the measurement of movement over the time period for it to be meaningful.  

If movement is uniform over time (always a fair baseline, although rarely accurate) our three year old house is going to have one inch of movement every twelve years at the locus of that crack and possibly much more over the entire house. After eighty years, that could be several feet if we’ve had a few of these cracks. That’s wildly unacceptable. A few quarter inch cracks over eighty years is a yawn because you can expect that the next eighty years will be about the same and, more to the point, the next ten years won’t produce anything surprising. Just a few more little cracks in the plaster.