Home & Garden Columns

About the House: Soft Stories, Line-Wire Stucco and Seismic Retrofitting

By Matt Cantor
Friday November 17, 2006

Before I ever look for a single foundation bolt there are a always a few other questions I always have about the building I’m looking at. Of course, I’m talking about earthquake readiness or seismic stability or whatever term-du-jour we’re currently using. 

Houses are not all alike in their inherent earthquake vulnerability and I’d like to bring up just a handful of special conditions that might be present in your home. Hopefully, this will get you thinking about what might have been missed (or could get missed) in the retrofitting of your house, resulting in a really big difference when the earth does the Watusi. 

Line-Wire Stucco: This is a generic term that I’m really not too fond of because it focuses on one aspect of a set of conditions that actually extend far beyond the use of line-wires. Back in the 1940’s some clever but not very smart contractors began installing stucco directly over framing without the use of any sort of backing. Building paper (usually called felt) was installed over the framing but the only other thing behind the stucco was a network of wires that were strung across the studs and stapled in place to provide a semi-rigid backing for the paper so that the wet stucco would have something to push against and wouldn’t just tear through the paper. It was minimalism (and capitalism) at it’s most profound. Prior to this period, stucco had been installed over horizontally installed boards (usually something like a 1 by 10) that would get nailed to the studs. The felt was then stapled over this, along with a metal mesh such as chicken wire to reinforce & hold the stucco in place (that last part, the chicken wire, is also used in line-wire too). 

In an earthquake, even these simple horizontal boards would provide a fair amount of shear resistance (the force that resists tearing and the resulting collapse of walls). This means that when the stucco cracked and detached, the boards would help keep the walls upright. If you take those boards away and you have a skeleton of uprights, they can easily get pushed over during an earthquake (helped enormously by whatever weight is resting upon them, such as the main floor, second story or roof). 

If you happened to have diagonal boards behind your stucco, you really lucked-out because they provide a much higher level of shear strength. You can usually see these diagonal or horizontal boards from the garage or crawl space and occasionally in an attic along one of the gable (triangular) ends. Today we almost always install stucco over plywood or a similar structural panel such as oriented strand board (OSB). OSB is made up of small pieces of wood, lots of glue and a very clever assembly that creates great strength. 

Once again, if you take the boards away and install stucco with no attachment except for a small number of nails, you may be in for lots more damage. Engineers seem always to disagree on the severity of one thing or another but there seems to be a consensus that this is far less serious in a one-story house but may be quite serious with additional stories, especially when combined with large openings such as sliding glass doors, large (or many) windows or garage openings. 

Another very similar condition involves the substitution of line-wires with soft wooden panels that have just enough rigidity to throw the wet stucco against without breakage. Celotex made most of the material I’ve seen and it’s so soft that you can push a pencil through without much effort. This looks like you’ve got some bracing but, in fact, it’s really about the same as the wires alone. It did provide a little insulation but may increase flammability due to its soft pithy makeup. 

If you have a house like this (either line-wire or soft backing panels), it’s best to talk to an engineer. You may want to add some extra bracing. 

Another common condition that I occasionally see is the all-glass house of the 50’s. Glass is not a structural material and if you have a lot of glass around the exterior of your house, you don’t have much shear resistance and that’s a bad thing. While there are numerous solutions to this vulnerability, it’s important to retrofit these houses. I fear that some of our “architect designed” beauties of the 50’s and 60’s will end up either fully collapsed or at best, so badly damaged that escape may be impossible. A house like this can be retrofitted by a clever engineer so it’s important to look into it. 

Eichlers fall into this category too. They’re the very modular looking homes from the 1950’s through the 70’s that have lots of floor-to-ceiling glass, stone fireplaces and simple wood panels. Though a popular “retro” style, these houses are likely to be seriously damaged in a moderate to large local earthquake. 

Wooden houses tend to perform really well in earthquakes if they’re effectively connected to their foundations but the more you remove the wooden walls, the weaker they get. 

Any house that has a lot of holes punched through the exterior walls (windows, doors, garages) tends to get pretty floppy. Since earthquake shear forces are greatest at the bottom of the structure, where the ground is trying to shake the house free (imagine a dog shaking off a flea), any big holes near the base become places for the walls to tear and begin the deformation that eventually can lead to a collapse (partial or complete). 

Imagine you’re sitting on a nice new thick cardboard box. You can sort of shift around and it stays pretty solid. Now cut some holes for windows. When you shift around you’ll start tears at corners of your windows and pretty soon, you’re beautiful house is suffering from an identity crisis. If you cut holes near the very bottom, this happens faster and collapse occurs quicker. 

Step outside your home and consider it as if it were the cardboard box. Consider every door, window, garage opening or other medium to large penetration as a big hole cut in the box. Chimneys are included (in addition to the fact that they’re the first things to come down) because they are often installed over framing without the benefit of wood planking or structural panels behind them. If you can visualize well, you may also note that one or more sides are weaker than the rest.  

One such case that I often see is a building with garage openings all across one side (usually the front). These are, of course at the base and constitute one of the most serious earthquake weakness we see. This is called a soft-story and means that there is almost no resistance to tearing or collapse on that side. Many multi-unit buildings feature this unfortunate element and, once again, an engineer really ought to be consulted since a) it can mean a very nasty event and b) it’s avoidable with the proper alterations.  

I remember so clearly that in 1989, when the Loma Prieta earthquake hit the Marina district in San Francisco, one of the primary images that poured over TV screens again and again was that of multi-story apartment buildings that had collapsed over their front garages. 

There are more special conditions that merit attention including steep hillsides (which change the dynamics of movement in the house), brick walls (as well as other masonry building materials), the soil type you’re resting upon and the weight of your roof. The list really gets sort of ridiculous. I don’t mean to make this unduly complex but it’s actually … complex. 

Nonetheless, the basic principle does work. If your house has a lot of “holes” (windows, doors or garages) or if your house lacks inherent bracing (as in the case of line-wire stucco), it’s more likely to get wanked out of shape when our big one hits. So what do you do? Get professional help, sleep later on Saturday and eat more ice cream. That’s about as helpful as I can get. 



Got a question about home repairs and inspections? Send them to Matt Cantor at mgcantor@pacbell.net.