Soft-storey and the old housing stock

The shape of the problem

Walk down almost any residential street in this city and you will see the same building repeated with variations: two or three storeys of wood frame over a ground floor with a garage opening, a front door and not much else.

It is an efficient way to build on a narrow lot and it has one serious structural weakness. An earthquake pushes a building sideways, and what resists that is wall — specifically, wall that runs in the direction of the push and is properly connected top and bottom. A ground floor that is mostly garage door has very little of it, while carrying the weight of everything above.

That is a soft storey, and in a large earthquake the characteristic failure is that the ground floor collapses sideways and the building sits down on it. It is one of the better-understood failure modes in residential construction, it was visible in San Francisco in 1989, and it is the reason the city has spent a decade running a mandatory retrofit programme for qualifying buildings.

What a retrofit actually does

It gives the ground floor something to resist lateral force with, and gives that something a path into the ground.

In practice that means one or more of: plywood shear walls, where a wall is opened up, sheathed with structural plywood to a specified nailing pattern, and anchored at top and bottom with holdowns; steel moment frames, where there is no wall to work with because the opening has to stay — a garage door, typically; and connections, because a perfectly good shear wall that is not tied to the floor above and the foundation below does nothing at all.

The connections are the part that is invisible and the part that most often decides whether the work was any good. A retrofit is a chain, and a chain that is not continuous from the roof to the soil is not a chain.

The foundation underneath it

This is the part that surprises owners and doubles estimates, so it is worth reading twice.

A shear wall or a moment frame transfers force into the foundation. That assumes there is a foundation capable of receiving it. In this city, depending on the age of the building, what is actually down there might be reinforced concrete — fine; unreinforced concrete — frequently not; brick — no; or, in the oldest stock, redwood mudsills laid on soil, which is a foundation in the same sense that a raft is a boat.

Where the foundation cannot take the load it has to be replaced, in sequence, in sections, under a building that is still standing and frequently still occupied. That is careful, slow, expensive work, and it is routinely the larger half of a retrofit budget.

Which is why the single most useful thing on a first visit is to open up a small section and look at what is there. An estimate written without doing that is an estimate with a large number hidden inside it.

What is usually behind the stucco

A partial list, none of which is unusual, all of which we have found repeatedly.

Knob-and-tube wiring, still live, in a wall nobody has opened since before the war. A previous addition that was never permitted, holding up something that matters. Dry rot at the rear, where a deck has been draining into the wall for thirty years, usually worst exactly where the new work needs to connect. Cut joists, where somebody ran a pipe through the middle of a beam in 1962. Asbestos and lead, which are near-certain in a pre-1980 building and have their own regulatory paths and their own schedule. And no shear connection anywhere, because in 1912 nobody was thinking about it.

None of these is a disaster on its own. Discovered in week two they are line items. Discovered in month six, with a crew waiting, they are change orders and they are arguments. That is the entire argument for spending money on investigation before the contract is signed.

Why this work is worth doing even when nobody makes you

A retrofit is unglamorous, it is invisible when finished, and it makes your house neither bigger nor nicer. It is also, on the arithmetic, one of the better things you can do with the money.

The purpose is not that the building is undamaged. It is that it does not collapse, that the people inside get out, and that after a design-level earthquake the building is repairable rather than a total loss. Anybody who promises more than that is overselling, and it is worth being clear-eyed about the difference: damage is expected.

Set against replacing a building, or against not having somewhere to live, the cost of a retrofit is small. It is also very much cheaper done deliberately, while you are doing other work, than done alone and urgently.

What to do first

Find out whether your building is in the city’s mandatory programme. If it is, you have an obligation and a deadline, and the conversation is about how rather than whether.

If it is not, have somebody look at the ground floor and the foundation anyway. Thirty minutes and a torch will tell an experienced person most of what matters, and a small exploratory opening will tell them the rest.

Then, if you are planning other work on the building, sequence the structural work with it. Opening a wall once is substantially cheaper than opening it twice, and a retrofit combined with a renovation costs far less than the two done separately.