Friday, 25 August 2017

How to build a house in Japan Part two: Two-by or zairai?

One choice that you probably won't be given by any architect or builder in Japan is whether to build in zairai koho or two-by-four. If you're building a wooden house it's a choice that is made early in the project.

Zairai koho is the traditional wooden building technique in Japan. It consists of a framework of pillars and beams,​ fit together with joints carefully designed to avoid excess stresses, and​ originally held together without any nails or screws, hence the nickname nail-less construction. The standard section size of the pillars is 120 by 120 millimetres, and the beams are multiples of this.

Two-by-four refers to a building technique using beams of two-by-four inches. Rigid panels such as plywood or OSB are added so the structural strength is based on the walls, where zairai traditionally relies on the pillars and beams for the structure. Just to add a little confusion, 2 x 4 beams measure half an inch less by the time they've been cut and dried, measuring 38 x 89 mm rather than around 50 x 100 mm that you might expect.

The two-by-four construction technique developed from balloon framing in the 1830s in the US. It allowed standard sizes of timber to be put together with mass-produced nails by relatively unskilled wood workers.

Compared to zairai, two-by-four constructions is probably cheaper, stronger, easier to build, and easier to insulate. Since the beams are rectangular rather square in ​section, the building more efficiently derives structural strength from the wood. Insulation can be added between pillars and studs, and the wood make​s​ up a smaller proportion of wall, so the insulation performance will be better than a building with square pillars. If the same technique uses 2 by 6, 2 by 8, 2 by 10 or​ even​ 2 by 12 beams, a suitable thickness of insulation can​ easily​ be added between the beams.

It's difficult to find tangible advantages to zairai construction, but I will try.

Zairai construction is traditional.

That ​is probably enough to illicit approving nods from fans of tradition, and disparaging scowls from anyone who has been paying attention since the Age of Reason.

Zairai construction is based around standard sizes and scales that suit the human body. Measurements are in the traditional units of shaku and sun. One shaku is within a hair of an imperial foot, and was standardised in 1891 to 10/33 of a metre. Traditional Japanese units are decimal, so a sun is one tenth of a shaku. Traditional zairai beams are 6 shaku, or 180 cm, from the floor. That's around my height, and​ after a few years living ​in a traditional Japanese house I was beginning to develop calluses on my forehead and a stoop in my back. The average height in Japan increased 8 cm in the second half of the twentieth century, so I suspect for most of the history of Japanese architecture, the lintels were at an appropriate height. ​In modern houses they are higher. Of course there is nothing to stop you from using human-scaled dimensions in a two-by-four construction. Also, you may have noticed that the shaku is remarkably close to the imperial foot, and the standard lengths of two-by-four (inch) beams are all in feet.

Zairai construction is based on a woodworking tradition at least a thousand years old, which can be seen in the oldest and the largest wooden structures in the world. By building a house in zairai you are helping to keep this tradition alive. But what exactly is being preserved? Why do home builders have to pay more to preserve it? They still make temples and shrines, so couldn't the fantastically wealthy priests preserve their tradition?

The joints of zairai are all supposed to fit together without any nails, except now they do use nails.
bolts​ or other connectors for the joints. And since the traditional pillar-and-beam structures do not meet modern earthquake regulations, to get planning approval for zairai buildings, you need to add structural walls, just like they do in two-by-four construction.

The square beams were traditionally prepared locally from​ round​ trees. Now timber is usually cut in saw mills, often using state-of-the art CNC machinery.

​So is your modern zairai building just a two-by-four construction with more wood in it, and more complicated joints?

I guess you could see an advantage in it being more difficult to build, since that means you have more highly-skilled carpenters. You have to squint a bit to see this, since you are also making the job more difficult, but there are some places where more highly skilled wood workers will make a tangible difference to your house.


Our house uses zairai koho, and it is on the list of things I would probably have done differently. Luckily ​that​'s a short list! The point when I realised that there had been a different option to this vast array of square-section wooden pillars was at the stage in the process where it was not possible to change the building technique.

You're always at some ​stage in a process.

There had earlier seemed to be a great rush to get the structure all sorted out, coinciding with a busy time in my day job. I was a bit disappointed as I was quite interested in structures, and would have liked to have had some input into it. It seemed like a lower priority than the insulation work and the systems we were considering, so that was a battle I chose not to fight. Qualified architects in Japan, or anywhere else, can be trusted to make structures that will not fall down.

After this urgent decision had been made​ to finalise the structure​, there seemed to be a couple of months when absolutely nothing happened. Ben talks about a similar artificial deadline in his retire Japan Blog and I think this is a common technique in the building trade.

When we were looking at ways to fit at least 250 mm of insulation into walls with 120 mm pillars, I had an idea of using two-by-tens as studs between the ​load-bearing ​pillars, which would have allowed one insulation layer rather than the three we have ended up with. This seemed like a bad idea as it was mixing two different techniques, and would leave a few awkward sized gaps. So I wondered about getting rid of the square pillars ​altogether ​and just using two by tens throughout.

As you will remember from lesson 4, the calculation of the thermal performance depends on how much wood there is in the insulation layer. This information needs to be added into the Passive House software, and I was checking the figure of 18.1% that we had. A more conservative estimation put it more like 25%, so a whole quarter of the wall was made up of wood, much of it by square pillars. This could have been halved by switching to rectangular sections. That would also have meant less wood​ to pay ​for​.

I suggested this to the architect who said it would take a month or two to change the structure, and he'd need to get someone else to calculate the stresses.

So we have a house beautifully built in wood, but ​we can't see any of it it, since it had to be covered to meet fire regulations.