JAPANESE MINKA XIX - FLOOR STRUCTURE 2: RAISED FLOORS 3

September 22, 2023

Any region blessed with a high-quality building material in abundance, be that timber, stone, clay, etc., will naturally develop extraction, processing, and other value-adding commercial industries around this resource, for ‘export’ to surrounding regions and further afield. In isolated mountain villages and on remote islands, however, there may be no economical or practical way to get the resource or its products out to the wider world. This ‘landlocked’ condition, combined with the resource’s abundance, may mean that it has little or no commercial value, and so it will only be used locally, and in ways that might be considered wasteful in other circumstances, because there is no economic motivation to maximise yield and therefore profit. This has historically been the case in some regions of Japan in regards to timber, and has resulted in a floor framing method known as dai-neda-zukuri or о̄-neda-zukuri (大根太造り), ‘large joist construction’. In this method, the time-consuming work of rip-sawing and finishing many standard-dimension bearers (大引 о̄biki), joists (neda, 根太), and stumps (yuka-tsuka, 床束) is foregone in favour of fewer, larger-section joists, notched into similarly oversized, beam-like bearers which require fewer or no stumps to span between walls. Thick floorboards or planks are then fixed to these bearers and joists.

Floor framing showing large-section, beam-like bearers with few or no stumps supporting them, notched out to receive thick joists, which have been removed in this image.

An interesting comparison to о̄-neda-zukuri construction can be made with another variation in floor framing, this time a modern one only developed in recent years, known as neda-resu (根太レス) or neda-non (根太ノン) construction. Here, joists are entirely absent, replaced by 24, 28, or even 32mm thick structural plywood sheets fixed directly to a ‘lattice’ of bearers at 910mm centres in both directions.

On the left: standard modern Japanese floor framing consisting of bearers-joists-floorboards. On the right, a recent innovation, ‘joistless’ construction: thick structural plywood sheets laid directly on bi-directional bearers.

These two floor framing systems represent solutions to what are essentially inverted material and technological conditions, and could further be taken as representative of a characteristic difference between pre-industrial and industrial worlds. Whereas the conditions that gave rise to о̄-neda-zukuri method were the abundance of a resource (high quality, large-section timber) and the lack of technology required to fully exploit it (specifically the lack of technology required to extract and transport the timber economically), in the case of neda-resu construction, it is the scarcity of the resource, and the presence of the relatively sophisticated technology (peeling lathes, defect scanners, modern adhesives, hot presses, etc.) required to produce the structural plywood that makes the system both possible and economical.

JAPANESE MINKA XVIII - FLOOR STRUCTURE 2: RAISED FLOORS 2

September 15, 2023

Last week’s post presented the typical method of raised floor framing used in Japan, with joists laid on bearers and either subfloor or finish floor boards laid on top of the joists. However, for reasons of custom, sumptuary laws, economy, availability, or climate, it was common in many areas of Japan to forego floorboards and instead lay sugaki (簀掻), lattices of bamboo, reed, or timber lath, over the joists, to form the sugaki-yuka (簀掻床) or ‘lattice floor.’

In contrast to tight-fitting floorboards which prevent heat loss in winter and (most) drafts from coming up from under the floor, the open structure of the sugaki lets air pass freely in both directions. In the warmest subtropical regions of southern Japan, this could be desirable, as the sugaki allowed cool air from the shaded space between the floor and the ground to be drawn up into the house to replace warmer interior air as it rose into the roof space.

A bamboo *sugaki-yuka* with an inset *irori* hearth. From the Kawano house, originally in Ehime Prefecture, Shikoku, first half of the 17th century.

In colder areas where the sugaki-yuka was used but drafts were not welcome, the subfloor space was sealed off by infilling the gap between ground and floor level in the exterior perimeter walls with stones, then rendering the stones with daub, giving an external appearance very similar to the raised earthen floors (taka-doza-yuka, 高土座床) previously discussed.　 In these minka, the sugaki would also be covered with mushiro mats; even in warm climates, ‘local’ mats were still necessary, as lattice floors of any type, but especially bamboo lattice with its raised joints, are uncomfortable to sit on.

A bamboo *sugaki-yuka* partly overlaid with *mushiro*.

This type of floor is also seen in the upper ‘attic’ storeys of the famous gasshо̄-zukuri 合掌造り (literally ‘praying hands construction’) minka of Gifu Prefecture. The upper levels of these houses were used to raise silkworms by feeding them on mulberry leaves, requiring a well-ventilated environment.

Exterior view of *gasshо̄-zukuri minka* in Gifu Prefecture.

Interior of a *gasshо̄-zukuri* showing a timber lattice floor.

Interior of a *gasshо̄-zukuri* showing a bamboo lattice floor.

JAPANESE MINKA XVII - FLOOR STRUCTURE 2: RAISED FLOORS

September 8, 2023

The raised floor, or taka-yuka (高床), refers to the arrangement where the plane of the ‘living floor’ is raised above the ground level. It can describe not only the floors of residential dwellings but also those of granaries and storehouses (these non-residential structures were likely the earliest examples of the taka-yuka, due to their obvious advantages in preserving grain and other perishables from rot and vermin); it may also refer to the subtype of earthen-floored dwellings covered in last week’s post, where an earth podium is built up well above the natural ground level. Here, however, we will be primarily discussing what most people understand by the term taka-yuka: a timber floor structure of stumps, bearers and joists, with a subfloor void between the floor and the ground.

In Japan, raised floors are typically 400 to 500mm above ground level, though there are examples of floors up to a metre off the ground. Historically they have been most commonly associated with and found amongst the residences of the aristocratic and upper classes, in low-lying marshy areas and wetlands, and in the warmer and more humid regions of the country, from southern Honshū to Okinawa.

The most common floor framing (yuka-gumi 床組) construction system in modern Japanese timber-framed houses, at least until relatively recent times, is this: 90 x 90mm bearers (о̄biki 大引) spanning the area within the ground sills (dodai 土台) are laid down at a pitch (spacing) of 910mm, on timber stumps (yuka-zuka 床束) that are also 90 x 90mm in section and set at a pitch of 910mm. These stumps are tied together near their bases with thin ties (ne-garami nuki 根がらみ貫) of around 90 x 12mm, whose purpose is to prevent the stumps from slipping off their pads. To brace the floor structure in the horizontal plane, diagonal 90 x 90mm members called hi-uchi dodai (火打ち土台) are inserted in the internal corners and in other locations, in the same plane as the dodai and о̄biki. On top of and perpendicular to the bearers are laid joists (neda 根太) of around 45 x 45mm or 60 x 45mm, at a pitch of either 455mm or 303mm, depending on the floor covering/load and the strength and depth of the member. Joists are doubled under internal walls. In minka, tatami mats were usually only laid in the formal zashiki room; in this case, joist spacing was the closer 303mm, because the thin subfloor boards typically used under tatami can’t span the 455mm between joists that the 20mm-30mm thick finish floorboards used elsewhere can.

Diagram showing the elements of modern Japanese floor construction.

Rip saws (oga 大鋸) did not appear in Japan until the 14th century, and spread only slowly.

A print from the 1830s depicting men cutting a large section timber with an *oga.*

Before that, the only way boards and planks could be made was by splitting logs longitudinally with wedges, then finishing the surface with an adze (chо̄na 釿) or spear plane (yari-ganna 槍鉋).

Relatively ready availability of large section timbers, practical limits to how thinly logs could be split, and the labour involved in finishing the boards all meant that this method tended to produce thicker planks. Adze-finished timbers have a beautiful undulating, wavelike finish, and genuinely adzed floorboards are still an option today the for those with the money; for those without, machine-finished ‘mock-adzed’ floorboards are also available.

JAPANESE MINKA XVI - FLOOR STRUCTURE 1: EARTHEN FLOORS 2

September 2, 2023

This post is a continuation from last week’s examination of earthen floors (土座床) in minka.

Even after the transition from the post-on-foundation stone method of construction to the use of a ground sill (dodai 土台堀立て柱) between posts and foundation stones, the tendency in doza-yuka dwellings was to use ‘half sills’ (han-dodai 半土台) internally, so that these members projected as little as possible above the ‘finished floor level’ of woven mats (mushiro 莚).

*Doza-sumai.* The earthen floors are covered with woven straw mats called *mushiro.*

The construction of the typical ‘floor living’ (doza-sumai 土座住まい) floor was a often more sophisticated than simply placing mats straight down on the earth. The ground was first dug out to a depth of around 100 to 200mm, then a soft ‘underlay’ layer, often of rice husks (籾殻, usually read momi-gara, but here read nuka), but alternatively some variety of straw (wara 藁) or thatch (kaya 茅), either of rice, Cyperacea species such as sedge, Miscanthus, speargrass (Imperata cylindrica) etc., or reed (yoshi or ashi, 葭), or millet husks (hie-gara or fue-gara 稗殻) was put down.

Since straw and reed are hollow, they have an insulative effect and prevent damp, and are also unlikely to harbour fleas. Over time as they are walked on, however, the individual straws or reeds are broken and crushed, meaning both a relatively noisy floor and, as the subfloor packs down, a gradual subsidence of the finished floor level in the most trafficked areas. Because of this, and the vulnerability of these materials to insect damage, traditionally the subfloor was replaced every year.

On top of the subfloor layer went the ‘finish’ floor: mats (goza 茣蓙) of woven straw, thick mushiro (atsu-mushiro 厚莚) known as nekota or nekokata, bullrush or cattail mats (gama-mushiro 蒲莚), sedge mats (suge-mushiro 菅莚), or occasionally the rigid tatami (畳) mats that are still a characteristic feature of Japanese houses.

To minimise damage caused by flooding and the effects of ground moisture, a platform of compacted earth was sometimes built up above ground level within the perimeter of the external walls, to a height somewhat lower than or even as high as the typical timber-framed taka-yuka floor; indeed if the minka also had adjacent raised-floor areas such as a zashiki, building up the doza to this same level was logical and convenient. At first glance these raised earthen floors (taka-doza-yuka 高土座床) might appear to be timber-framed themselves, but lifting the mushiro and underlay or looking at the subfloor from the exterior would reveal an earthen base. Building up the floor in this way has the advantage that the underlay of husk or straw can be omitted, since raising the floor is similarly effective in reducing damp; it also eliminates the work of replacing the subfloor annually.