“The high common space ratio was perhaps one of the principal reasons why the high density apartment houses on Gunkanjima seemed like a 'good place to live'”. Common space ratio is the percentage of common space to total floor area of a given building. Common space is the portion of the total floor area which is left over after the subtraction of private space and space assigned for other specific functions. It isn't necessarily the compliment of the rentable ratio, which is used to evaluate rentable offices or apartment houses.

Common space ratio = sum of common space / total floor area X 100

Rentable ratio = sum of apartments for rent / total floor area X 100

Total floor area as defined here is derived by the estimation method set out in the Building Standards Act. It includes not only internal space but also external space such as the space occupied by the structure, the space under the eaves if they project further than 1 metre, verandas, piloti, and stair wells. Common space does not include subordinate equipment space the function of which is simple and clear such as an elevator-shaft, a toilet or a storehouse even though they may be used in common. Let the ration of the sum of the floor area of subordinate space relative to the total floor area be alpha, the sum of the common space rentable ratio, and alpha makes 100. Therefore, common space as defined here means public space such as a corridor or a landing which can be used flexibly not only as a passage but also for other purposes. The concept of common space indicates flexible space which is used differently at different times, and where the boundaries between the private and the public remains in imprecise; some people refer to it as semipublic space. The common space contained in the residential buildings on Gunkanjima was rather different in quantity and in complexity in use form that of ordinary apartment buildings such as Housing Corporation apartment buildings, etc. Firstly we have tried to compare it quantitatively with other examples.

Table 1 lists the common space ratio of the apartment houses on Gunkanjima which we calibrated from all the plans we had made. For comparison, table 2 shows the common space ration of the Dojunkai apartment houses in the later years of the Taisho period. Table3 shows the common space ration of a representative selection of housing corporation apartment buildings. Table 4 is the chronological graph of the 3 tables. (Four tables are omitted here).

Extrapolating from these tables and the graph above, the following conclusions can be drawn.

1.Common space ratios on Gunkanjima are the highest off all by a comfortable margin.

2.There is a considerable variation according to structure and plan type.

3.The common space ration of high apartment houses exceeds that of low ones.

Points 2 and 3 seem to be directly linked and the reason for this is perhaps the fact that the so-called battery-type arrangement is suitable for low apartment houses. The term “battery-type” refers to a type of apartment house with apartments connected to the ground in twos or threes only by means of a vertical circulation element (stairway shaft or elevator). As a result, individual apartments have a high degree of privacy but tend also to be enclosed and exclusive, and there is barely any neutral zone between private space and public space. In most cases, staircases function as the core of the bearing wall. In the case of the medium-rise or low-rise apartment houses, a box-frame structure can be used, keeping keeping the constructions costs low. In terms of planning, too, because the rentable ratio is high, such structures are highly profitable to run. On the other hand, in the case of high-rise apartment houses with a lot of apartments, a battery-type arrangement increases the need for vertical circulation lines requiring extensive elevators, so the side-corridor type has a higher degree of profit efficiency. In the Takashimadaira housing project, constructed in a city with high land prices, most building were of the side-corridor-type because they were high-rise apartment houses. Dojunkai apartment houses before the War and suburban housing projects with comparatively low land prices even after the war were almost all of the battery-type. A building of this type, if it has a box-frame structure, is relatively inflexible to changes in the quantity and quality of supply and demand, and moreover the division between public and private space is particularly abrupt.

From the apartment blocks on Gunkanjima, three large scale examples, which also have an important place in technological history, have been chosen for analysis: building No. 30 (Taisho 5, 1916), the Nikkyu Flats (Taisho 7, 1918), and building No. 65 (Showa 20, 1945). The common space ratio of building No. 30 was 36.4%, that of the Nikkyu Flats was 32.9%, and that of building No. 65 was 31.9%. In contrast to these three examples, the common space ratio of the Aoyama Apartment houses (Taisho 14, 1925) put up by the Dojunkai ranged from 11.1 ~ 15.3%, whilst that of the Daikanyama Apartment houses (Taisho 14, 1925 also by the Dojunkai) ranged from 7.8 ~ 11.2%. These buildings were erected as part of the reconstruction policy after the Great Kanto Earthquake and are still in use today. The Takashimadaira housing project is regarded as typical of urban high-rise high density housing projects undertaken by housing Corporations after the War. Building G-3 in the Takashimadaira housing complex has 10 floors, and the common space ratio is 21.3%. The common space ratio of the Housing Corporation's low-rise apartment houses is 7.4 ~ 8%, which, at under 10%, is a very low proportion of the total. Besides the enclosed character of the buildings, the low common space ratios of Housing Corporation apartment houses have increased the sense of alienation experienced by those living in modern apartment houses. The high common space ratio was perhaps one of the principal reasons why the high density apartment houses on Gunkanjima seemed like a 'good place to live'. As will be shown below, because the existence of common space helps to increase community consciousness and the feeling of unity, the difference of common space ratios is an extremely important social.