Kento-kunThis memo provides a clear summary of the laws frequently used in the design of new wooden houses and small-scale buildings.
Kento-kunI’ve got to study hard again today!
Today, let’s learn about “areas exposed to fire spread.”
Today’s basic concept in wooden house design is “areas exposed to fire spread.”
It depends on the zoning area, but when building in an urbanized area designated under Article 22 of Japan’s Building Standard Law, the exterior walls, eaves, and similar parts of a building, or portions of a building, that fall within an area exposed to fire spread must have a certain level of fire resistance (Article 23 of Japan’s Building Standard Law). In addition, in densely built-up urban areas such as Fire Prevention Districts and Quasi-Fire Prevention Districts, openings such as windows must use fire-resistant sashes or similar components (Article 61 of Japan’s Building Standard Law).
When a certain level of fire performance is required, it affects the scope of the regulation, design flexibility, and construction cost. That is why understanding areas exposed to fire spread is important in wooden house design, so let’s learn it together.
(Definitions)
Article 2 In this Law, the terms listed in the following items shall have the meanings prescribed in each item.
(vi) Areas exposed to fire spread:
The term refers to the parts of a building located within 3 meters on the first floor and within 5 meters on the second floor or above from the adjacent property line, the centerline of a road, or the centerline between the exterior walls of two or more buildings on the same site (where two or more buildings whose total floor area is 500 m² or less shall be regarded as a single building) (these are referred to as “adjacent property lines, etc.” in item (b)). However, the following parts shall be excluded:
(a) Parts facing a park, plaza, river, or other open space or body of water that is effective for fire prevention, or a fire-resistant wall or any other similar feature
(b) Parts designated by the Minister of Land, Infrastructure, Transport and Tourism as not being likely to ignite from the heat of an ordinary fire occurring around the building, in accordance with the angle between the exterior wall surfaceof the building and the adjacent property line, etc.
Source: Article 2, Item 6 of Japan’s Building Standard Law
Kento-kunThe basic rule is that areas exposed to fire spread are the parts of a building located within 3 meters from the adjacent property line or the road centerline on the first floor, and within 5 meters on the second floor and above. In addition, when the total floor area on the site exceeds 500 m², buildings should be grouped in units of up to 500 m², and the relevant range is measured as 3 meters and 5 meters from the centerline between the exterior walls of those building groups. The diagram below summarizes this rule.
Kento-kunWhat kinds of things are listed in item (a)?
Kento-kunItem (a) includes places where buildings are not expected to be constructed in the future, such as parks, plazas, rivers, waterways, open spaces, railway land, greenways, and paths. In addition, it is also possible to exclude a part from the application of areas exposed to fire spread by installing wing walls or similar elements with fire-resistant construction, such as fire-protective construction, quasi-fire-resistant construction, or fire-resistant construction.
Kento-kunWhat are the parts designated by the Minister of Land, Infrastructure, Transport and Tourism in item (b)?
Kento-kunItem (b) is a new standard introduced in 2020 as a relaxation measure for areas exposed to fire spread. Depending on the distance from the boundary line and the angle, certain parts may be excluded from the area exposed to fire spread.
Kento-kunI see. So basically, I should use the standard rule as the general principle, and only consider the relaxation measures when I really need to avoid having a part fall within the area exposed to fire spread.
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