<p> The concept of fireproof construction in Japanese building regulation was redefined in 2000 to enable large wood based buildings to sustain permanent load bearing capability even after enduring the effects of a fire. The target of this study is the fireproof layered structure composed of:</p><p> a) a load bearing part at the core</p><p> b) a barrier (fire-retardant impregnated wood)</p><p> c) a sacrificial layer</p><p> This type of layered structure is the furthest behind in terms of development in respect to other types of wood-based fireproof construction.</p><p> The authors clarified the condition of ability to self-extinguish, to the point of no further glowing combustion, after the fire heating of this test target. In the previous report, by designing the thickness of the sacrificial layer so as to reach the pyrolysis temperature of the fire retardant agent during heating, the fire retardant performance of the barrier can be minimized. From the pyrolysis temperature of the fire retardant agent, the sacrificial layer thickness may be increased in proportion to the fire resistance time.</p><p> However, if there is an optimum thickness of the sacrificial layer at each fire resistance time then the sacrificial layer, which is exposed to fire heating shorter than the designed fire resistance time, cannot self-extinguish. Therefore, the following 2 tests were conducted in order to clarify that the sacrificial layer thickness does not depend on the designed fire resistance time.</p><p> (1) Bench-scale tests were conducted using small specimens modeling the wooden fireproof structural elements in order to confirm whether they self-extinguished when they were heated for a time shorter than the designed fire resistance time.</p><p> (2) From series (1), it was estimated that 25mm is the optimum sacrificial-layer-thickness independent of the designed fire resistance time (the specification had fire resistance for 1 hour). Therefore, the specification was confirmed through further testing that it self-extinguished even when exposed to heating for less than 1 hour.</p><p> From these tests, the following conclusions can be drawn.</p><p> 1. The sacrificial layer, with a thickness designed according to the fire resistance time, may not be self-extinguishing in some cases if the heating time is shorter than the designed fire resistance time. The reason are as follows. First, since the sacrificial layer is too thick to allow the heat to penetrate into the barrier layer by the end of heating, meaning the barrier layer will not reach pyrolysis temperature. Second, the sacrificial layer is kept hot by the carbonized layer on the surface, meaning heat loss on the surface does not cause it to self-extinguish.</p><p> 2. Regardless of the heating time and type of the fireproof performance, the optimum thickness range of surface layer is 25 mm or less. In the range where carbonization is completed during heating, fire proof performance is not greatly affected without using fire retardant wood as the sacrificial layer. Therefore, an optimum design method for wooden fireproof structural elements is to set the sacrificial layer thickness to the upper limit value so that it does not burn by glowing combustion after the end of heating.</p>