Within the latest decades, the research community has been focused on changes that may take place in qualitative characteristics of wood after long-term use of wooden structures used in buildings and structures. These are research papers by such scientists, as Varfolomeev Yu. A., Gusev B. P., Keskulla T. E., Kisternaya M. V., Okamoto K., Pishchik I. I., Terentyev V. Ya., Fefilov V. V., Fukada E., Shapovalova L. G. and others. Such papers have shown major impacts produced by long-term natural ageing upon physicochemical, mechanical and other properties of wood due to structural and chemical transformations taking place in the wood. Today, impact produced by long-term natural ageing of wood upon its fire hazard rates continues to be under-investigated. As shown by analysis of conflagration accidents that have taken place, facilities having long useful-life and containing wooden structures behave unusually when exposed to high temperatures. Temperatures abnormally high, rapid propagation of fire and high-rate thermal damage to structures affected are most typical for such conflagrations. Non-standard behavior of long useful-life wooden structures in case of conflagrations and their fire hazard have been dealt with in research papers by such scientists as Aseeva R. M., Naganovskiy Yu. K., Pokrovskaya E. N., Serkov B. B. and Sivenkov A. B. In such papers, their authors have provided evidence of high-rate combustion hazard wooden structures acquire due to their long-term natural ageing. However, effects produced by long-term use of wooden structures used in buildings and structures upon their fire resistance continue to be poorly understood, and approaches best fit for making their fire-safe are still in need of investigation. Lack of scientific research in this area and the actually neglectful attitude to fire safety regulations and references system, as applicable to such factor as wooden structures' terms of use, provide a paramount importance to findings made by the study. The principal purpose of this study was investigation into the impact of their long-term use upon fire resistance of wooden structures (WS) and upon their carbonization specifics combined with loss of WS sustaining capability in case of a conflagration. It was the first time that investigation was ever made into behavior displayed by long useful-life wooden structures when exposed to a conflagration. It has been shown that time is a major factor contributing into carbonization rapidity (in the course of the use, such carbonization rapidity grows 2-fold on the average) and heating extent across the thickness of wooden structure affected. It has also been shown that wooden structures used for 150 years have higher wood density rates and physico-chemical properties. However, high temperatures cause loss, by such structures, of their initial durability rates, even when temperature is 80 °C only. It has been shown that carbonization sets on at lower temperatures in wooden structures that have been longer in use; investigation has been made into properties and structure of wooden structure coal layers and their oxidizing power. It has been found that specific properties and structure of coal layer produced in any wood long-advanced in its natural ageing make such wood more fire-resistant. However, this is combined with high-rate cokes oxidizing power and heat output; high-rate thermal damage to the wood structures is also evident. Interrelation has also been shown between findings obtained by thermal analysis method (TG, DTG and DSC) and carbonization extent, which is the principal process accompanying loss of their sustaining capacity by wooden structures. Such interrelation allows making estimations for fire resistance in long useful-life wooden structures usable in buildings and structures. Carbonization rates obtained for various temperature ranges, and also temperature and time, when carbonization usually sets on, may be used in calculation methods applicable to estimations of the highest wooden-structure fire resistance rates. Estimations for the highest fire resistance rates of long useful-life wooden structures may be obtained for the existing buildings and structures and for those under reconstruction. Findings of the studies are proposed for use in Russian and foreign regulatory documents and in future, within studies of behavior displayed by wooden structures in case of a conflagration. Findings so obtained allow improvement of safety for humans in long useful-life buildings and structures, and also improvement of safety for fire-fighting teams and successfulness of their efforts in fire extinguishing at such facilities.