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The use of timber with other bio-based building materials is often restricted due to the fire hazard. The primary protection for timber against fire is provided by cladding. Currently, there is limited fire protection design data on clay and hemp boards since they are relatively new products. Furthermore, the mechanical performance of boards is of utmost importance in timber frame assemblies. This paper presents a test program that determines the tensile and bending strength of boards accompanied by a comparison to widely used gypsum plasterboards. The fire protection effect for timber is assessed by using a cone heater. Main results indicate that clay and hemp boards are adequate alternatives for the gypsum plasterboards as they can demonstrate similar performance. Hemp boards made by dry method perform significantly better compared to the ones made by wet method. Clay boards with special additive demonstrate increased fire performance compared to more ecological boards. This study contributes to further research that is needed in order to provide design parameters for sustainable building solutions in the future.

Summary Clay and lime plaster are traditional surface finish materials used on historic timber walls and ceilings. Today, hardly any fire technical properties or design parameters exist that consider such plasters as fire protection materials for timber structures. This hinders the fire assessment of existing building structures and disadvantages their use in modern design solutions where healthy and sustainable materials are increasingly favoured. This research follows the safety philosophy of EN 1995‐1‐2 to describe the fire protection ability of plasters. This paper investigates the temperature‐dependent thermal properties of historic plasters by presenting series of material‐specific tests and furnace tests carried out under standard fire exposure conditions. Experimental studies are supported by numerical heat transfer simulations. Results demonstrate particularities between the fire protection ability of clay and lime plaster, however, highlight the need for further investigations in terms of their thermophysical performance and standardisation at the European level. Design parameters are presented and discussed in future perspective. This work demonstrates a basic research to plan and design full‐scale fire tests according to EN 13381‐7:2019.

The primary protection against the charring of timber is ensured by protection materials. Today, there are only a limited number of materials given in design codes as fire protection materials for timber. Historic surface finish materials such as plasters have rarely been studied with respect to fire; no design values exist in the current fire part of Eurocode 5. Full‐scale fire testing is costly to assess the fire performance of material combinations, thus this study presents a useful tool that is specifically tailored to evaluate the fire protection ability of materials in small‐scale. A review of conducted tests demonstrate that the cone heater of a cone calorimeter is a dependable device to estimate the charring performance of protected timber specimens as the test results approximate the ones obtained from furnace tests. This work contributes to the assessment of fire resistance performance of various combinations and types of plaster systems found in existing timber buildings that often require an individual approach for an adequate fire risk analysis and design decisions to meet current fire safety regulations with respect to the load‐bearing capacity and compartmentation of building structures. Increased knowledge on the fire protection performance of traditional plasters is believed to facilitate their wider use in timber buildings, primarily to preserve their significance as part of the cultural built heritage.