Fire behaviour of facades in multi-storey wood-framed houses

The fire behaviour of wooden facades in multi-storey houses has been studied in a Nordic research project on the fire safety of wood (Brandsäkra Trähus). The results of the research have been used in pilot projects for multi-storey wood-framed houses and utilised in the reform of the structural fire safety regulations of the Finnish building code. Two different fire scenarios were studied: a relatively small ignition source outside the building and a flashover of a compartment fire. These test arrangements correspond to possible fire scenarios in sprinklered and unsprinklered houses, respectively. Fire tests of wooden facades with different materials, surface treatments and structures were performed on intermediate and large scales. In the case of an external fire, the most efficient way to prevent the propagation of flames to the next storey of the facade was structural modification of the facade profile, i.e. cantilevers and oriels. Special attention must be paid to the sufficient width and length of the protrusion, and the non-combustibility of its lower surface. In the tests of fire retardant (FR) treated specimens, the flame spread was considerably delayed or even stopped depending on the FR treatment used. However, the long-term stability of FR treatment is generally undefined. When the facade was exposed to the flashover flames of a compartment fire, the fire spread could not be prevented by a 400 mm wide cantilever or special ventilation structures of wooden eaves. On the basis of the test results, the use of wood in the facades of multi-storey houses with unsprinklered apartments should be limited to 20 - 50 % of the total surface area depending on the location of the wood material. In sprinklered houses, however, wood can be used more extensively. Simple criteria based on heat flux levels to the windows of a facade and falling pieces have been proposed for sprinklered and unsprinklered buildings. According to these criteria, the fire safety of wood products to be used in facades of multi-storey wood-framed houses can be estimated.