Flame height correction model and over-wall burning criterion for pitched roof–firewall systems

The spread of flames across a firewall is a critical process in the propagation of fires from pitched-roof buildings to adjacent spaces. This study conducted experiments on pitched-roof-firewall systems under still air conditions, varying the roof pitch ( α = 20 ° - 40 ° ) and burner-to-wall distance ( d = 0.1–0.4 m) and heat release rate ( Q ̇ = 30.8–61.6 kW) on an adjustable-slope platform. A total of 48 tests with firewalls and 12 baseline tests without firewalls were conducted to measure flame height, deflection angle, and the flame attachment length along the slope. The results indicate that slope angle has the most significant suppressive effect on vertical flame height, followed by heat release rate; wall distance has a weaker direct effect on flame height, primarily modulating flame deflection and the flame attachment length along the slope by altering the entrainment conditions on both sides of the flame. Using the wall-free reference height H f,0 as the characteristic length and introducing the dimensionless wall distance η = d / H f , 0 , a flame height correction model for wall-present conditions was established; furthermore, a confined-transitional-over-wall burning criterion centered on H f / h w was proposed. This criterion agrees well with experimental observations and the frequency of over-wall burning, and can serve as a reference for the performance-based design of firewalls on sloped roofs and for assessing the risk of over-wall burning.