  The development of thermal boundary layers and plume near a
section-triangular roof under different isothermal heating conditions have been
the focus of numerous numerical studies. However, flow transition in this type
of flow has never been observed experimentally. Here, phase-shifting
interferometry and thermistor measurements are employed to experimentally
observe and quantify the flow transitions in a buoyancy-driven flow over an
isothermal section-triangular roof. Visualisation of temperature contours is
conducted across a wide range of Rayleigh numbers from laminar at $10^3$ to
chaotic state at $4 \times 10^6$. Power spectral density of the temperature
measurements reveals the type of bifurcations developing as the Rayleigh number
is increased. This flow transition is characterised as a complex bifurcation
route with the presence of two fundamental frequencies, a low and a high
frequency. We found that the thermal stratification in the environment plays a
significant role in the flow transition. The spatial development of flow is
also quantitatively and qualitatively described. In addition to clarifying flow
transition in experiments, the work demonstrates the implementation of
phase-shifting interferometry and punctual temperature measurements for
characterisation of near-field flow over heated surface.
