Analyses were conducted to study the problem of natural convection underneath a hot and isothermal horizontal infinite flat strip. It included a numerical and an analytical investigation of the problem. The work offers simple closed form solutions for the critical flow depth, boundary layer thickness and heat transfer coefficient. The governing equations were solved by the integral method. The justification for applying self-similar boundary layer profiles was demonstrated both numerically and from analyses of published experimental results. The solution was improved through the use of numerical analyses as well as from analytical inspection of limiting cases. The results were successfully tested against published experimental data. Furthermore, the work offers an explanation for the discrepancy that exists amongst the various heat transfer correlations found in the literature.