Structural timber is gaining popularity in the construction industry; however, there is an urgent need to examine its safety in terms of strength and sectional properties. This paper seeks to examine the effect of flexural/bending strength of I and box African birch built-up beam sections on nail spacing. The air-dried African birch (Anogeissus leiocarpus) timber was obtained from a local timber market in Kaduna, Nigeria, the Pantaka market. The timber was sawed into sizes before being assembled into I and box built-up sections. Using a 2 mm diameter, 2-inch steel nail, forty (40) built-up beams were created. The dimensions of the I and box built-up beams were 500 x 75 x 20 mm and 500 x 40 x 20 mm, respectively. The samples were then subjected to five-point flexural loading at 6.6 mm/min using the Digital Board Universal Testing Machines (UTM). The nail sizes and spacing were chosen in accordance with BS 5268 Table 60. (2002). The flexural strength properties were determined at a moisture content of 18%, in accordance with the British Standard BS 373 (1957). Under the ultimate limit state of loading, statistical analysis, structural safety analysis, and probability of failure were established for the African birch timber as structural members in bridge beams and general construction works in accordance with BS 5268 (2002). A reliability and sensitivity analysis were also performed. Conclusively, the box section has a higher safety index than the I section, while the I section has a higher safety index than the solid section. The obtained results will serve as a guide for structural timber designers as well as other timber users. As a result, additional research can be conducted on other mechanical properties of African birch timber, such as compressive, tensile, and shear strength to determine structural safety.