This paper presents the effect of brick and kiln wall roughness on the fluid flow, pressure drop, and convection and radiation heat transfer in tunnel kilns. The surface roughness of 0–4 mm is investigated for bricks and tunnel boundary. Another wall roughness of 10 mm is considered to explore the effect of significant defects in the tunnel boundary. The study is conducted using a three-dimensional computational fluid dynamics (CFD) model based on the finite volume method with the k – ω turbulence model. The convective heat transfer coefficients enhance by 45% and 97%, and the pressure drop increases by 25.1% and 80.4% as the brick roughness is increased from 0 to 1 mm and 0 to 4 mm, respectively. The ratio of heat transfer rate to pumping power reaches its maximum at a brick roughness of 2 mm. These results provide essential knowledge about the acceptable range of brick roughness for manufacturers. As the tunnel boundary roughness is increased from 0 to 1 and 0 to 10 mm, the heat transfer rates increase by 1.34% and 3.88%, while the pressure drops increase by 7.5% and 18.2%, respectively. These results are supportive of scheduling the maintenance of tunnel kilns’ interior structure. Moreover, the enhancement of the radiation heat transfer depends on the brick emissivity and the area ratio of rough to smooth surfaces.