Interface roughness effects on electron transmission in tunnel junctions are investigated theoretically in the limit of thick barriers. The barrier roughness is described in terms of self-affine fractal scaling by the roughness exponent H, rms roughness amplitude w, and correlation length ξ. For realistic parameters diffuse transmission usually exceeds specular transmission. It is shown that for small roughness exponents (H<0.5) the transmission coefficient increases with decreasing ratio w/ξ. For large roughness exponents (or smoother interfaces at short wavelengths) the transmission coefficient has a maximum at a certain value of the ratio w/ξ. With increasing w/ξ the tunneling current behaves similarly as the transmission coefficient