A log structure for prefabricated log sections suitable for constructing log buildings is shown herein. The log section has a substantially flat longitudinal upper surface. A pair of spaced parallel longitudinal tongues is formed in the middle portion of the upper surface and these tongues extend radially upwardly from the upper surface. Also, two raised platforms are formed on the longitudinal edge portions of the upper surface. The platforms have a substantially flat upper face. The log section also has a flat lower surface with a pair of spaced parallel longitudinal channels formed in the middle portion therein. The channels have a complementary cross sectional configuration equal to that of the tongues and have cross sectional dimensions at equal to or larger than that of the tongues. A center bore is formed throughout the entire length of the log section. A circular groove and two diametrically opposite radial grooves are formed at the end surfaces of the log section. Two log sections may be joined longitudinally by interfitting the tongues of the lower log section with the channels of the upper log section. A space is formed in the joint, which is filled with sufficient sealing material to seal and bond the joint. Two log sections are joined end-to-end by the provision of a coupler fitted in the circular and radial grooves of the log end. The coupler forms a seal and barrier in the end joint.