This paper shows the assessment process of a 100 year-old historical over-capping timber structure. Visual observation, destructive, and non-destructive tests were performed to obtain the material properties of the timber structures. Based on the visual inspection, several of the element timber structures experienced deterioration. The destructive samples were obtained to observe the mechanical properties of timber material. The samples were acquired at a location that did not disturb structure stability. The mechanical properties of timber material, such as bending stress, tension stress parallel to the grain, compression stress parallel to the grain, and compression stress perpendicular to the grain, are examined to gain the actual strength of the material based on disturbed samples. Ultrasonic Pulse Velocity and humidity tests were performed on-site as part of non-destructive tests. The structure analysis was performed based on existing material data. To obtain nominal stress, this basis design stress was multiplied with several coefficients such as wet service factor, temperature factor, beam stability factor, size factor, flat use factor, incising factor, repetitive member factor, and column stability factor. The structure analysis showed that the columns’ structure needs to be retrofitted. The proposed design for timber retrofitting is shown in this paper and has been conducted on the site.