The structural integrity of existing buildings, particularly those of historical significance, is a critical concern, often complicated by limited knowledge of the original material properties and the effects of aging and changing use. This study explores the use of non-destructive testing (NDT) methods as practical, reliable alternatives to destructive techniques for assessing timber structures. In the first phase, 100 Irish-grown Sitka spruce specimens were tested in a laboratory setting to evaluate the correlation between the dynamic modulus of elasticity (Edyn) - derived from density and Time-of-Flight acoustic measurements - and key mechanical properties such as bending strength, and the local and global bending moduli of elasticity. Bending tests were conducted in accordance with the EN 408 standard. Regression analysis confirmed that Edyn is a reliable indicator of the mechanical properties of timber, supporting its use in the evaluation of existing timber structures. The second phase involved an on-site evaluation of a historic timber roof in Ireland, composed of queen post trusses. Acoustic NDT was used to estimate Edyn and infer mechanical properties, while moisture content (MC) measurements helped adjust readings and identify potential decay. This assessment provided insights into the condition of the structure and highlighted practical limitations of in-situ testing while recommendations are provided for future inspections. Overall, the study demonstrates that the NDT methods can be effective for evaluating the mechanical performance of timber in both laboratory and real-world contexts, offering a viable approach to support the preservation and maintenance of timber structures.