•In historical structures, wooden elements are often degraded by superficial insect attacks.•Neglecting the contribution of degraded areas to the load bearing capacity of the beam can be an over-conservative approach.•A reduction coefficient of degraded wood is defined based on theoretical calculation and experimental investigation.•The reduction coefficient can be applied to the mechanical properties of degraded areas so to include them in structural calculation.•A value of n = 5 is proposed for the reduction coefficient. Existing timber structures are often affected by peripheral degradation due to insect attack. The contribution of the degraded areas of the beam cross-section to the load bearing capacity is usually neglected, leading to an over-conservative approach to the structural verification of historic wooden constructions. This work analyses timber elements dismantled from a nineteenth century building by means of a comprehensive geometrical survey, the evaluation of the degraded portion of the cross-section and by destructive bending tests. An iterative procedure based on the linear elastic theory of composite beams was set up to define a reduction coefficient of the mechanical properties of degraded wood, in order to include the contribution of the degraded parts during the structural calculation. The analytical procedure proved to be effective only for timber elements without localised major defects or anomalies, such as woodworking or internal decay, which may alter the linear elastic flexural behaviour of the beam. The values obtained of the reduction coefficient ranged from 1.7 to 5.0 and were successfully verified by numerical models.