A method for determining the state of preservation of a wooden support and establishing a diagnosis in order to find out whether the wooden support is in a state which is suitable for use, in a state requiring a maintenance operation or in a state requiring its elimination includes measuring the hardness of the wooden support and the hygrometricity of the wooden support. The method further includes determining the species of the timber of the wooden support, determining the treatment previously applied to the wooden support, calculating the resulting safety residual stress, comparing the value of the calculated safety residual stress with an observed average safety residual stress and establishing a diagnosis depending on this comparison.