The purpose of this paper is to discuss the torsional rigidity of nailed wooden members such as rafters or joists. Rotational rigidity of the nailed members which has a rectangular section can be evaluated by considering apparent withdraw rigidity of nails. The design formulae for calculating the shearing rigidity of a horizontal structural element such as floors or roofs are proposed using the rotational rigidity of nailed members. The results obtained in this paper are as follows. 1) Rotational rigidity of nailed wooden members can be evaluated by Eq. (7) with using apparent withdraw rigidity of nails Km. Km is evaluated for existing or not existing of device to prevent rotation of nailed members respectively. 2) Shearing rigidity of rafter and joist, which is concerned in the torsion of nailed members, can be evaluated with Eq. (28) and Eq. (29).
