Case Study: A Sagging Roof
The roof of this barn exhibits a sag (featured photo). The roof is framed with rafters (Figure 1). The rafter ties are set very high to allow clearance for interior activities; however, this prevents the ties from adequately restraining the bottom ends of the rafters from thrusting (or pushing) outward. The result is the outward lean of the bearing walls and the sweep (side to side bow) at the eaves (Figure 2).
Is the sag and thrusting recent?
The answer may be found in the separations between the top ends of the raters and the ridge board (Figures 3 and 4). The separations are due to the outward leaning of the bearing walls and the sag at the ridge. The heavy corrosion on the nail shanks exposed in the separations indicate that the outward leaning of the walls and the sag at the ridge have been present and progressing for years.
So there you have it!