It is required that a Purlin, which is installed to support over spanned Rafters be the same dimensional size as the Rafter being supported. In this case with 2x6 Rafters, 2x6 purlins not 2x4's as pictured should be supporting the 2x6 Rafters. The braces (struts) supporting the purlins can be 2x4's. Those braces can be up to a 45 degree angle and need to be installed at a maximum distance of 4' apart. The collar ties shown are properly installed in the required upper one third of the roof frame and attached at every other rafter as needed. The 2x4's used for the collar ties seen here are adequate since 1x4 is the minimum size required. The ceiling joists are running parallel with the rafters and are attached to them at the top plate, thus having them also serve as rafter ties which are required at the bottom third of the roof frame area.
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Photo by JWK Inspections, Joseph W. Keresztury