fractures at opposing wood rafters (near ridge)

[PSUengineer1]

Hello. I have two rafters in a log cabin structure that are fractured near the ridge. The two affected rafters oppose each other. The rafters are not water-stained and fractures are not in areas of knots. The rafters span about 17 feet on a 12:12 pitch roof. The rafters are about 3 wide, 7 1/2 inches deep and spaced about 6 feet 5 inches apart. One exterior wall (in near alignment with rafters) has wood rot near its bottom where a deck was once erected (and inadequately flashed) adjacent to the exterior wall. Please see photos attached and let me know what you think caused the rafter cracks. Thanks.

 

[dhengr]

PSUengineer1:
In photo #6, I doubt that that 3"x7.5" rafter member would have been properly graded for that application and use. They probably were not actually graded members. They (NDS and grading rules) do not like grain slope like that which extends out to the tension edge of a bending member. And, this is right where the split starts. There are high horiz. shear stresses in that area of those members. Furthermore, any mis-cut of the ridge plumb cut, or geometric (settlement or lateral) movement of the entire roof system, would/might cause those members to be loaded right at their upper tips, causing a tension/bending stress component across the grain at that end of the member. Finally, in photo #10, I think I see some cracking in some mortar joints in the chimney, right up near the ridge, and the roof sheathing is very tight to the masonry, pushing on it, probably indicating some movement. I would need much more detail about the actual conditions to say much more. Is the rafter 17' long, on the slope, or is the cabin 34' wide, with two 17' spans, on the horiz. projection?

 

[PSUengineer1]

@dhengr: the rafters are 17-ft long on the slope.