fractures at opposing 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.

 

[Mike Mike]

I don't do a lot of rafter systems but here are my thoughts: It looks like the wood split along the grain. It seems like higher quality lumber would have aligned beam and grain orientations better. Is it possible there were minor splits here since installation, and the bending stress along the bottom of beam is exacerbating them by having some component of tensile stress perpendicular to grain? What kind of snow load does this building see? The rafters seem a bit small for anything up north.

 

[Said the Sky]

interesting this ceiling is vaulted with no ridge beam, are the exterior walls holding up well from the thrust?

it looks like the rafters moved overtime as they dont appear to be perfectly aligned, could this movement have caused the splitting at the ridge joint?

as previoulsy mentioned the rafters look small