You mention corrosion in the hanger, so I assume the sub-purlin is also rotted? but was the failure in the hanger, or the sub-purlin due to rot?
If water is coming from above, then I think you have to investigate the sheathing, and also the purlins. I assume you have wood purlins (solid sawn or split lam).
I’ve seen conditions where the top of the purlin is rotted out at the center (below the sheathing joints), but you can’t see it from looking at the sides of the member—you have to remove the sheathing.
If you can convince yourself this hasn’t happened, and the areas affected aren’t large enough to worry about lateral load, I’d look at sistering a second sub-purlin next to the original and attaching at the ends with either an open end double face hanger, or a 2x ledger board placed underneath the sub-purlins. Rot in the existing members won’t continue if the source of moisture is stopped.
Another possibility to consider or watch for is a slipped-hanger, where because of shrinkage and construction tolerances with the panelized system, the top lip on the F hanger slips off the purlin. This results in a failure, though not due to corrosion or rot. If this is a possibility, email me off-line and I’ll send you an article with some further discussion of slipped hangers.
I am looking at some roof repairs (yeah, I've finally made the "Big Time") at a commercial structure. It is a typical panelized roof with 2X4 sub purlins spaced at 24" spanning 8'-0". These sub purlins are attached to the purlins with FN hangers. Somewhere along the line, moisture got between the roofing membrane and the insulation foil which allowed corrosion to attack the FN hangers. Recently, one sub-purlin failed. Fortunately, no one was in the space at the time.
The question I have is regarding the attachment of the replacement sub purlin to the existing roof diaphragm. Of course, the owner would rather not remove the existing roofing to re-nail from above. However, considering that the diaphragm is probably only 1/2" thick, I really don't see any other way. Some of the sub purlins are attached at the panel edge some are not but the only way to know that is to remove the roofing.
Does anyone have any ideas?
T. William (Bill) Allen, S.E.
Consulting Structural Engineers
V (949) 248-8588 • F(949) 209-2509