roof: 10 yrs old, rusty nails?

Question

I had to take off part of a roof that we put on about 10 yrs ago, and found a fair number (but not all) of the nails were showing considerable rust. It's a hip roof, about a 40-degree pitch, and the areas I looked at were around a dormer valley and along the dormer ridge.

Roof is 3-tab shingles (4.5" exposure) over felt paper. The valley is 'open', with generous roofing cement under the exposed shingle edges. Sheathing is 1/2" plywood over 3/4" slats. We get some snow (Syracuse NY), a fair number of freeze-melt cycles, and it can be pretty humid. I don't see any other evidence of leakage or water penetration.

So, I'm wondering why the nails are rusty. A roofer (a relative) said that electro-galvanized nails just rust in this climate. Someone else suggested that it's evidence of inadequate ventilation, where moist air is leaking from the house through the nail holes.

Any speculation would be appreciated.

TIA, George

thunder · Accepted Answer

"Considerable" varies with one's eye. Discolored isn't a problem, as all nails, galvanized or not, will eventually rust. Rusting to the point of failure, is another matter.

I would think this to be a stretch. Are there any other signs of inadequate ventilation?

Randd01 · Answer

Doubt it is a ventilation problem, but it is easy to check. The next real coldgo up in your attic and see if there is frost on the underside of the roofplywood. If there is it could mean poor ventilation. Do you have ridge ventsand soffit vents. Hips are tuff to get ventilated without them. Some poeple useturbines also. Also ck to see if the nails sticking thru the plywood underneathare rusty also. Most likely it is normal discoloration from high humidity days.I don;t think it is any reason for concern.

roof: 10 yrs old, rusty nails?

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