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Malcolm 07-15-2007 03:18 PM

Water rolling back and hitting nails
Has anyone experienced this before? Do you guys go on a lot of leak calls or see a lot of rotten sheathing because of water rolling back under the shingle and rusting out nails. Nail heads are covered by less than a half inch in most cases. I would think water could get back up underneath the shingle and cause problems. Especially on a 4/12 roof.

Ed the Roofer 07-16-2007 05:51 PM

Water going uphill in a reverse gravitational scenario is called "capillary action". which can occur on shingles and standing seam metal roofs as well.

The diagram below shows it being inducted by solar induced thermal gradiant. The other phenomenon that also occurs is differintial in interior and exterior pressure, which can cause water to flow uphill into the seams.

Also, just because the nails are rusty does not mean a moisture infiltration occurrence. It very likely can be too much interior attic humididy cintinually permeating the decking structure and being conducted to the metallic nail heads and shafts. I have seen more rusty and nearly disolved nails from improper attic humidity levels due to inadequate intake and exhaust ventilation, which unfortunately is more common than presumed.


Malcolm 07-16-2007 09:30 PM

Wow, thanks for the reply. I was ripping off my old 3 tabs and noticed water residue lines that backed up all the way to the tar strip. The roofers that previously did the roof were horrible with nail placement. They nailed in the tar strip and above the tar strip. I was just worried about leaks because I plan on nailing my new roof on the nail line like the manufacturer wants. Do you actually put the point of the nail through the nail line on the shingle (I am installing architectural shingles with less than a 1/2 nail cover)? If I nail a little higher, I will still hit both shingles. I'm talking about nailing a 1/8" to 1/4" higher than the nail line. I noticed my neighbors roof was nailed this way. I don't know if it was because the roofer didn't know what he was doing or he did it intentionally.

Malcolm 07-18-2007 07:09 AM

Any help. I am nervous about capillary action causing problems. Just curious as to where pros nail their shingles.

MJW 07-26-2007 06:48 AM

It's a secret of the pro's. :yes:

The lines you saw are probably leaks that formed a line above the shingle. I wouldn't worry about it that much. If there is a big problem it is probably like Ed said, a moisture problem in the attic. Make sure your soffit vents are open and make sure there are enough vents on the roof.
Feel happy to be a homeowner and not have any normal leaks as you haven't hired a contractor.

Ed the Roofer 07-26-2007 01:21 PM

The location for proper nail placement is written on the bundle wrapper of every bundle of shingles I have ever taken the time out to read.

All other proper installation instructions are written on the bundle wrappers as well.


the roofing god 08-01-2007 09:44 PM

nail w/ the nails in the lines provided,not only does it catch 2 shingles(headlap),but it also secures the 2 parts of the laminated shingle together,look at the shingle and you will see the double thicknes doesn`t extend much past the line,and if you nail 1/2" higher you may miss it,which may allow the strip part of the shingle that closes off the dentil pattern ,to blow off, etc

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