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Frost on roof deck

Scott Bachman

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New contributor. Hello to all.

I did a roof inspection on Friday for a lender. 1958 house, tongue and groove deckboard, with soffit, gable and ridge vents. There were two isolated areas of frost on deckboard in attic. One area of frost covered deckboard (3'x3') shows obvious leak stains (see photo). The area I am unsure is the other photo. This is a different location in attic, near the eave, above bedrooms. As the photo shows, the frost is on every other T/G deckboard. No signs of leakage/stains. The pattern is what throws me. This is my 1st frost in attic (moved from central Florida last summer). The shingle covering is poor quality, homeowner installed, several years ago.

Any info what be helpful.

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So, that's the weather side for NC, right? Seems like it might be a situation where some wind is moving through the roof somehow. Chad might be onto something - were these located on the cold north slope just about in the middle of the attic, so that ventilation at the bottom of the roof plane is almost nil near the eaves because the gable end vents have short-circuited proper eave-to-ridge ventilation.

Still, if that's all that's developed on the underside of that roof after half a century with the existing ventilation, I don't think it's going to be a ventilation issue, unless they've made changes to the ventilation configuration. If not, I think you have to figure out whether it's a long-standing non-issue or a recent development caused by something external that is going to become more serious as time goes on.

Now, unless the lender hired you as a ventilation specialist, or unless you've got a lot of confidence in your own ability, based on plenty of experience, and are ready and willing to stand behind any correction that you recommend, I hope that you are doing this as a mental exercise only and aren't trying to diagnose this for the lender, because you've done what a home inspector is supposed to, spotted an anomaly and have reported it. Now, recommend they get it checked out and repaired as necessary by a competent roofer - then follow up to see what the roofer found and had to do to repair it - otherwise, you're liable to find someone pointing to you later and saying, "Hey, don't blame me, I didn't diagnose what was causing it, I only did the repairs based on Scott's diagnosis and recommendations for how to repair it."



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The first photo is near the ridge and near the east gable vent. That one I can attribute to moisture leak.

The second photo is at the eave near center of attic. It is the cold north slope. Have seen mostly NW winter winds prevailing. The ridge almost certainly was opened up at the last shingle replacement (approx. 5 years) or the one prior.

This is for my information only. Other issues with the roof warrant roof repair/replacement. Last install was a poor attempt by homeowner.

My curiosity keeps going back to the pattern on the eave photo. It was very defined, every other T&G deckboard.

Thanks for all replies,


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As others have mentioned above as well, when I see homes with soffit and ridge venting as well as gable vents, power vents, etc. I note the attic ventilation was "Mixed Ventilation Types" and tell the client that some roofing shingle manufacturers (like Certainteed) require removal or covering up the older vents, and keep only the soffit and ridge venting. Certainteed puts that requirement right in their install manual; other companies, like GAF, Tamko, and Owens Corning "highly recommend" doing the same.

I also tell the client that should an issue arise in the future and a Certainteed rep comes out to investigate a claim that Certainteed may void the warranty on the spot because the ventilation requirements were not met. I have personally seen this happen in a home I inspected 4 years ago, I noted this condition, the new buyer did not heed my suggestion, and when they called the shingle manufacturer to file a claim and their rep came out, he noted the improper attic ventilation and did void the shingle warranty then and there. The home owner than called the roofer who installed the roof, but found out he had gone out of business 2 years earlier.

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This should be considered strictly anecedotal information. I saw a similiar situation several years ago and asked my Father if he had any ideas why it would happed, Realize my Father was a very fussy carpenter and built many houses.

His thought was that southern yellow pine often is booked after sawing and was often shipped with thin stickers in the bunk. As heat drove the sap to the interior of the board, the reversed board would have the sun side toward the attic space and would have more sap and consequently would be less prone to acumulation of moisture (frost).

Even if the explaination is wrong, at least it is a talking point and gets the client thinking about ventilation.

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