Wind Mitigation - two layers of decking

Is two layers of decking an issue for wind mitigation inspections? If the 8Ds only come through 1.5" are they still consided 8Ds since they have the effective penetration of 6D nails. Since there is a deck thickness accommodation made for dimensional lumber, wouldn’t 2 layers of decking be okay too?

As far as I know, 10d nails are only required for roof decks between 1.125" and 1.25" thick. How did you determine there were two layers of 1/2" anyway?

Were you able to slip a ruler between the deck?, I have never encounter two layers of decking.

Here it is - they were gold ring shank nails:

Mike, that looks like T & G decking, which if it less than 6 inches its one nail per board greater than 6 inches its two nails per board.

Just has to be a nail. It looks like an 8D.

Thanks Shawn. What I’m trying to ask about is if there is another layer of plywood on top of the T&G which is probably 1/2" then the full deck thickness would be 2x a normal roof decking, right? So the 8Ds would only go into the trusses 1.5" making them only as good as 6Ds.

I saw this twice in a day - the other one was around a roof vent where I could see two layers of plywood. But if it is not that common then it probably isn’t worth getting excited about.

There won’t be an extra layer of plywood on top of the T&G as this would probably not pass inspection. They might build up the roof vents with plywood but the whole deck would not be two layers. The T&G is all that matters to the Underwriters. Just has to be a nail bud, remember the spacing in my OP, if your measuring the nail at 1.5" then add .75" and you will get 2.25" total nail length which makes it an 8D.

We used to require(going back to MSFH) 10d for a plywood layer on battens. Without some calculations form an engineer, I would require 10d.

Yes, it may have been a one-off situation with this roof which was installed in 2009. The T&G was pretty old/stained/deteriorated and the house was a 1924. The ring shank nails were only coming through 1.25" at each shiner (and there were many).

If there were many shiners, as you say, then the roof was not nailed to code and may not have proper uplift. That would Nick out two credits. This wa not an open beam ceiling, was it?

Just saw your pictures. Not open beam ceiling. But just from that picture there are way too many shiners. If you do have plywood on top of the plank, what is holding it down? How can you check nailing pattern with that many shiners. And becuse the roof is a system, how can you check roof covering (although I already know the answers I will get).

You have more going on here based on the described situation, the devil is in the details! Specifically, the OIR-B1-1802 only references common nails not ring shank nails, and you have ring shank nails in the photo. Also, if there are visible fasteners for the deck showing as shiners they are code violations and you are documenting it in a verification form. On one hand you are saying the decking is attached in accordance with the listed provisions, while showing it is not in the photo. Going back a bit you said the plywood was installed over top of the T & G decking that was deteriorated and stained, this would also be a code violation as the substrate for the new decking is required to be adequate for attachment:

**2010 Florida Building Code: Existing Building 611.2 Structural and construction loads. ****
The structural roof components shall be capable of supporting the roof covering system and the material and equipment loads that will be encountered during installation of the roof covering system. **
611.3 Recovering versus replacement. **
New roof coverings shall not be installed without first removing all existing layers of roof coverings where any of the following conditions occur: 1. Where the existing roof or roof covering is water soaked or has deteriorated to the point that the existing roof or roof covering is not adequate as a base for additional roofing.

5. Where the existing roof is to be used for attachment for a new roof system and compliance with the securement provisions of Section 1504.1 can not be met.

Your original question was “is two layers of decking an issue for wind mitigation inspections”? You bet, single layer or double layer, it has to be done correctly and fastened in accordance with the Florida Building Code. Period! Based on the information you provided, both in pictures and verbal, it is not fastened correctly and was applied over material not suitable for reattachment of the decking. Also, the application of decking over top of another requires calculations for increased load on the roof structure. Were these calculations part of the roofing system replacement submitted for permit?

You can apply another layer of decking over an existing layer, that’s not a violation. But, any new layer of material must at minimum meet the provisions of the current building code for application….and must be adequately attached to the roof structure through the decking in compliance with the Florida Building Code. At the point of adding the new deck, chapter 9 of the Residential Code applies and states qualifying nail length for 8D ring shank nails is 2”:

**2010 Florida Building Code: Residential R803.2.3.1 Sheathing fastenings. ****
Wood structural panel sheathing shall be fastened to roof framing with 8d ring-shank nails at 6 inches on center at edges and 6 inches on center at intermediate framing. Ring-shank nails shall have the following minimum dimensions:

5. 2 inch nail length
All things being equal, if the qualifying nail length for 8D is 2” and you are showing 1-1/2” of fastener at the bottom side (no way of knowing if the nail is counter-sunk or under-driven I might add) then it would deem to comply with chapter 9 of the Florida Building Code for proper decking attachment. The issue then becomes, the use of ½” wood decking if the original decking is not sufficient for attachment. The Florida Building Code requires use of minimum 5/8” roof sheathing:

***R803.1 Lumber sheathing. *Allowable spans for lumber used as roof sheathing shall conform to Table R803.1[FONT=“Times New Roman”][/FONT]


All things considered, you have a pretty big issue here……go to the building department and investigate further, that is my advice.

But hey, what do I know, it’s all cut-n-paste with underlining!

Robert R. Sheppard*** ***
Licensed Home Inspector HI #3289
ICC Certified Residential Building Inspector B1 #8325007
ICC Certified Residential Electrical Inspector E1 #8325007
ICC Certified Residential Plumbing Inspector P1 #8325007
Registered Professional Inspector RPI #0796
Florida Association of Building Inspectors, Board of Directors/Legislative Chairman
Miami Dade College: Field Instructor, Home Inspections

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This is common with the older homes plywood over dimensional lumber. You just have to find a way to measure total thickness along with the nails so you can come up with a logical deduction. I would hope they nailed with 10’s.

Well a 10D nail is 3 inches, so…

.5" for the plywood and .75" for the T&G and 1.5" showing for the shiner would be 2.75" total. close to a 10D

8D is 2.25"

Don’t hold my plywood and T&G measurements I know they are more metric.

But if there is plywood over T&G what would we go with, T&G just requires one of two nails per board over plywood requiring nail spacing??

Oh jeez, the form does not specify which type of nail in the case of T&G (no second layer of plywood):

But I would guess 6D would disqualify a normal T&G roof from getting C anyhow, right?

nope :stuck_out_tongue: call it what you see it in there eyes, T&G :cool: that is the way this form works, till they let an HI or Contractor redesign the form.

I would think the plywood is now the primary decking. If you loose the primary decking then the rest is vulnerable to water intrusion and wind damage. So the plywood has to be installed to code regardless of what’s underneath.

Looking at the pictures, you have tongue and groove wood with one or two nails, so far, a ©. Then you have at least 15 nails that missed the truss which one would have to assume was for the plywood overlay. Since those nails missed the truss, they are non-compliant with the re-nailing guidelines.

The form asks for the weakest form of attachment. Nailing into thin air would satisfy that requirement, so, I would mark (E) Other and state improper nailing.

I would have the homeowner contact the building department and have them come out and do an inspection. When they cite the improper nailing, then whoever did this can come back out and try again.

I usually would mark E for that one. Missing that many nails you have too. Hate to fail it, but sometimes we have to do it.

Eric and I agree!

Also, ring-shank nails have roughly 3 times the uplift rating as common nails (reference Prevatt 2009)…carry on.

Look at that, no cut-n-paste and no underlining…:mrgreen: