Two Layers of DECK ARMOR for Long Exposure and Rough Weather


Opinions vary on whether DA can withstand heavy and wind driven rain (and no discussion of snow that I could find.)

I have observed that at horizontal laps, the top layer will look soaked while the bottom layer is dry.

Also, after a rain, DA dries like cloth: the water wicks downward so the upper portion will be dry while the bottom will be soaked - the bottom edge the last to dry.

It seems to me that two layers would address all these concerns - including UV where the top layer would protect the bottom layer and extend its live beyond the 90 or 180 (I don’t understand the two specs for DA.)

I have some splice DA areas that will be exposed over winter (Pittsburgh, PA) and I’m thinking just another layer instead of a tarp.

Regarding PERM: One might argue that 2 layers cuts the perm spec in half and is undesirable. Since asphalt shingles’ perm of about 1 makes them almost a vapor barrier, a double layer will not have much effect diffusion of water vapor through the shingles - half of nothing is still nothing. Evaporation of trapped water into the structure would get the same argument. By the time you make it through the sheathing, even a double DA layer would still be a high perm.

NOTE: I may try one of the SIGA underlayments for the 2nd half next spring.


If you want the strongest toughest underlayment out there get some Titanium UDL 30/50 and secure it with hand driven hot dipped galvanized roofing nails, this will last much longer than it reasonably should.

The next best method of attachment would be plastic cap nails followed by plastic cap staples.
Cap nails are better than cap staples because the staples sometimes cut/blow through the cap and then the cap falls off and that spot is a potential leak.

Synthetic underlayments don’t have any self healing or fastener sealing properties so it is extremely important that the deck is clean of any debris and any old fasteners or proud nails that could poke holes in it from underneath.

I avoid GAF products like the plague that they are so I can’t comment on Deck Armour except that it is probably one of GAF’s better products.

IMO 2 layers is a complete waste of resources.


I know the original Titanium is a vapor barrier which is verboten in a house with no ventilation (as is printed on the package!) - this is a 30’s stucco house that “leaks like a sieve” and my plan is to keep it that way. There is no VB in the walls (or insulation for that matter) and while the stucco was painted 50 years ago, the paint has not spalled off since moisture behind the paint was able to diffuse into the interior before freeze/thaw damage could develop.

I am 100% against any vapor barrier except under a concrete slab. And even that creates a soil “abscess” which does not have a healthy eco-system.

When (not if) water finds its way behind a VB, mold follows.


Use 1 layer of Deck Armour and secure it with hand driven roofing nails.


I would not double up deck armor. It is breathable, but not that breathable, 2 layors would probably come close to making it a vapor barrier, totally canceling out paying a premium for it twice. Just my opinion.


Hey Axiom!
Could you explain to me why you like the hand driven roofing nails instead of the hand driven simplex?

Is it because the simplex caps can sometimes make the shingles not lay as flat?
And you have had great success with just the roofing nails?


How big is the area you are concerned about leaving exposed over the winner? I know it is your house you are working on. If it was my house and not too big of an area I would probably throw a layor of 90# on it with steel simplex and pop it off in the spring. Haven’t bought DA in a while but know it’s no cheap.


DA has a Perm of 16, Shingles 1. A double layer of DA would be 8 perm.

I lifted this from a search: “According to John Straube, published permeance values for #30 asphalt felt range from 0.5 perm to 3.0 perms. Straube suggests using 1.75 perm as a reasonable average value.”

So double DA would still be better than 30# felt.

RE: Nails in DA: I did DA on a 6 in 12 garage roof with Miami Date tin caps w/1in crown staples. The problem was when walking on it, the pinch was not enough to keep the DA from pulling on the staple legs at the hole. Same thing happens with roofing nails. With the caps, there is enough perimeter grip to keep the walking loads from reacting at the nail shank. I HATE CAPS because they are lumps under shingles - used to be a real No-No in bygone days…


Tin caps are another Florida code thing that arnt commonly used in the rest of the country. I have tried multiple ways to install synthetic and plastic cap nails in my experience are the only water tight way to install it.

If you do end up doing double DA make sure to do half roll stagger like the old slate double 30# specs.

If you haven’t figured it out yet I am not a big fan of synthetic underlayment.


In DA’s defense, it is not slippery when wet and it will not tear - I use strips of it for cordage! And I wouldn’t use Titanium under any circumstances - or any other vapor barrier.

Cap nails may be OK in plywood, but in old 1-by sheathing, I gave up trying to get them perfect. It’s almost like a hand nailed roofing nail is the only way to get them perpendicular - when you start driving the nail, you see which way it’s drifting and you compensate.


If using loose roofers would still nail through a plastic cap disc. The outer bead does a good job of diverting the water around the penetration and also helps to prevent tearing from wind.


IMO if one intends to leave the roof with nothing but underlayment on it for extended periods hand driven roofing nails will hold just as good, they sit tight & flat against the roof.
Simplex nails are fine also, I only hand nail the stuff when doing something small.

That being said I just use DiamondDeck (Certainteed) or Secure Start Plus (Malarkey) and secure it with my Stinger Cap Nailer, I only use 1/2 the amount of fasteners that the marks on the underlayment call for.
This has held up to 100+ mph winds.

This is a house I was working on while a storm hit, facing West on Lake Michigan all these trees got blown down, they were very large healthy trees.

My DiamondDeck stayed on the roof, I was completely amazed, some of the old shingles blew off.


We use the stinger cap nailer also
Because that gun is the best.
It doesnt jam. And doesnt waste caps.
I dont like the fasteners as much as i like the hitachi "pro pack"
But unfortunately the hitachi gun waste a lot of caps
And jams pretty easy.
It is really picky about its shooting position angle and its speed.


I have a cap stapler also but it blows through the caps because they are cheap, if you lower the pressure it leaves behind proud staples.

I didn’t have a problem with this when the blue cap DuoFast cap staples were available, it became an issue when they switched to the green cap knockoffs.


I bot an AJC Magnetic Hatchet and am hand nailing the “Orange Caps.”

Funny about the Stinger. Their hammer stapler shoots only 3/8in staples
which is just too short for caps. I believe that Stinger cap nailer has
been around for many years under at least 2 different names/manufacturers.


The Stinger cap nailer is just like the old SpotNails ones.


One layer or 1 layer double coverage its overlap is half a roll with nap nails will work thru bad winters done many


Aferrao319: Making sure I understand what you said: Either 1 layer or 2 layers will be OK for a winter exposure, right? What are nap nails?


If your planning on leaving it all winter I would halflap the underlay, and throw a tarp on it.


Nap nails are whatever nails the guys decide to use use while you are in the truck taking a nap.