“Regular” vs composite deck screws

JHZR2

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Is there any practical difference?

What I’ve seen is that there’s additional/ different threads up towards the head, I guess to get more bite in the softer composite material. I’d suspect that for some brands who sell both (GRK,deck mate), a #8,9,or 10 will be the same, screw to screw, the shear strength would be the same, pullout maybe superior for the composite type.

I’d think that when doing trim, be it with composite or wood, the composite type screws would pull the trim board tighter to the underlying structure. But is that a bad thing if my construction is fully wood?

For most stuff where Im not using nails, Spax is my go to. I do have some GRK structural screws. But for (wood) trim boards, I’m looking at other screws. Any reason to select/avoid the composite type?
 
Let me explain what they do because "better" is a judgment call depending on the specific application.

First, I have built 2 decks and 3 fences in my life so make no claim whatsoever in being anything more than a decent carpenter for structural "stuff"- but I know threads, threading, fasteners and joints at an SME level so that's the perspective I'm addressing this from and machinery's Handbook is the reference.

Funny because you can also find the physics in searched for screw conveyors either in CEMA or ANSI ( same concept)

The tunneling pitched screw thread portion is for fast moving media and pulling through- this is for overall loading along the axis thus the "strength"

The closer end pitch slows that velocity and creates bunching ( makes more dense) which can act like a seal or even a top cap on the fastening zone ( depending on the material density and grain its going into)

Some can have a "reversing thread" at the very end which acts almost like a "counter sink" thus adding rigidity to the head ( something I believe would be beneficial on a deck) and even a moisture seal.

So they do more and "bring more to the table" but "better' is more along the lines of how "good' you want it. I personally recommend using them in those applications to the exclusion of all other like products but that's a personal view.
 
The more recent installations I’ve witnessed using composite have been with the clips and screwed into the joists versus directly screwing the composite to the joist. The installer told me that some of the failures they saw were attributed to the screw. Rain water would seep slowly past the screw head and into the composite material which is part plastic resin and part wood. The wood part would expand due to getting wet this causing premature failure.

I would look into using clips if possible.
 
The installer told me that some of the failures they saw were attributed to the screw.

For discussion fodder only (nobody has ever commissioned me to do a failure analysis or engineer a wood joint so this is purely observation and speculation on my part)

I strongly suspect that these 'alleged" screw failures are more due to the screw proper being "inferior" to the loads and chemistry it is being tasked to withstand.

You basically don't have grade 5,8,B-7 etc. for them, no torque spec, coatings versus wood treatment chemicals reaction is largely questionable and material characteristics are all over the place.

I have been told by various manufacturers over the years that most "wood screws" are historically under designed because a properly designed one that was rated for a load basically couldn't be profitably sold.

I don't immediately accept that as more than the individuals opinion but I don't discount it entirely because if wood screws were held to the same standard that graded fasteners are- they would be radically different.

Just curious if you or anyone else has ever really looked into this issue or have specific observations other than a few obscure articles out there.
 
The conversation I had with a contractor was a few years ago. Here are some points I can recall;

The type of screw is very important. I don’t remember the name.

Pre-drilling was important to avoid any damage outside of the screw hole.

The screws are flat head for obvious reasons. If they are screwed down tightly where it creates a place for water to sit then that’s a problem.

The installation I saw used clips that fit into the groove on the side edge of the deck board. The clip was screwed directly to the joist using stainless flat head screws. The tongue on the next board installed hides everything. So far it has held up versus the previous installation which showed signs of failure around the three year mark. The company she bought the decking from approved the warranty claim and supplied all new material.


I recall seeing a episode of This Old House that had a composite deck installation. That would be a good reference to search for.
 
When composite decking materials were first introduced, the existing deck screws caused a lot of ”mushrooming” of the decking around the screw head. Both ugly and a bit of a trip hazard.

The plastic was softer than wood and behaved differently as the screw was driven in. The composite decking screws were developed to keep the plastic from deforming as much and mushrooming around the head. The extra thread near the head, the different thread pitch along the depth, all designed to mitigate that.

Sure, you can pre-drill and mitigate that, but when building a deck, predrilling thousands of holes is a pain. I know. I built a deck with tropical hardwood, and the best of screws would snap without predrilling. I ended up buying a new (at the time) 14.4V Makita driver/drill set. Two tools to drill and then drive the screws. I can’t imagine swapping drill and driver bits to get the job done. Either way, far slower than just driving the screw.
 
Pre-drilling was important to avoid any damage outside of the screw hole.

In my few discussions as you and Astro posted- this right here is what I have been universally told is the problem.

Never done an FEA on wood but have done some on synthetic materials for joints but I can see a "properly engineered fastener" in terms of load and all that damaging the wood without a pilot hole.

What currently exists is a "tweener"

True or false, I don't know but I can see the logic in it.
 
The more recent installations I’ve witnessed using composite have been with the clips and screwed into the joists versus directly screwing the composite to the joist. The installer told me that some of the failures they saw were attributed to the screw. Rain water would seep slowly past the screw head and into the composite material which is part plastic resin and part wood. The wood part would expand due to getting wet this causing premature failure.

I would look into using clips if possible.

I have nothing to clip. I’m looking to screw fascia boards to the rafter tails on my garage.

120 feet straight on each side. My new garage never had gutters, so I’m replacing the fascia and setting it up for gutters, so the roof dropping doesn’t splash up on the doors.

So a 1x6 fascia screwed to the rafter tails is what I’m dealing with. If the secondary thread intended for holding the thinner decking piece to be held down/in tight.
 
Do the composite deck screws countersink o.k. when used with real lumber?

I'm curious how real lumber that expands and contracts quite a bit reacts around the threaded portion of a composite deck screw embedded in the fastened (deck/facia) board.
 
I'm curious how real lumber that expands and contracts quite a bit reacts around the threaded portion of a composite deck screw embedded in the fastened (deck/facia) board.

It would have to react by exerting circumferential pressure all around it up to the limit the remaining mass and grain structure would rebound to the dimension of the inserted screw
 
Do the composite deck screws countersink o.k. when used with real lumber?

I'm curious how real lumber that expands and contracts quite a bit reacts around the threaded portion of a composite deck screw embedded in the fastened (deck/facia) board.

That was sort of the basis of my curiosity. Is there a downside, performance reduction, etc. This isn’t really structural beyond the fact that water of ice could be in the gutter.

This may help.


Thanks, I had reviewed that previously. It does have some good photos. I went through that when I had to look into structural screws that would install easier than toenails in certain specific retrofit locations.
 
It would have to react by exerting circumferential pressure all around it up to the limit the remaining mass and grain structure would rebound to the dimension of the inserted screw

I was thinking about the fascia/deck board expanding and contracting across it's thickness (length of the fastener), not the friction of it gripping around the fastener.

This company has a special system for composite fascia that allows for expansion/contraction: http://starbornindustries.com/deckfast-fascia-product

A regular deck screw is somewhat similar to the above system regarding the flat shank directly below the head.

A composite deck screw has reverse threads below the head that embed in the "attached" board/fascia. As the board expands and contracts in thickness, does this affect how tight it is held to the framing member? Vs. the screws above, the attached board cannot move on the screw shank to accommodate expansion/contraction.

I hope this makes sense. Probably a moot point that only us ODC'ers think about.

Conventional............................................vs. ..................................................composite
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IMG_0906.JPG
 
A composite deck screw has reverse threads below the head that embed in the "attached" board/fascia. As the board expands and contracts in thickness, does this affect how tight it is held to the framing member? Vs. the screws above, the attached board cannot move on the screw shank to accommodate expansion/contraction.

Makes perfect sense

That reversing part works just like on a conveyor screw- it "pushes" media back to keep it from bunching/mushrooming and the "head" pushes against this denser mass so it will seat tighter. That's no different than a mechanical application of the same.

The expansion/contraction part is a different matter as that allows the material to breathe ( in terms of moisture, mechanical relaxation, moisture etc.) without affecting the properties of a fastener.

Both are different ways to direct/manage material stress relative to a fastener- whats "right" or "best" is determined by the specific material in question. ( and what the desired end effect is to be) Its a floating answer.
 
So, to answer the OP question. The composite deck screws are designed to hold a horizontal composite deck board to the framing below.

They might work to hold a verticle wooden roof fascia board to the wood rafter end o.k., but it is not designed for that and one would be in uncharted territory. We don't have any evidence that it is better (or worse) than a conventional deck screw with smooth shank below the head.
 
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