Fasteners for Outdoor Projects

[Mike Goetzke]

It's been a few years sin I tackled a big outdoor project. I building a pergola and getting all the material. After watching a few videos I see many are using screws like Spax, FastenMaster,.etc. In the past I have built using 3/8" through bolts with nuts, carriage bolts, and lag bolts. With the wide selection of impact drivers are the high strength screws as strong as the bolts?

Thanks

 

[kevinculle]

If your design allows you will get higher attachment strength with through bolts and nuts provided you also use suitable washers under the bolt head and nut to provide adequate bearing surface.  These are most appropriate at the joints between load carrying beams and supporting posts.  As you step down in load carrying to joists that span the beams, rafters if there is a roof structure, railing attachments, etc. the self-tapping screws you describe will provide good strength.  I find Torx style heads to provide secure driving torque whereas Phillips style heads are more prone to camming out under driving torque.  Make sure all fasteners are rated for the lumber type, particularly treated lumbers.

 

[Packard]

The answer is:  It’s complicated.

Wood screws or any type have very high tensile (pull-out) strength.  Try using a claw hammer to claw out a nail, and it will generally pull out easily.  But try that with a wood screw and good luck to you.

But wood screws fare poorly in shear strength.  Construction bracket manufacturers have long insisted on nails for their installation and not screws for a good reason. The fixed hole size in the brackets limit the size of the fastener.  As I recall Simpson Strong Tie brackets call for 10d nails (0.162” diameter).  A wood screw with a similar outside diameter will probably have a core diameter (which relates to shear strength) of about 0.145 - 0.150”.

In addition, the screws get work-hardened during manufacture.  If you have ever taken a wire clothes hanger from the dry cleaners and bent it back and forth several times to break the wires you will understand that a work-hardened piece of wire becomes brittle.

All threaded screws become work-hardened to some extent.  The pressure of the rollers to roll in the threads does that.  With machine screws that is good.  Only the threads become hard and hard threads are good for durability.

But with wood screws, they are not only rolling in the threads, they are rolling in a taper.  So the threads become hard (a good thing) but the core diameter of the wire also becomes hard and brittle (a bad thing—especially in shear strength).

To make matters worse, screws are often zinc plated.  The plating process creates a situation called “hydrogen embrittlement”.  If you’ve ever assembled something made in China and the screw head snapped off while screwing it together, then you are aware of hydrogen embrittlement. 

Hydrogen embrittlement can be remedied by baking the screws at 300 degrees for a half an hour, but that must be done within 30 minutes of the end of the plating process.  Failure to do so will make the product very brittle.

A few years ago, Simpson (“Strong Tie”) came out with special screws for use with their brackets.  This was not done to save money.  They are much more expensive than nails.  And it was not done for strength.  It was done because there are situations where swinging a hammer is difficult (though a palm nailer generally works well).  It also makes disassembly easier (not usually a factor).

For use with construction brackets only use screws certified for that application.  Using Spax or other construction screws would be a serious error.

But if you need pull out strength, then screws are superior.

If you have ever witnessed the rehabilitation of an older steel bridge, you will notice that they are removing the old rivets and replacing them with high strength bolts and nuts.

It would appear that the rivets would have the advantage as there are almost always assembled with the fasteners in shear.  The rivets’ outside diameters are going to be larger than the core diameter of a bolt (the threads do not impact the shear strength.

But the rivets are simply peened over.  The bolts get torqued tight.  When they tighten the bolts it creates friction between the plate and the girder.  That friction creates more strength than the shear strength of the bolts.

So, if the structure you are building can benefit from the friction between the two pieces of wood, through bolts will work better.  But they may require periodic tightening as the wood shrinks with age.

Sorry to burden you with all this information.  It is complicated.

But always use screws that are certified for use with the construction brackets.  Or the nails that are specified.  Do not substitute construction screws for that application.

 

[dicktill]

Excellent reply [member=74278]Packard[/member], thank you! (Guessing there's some engineering in your background.)

 

[squall_line]

Since we're on the topic of "it depends", your local building code may also dictate construction style and fastener selection.  Wind loads, snow loads, etc. may apply in some places where it does not apply in others.

 

[Packard]

Excellent reply [member=74278]Packard[/member], thank you! (Guessing there's some engineering in your background.)

I was exposed to a lot of engineering on my job, but I am not an engineer.  I do feel I am on firm ground with my response above.

I hope it helps.

 

[Ron B]

Excellent reply [member=74278]Packard[/member], thank you! (Guessing there's some engineering in your background.)

I also appreciate your answer, Packard.  I always want to know "why".  If you know "why" then you don't mix bleach and ammonia, you don't throw gas on a fire, you don't stick your tongue on a frozen flagpole.  Even if it is a "double-dog dare".

 

[leakyroof]

  Building Codes in your neck of the woods will be an influence as well if that matters. As Packard noted, it's often the shear strength that many people forget about. Fine Homebuilding has done many articles over the years where they've touched on it, screws in construction, etc.
It's wet here in Chicago more than it's too dry... [wink] So, for heavier outdoor stuff like a Pergola, I'd favor Galvanized Bolts over construction screws for any of the primarily structural pieces . I'd leave screws for applying trim pieces. Especially for anything that's going to be over my head while I sit under it... [big grin]
Around here it's probably Code as well, but so much of that seems to be going by the wayside as builders try to hurry up very delayed projects due to Supply Chain issues. I casually look over new construction sites all the time around us and catch some amazing errors that either were present as being built, or slid into a problem shortly afterwards.

 

[Packard]

There are three basic types of galvanizing available for fasteners.

Note:  A standardized test is often used to determining the relative corrosion resistance of various finishes.  These tests do not tell you how long it will take to rust in the real world, but it will tell you how well it will fare compared to other finishes.  The most frequently uses test is ASTM B1117.  In this test they have a chamber which creates a environment that is particularly harsh for most finishes. The chamber has a highly elevated temperature, a high humidity created by using a saline solution.  This mimics a sea/ocean environment, but at a greatly accelerated corrosion rate.  These are rated by the time it takes to first see red rust.

I will give some approximate values that I recall:

Black oxide + oil.  100 + hours to red rust
Black oxide with the oil wiped off:  1 to 5 hours to red rust.
Nickel plating:  25 hours to red rust.
Bright nickel plating:  20 hours
Zinc plating with a clear chromate:  85-95 hours.
Zinc plating with a yellow chromate:  115 hours.
Commercial pre-galvanized wire (coating thickness of 0.0003” nominal:  20 - 25 hours.
Note:  There are higher grades of pre-galvanized material, but I am not familiar with those.
Hot Dipped galvanized:  250 - 500 hours (depending upon coating thickness.
Powder coat (paint): 200 - 400 hours
Dip spin (bulk paint/coating:  200 - 1000 hours
 

1.  Hot dipped galvanized.  The parts are dipped in molten zinc.  Time and temperature control the coating thickness.  Finish adhesion is excellent.

2.  Cold (or “impact” coated) galvanized.  The cleaned parts are put in a tumbler with balls of zinc/tin alloy. In theory, this should equal hot dipped’s performance.  However the adhesion of the finish is not very good.  It is a particularly poor choice for nails as hitting the heat with a hammer will often knock off the finish. The same can be done with screws as the bit could knock off the finish.

3.  Electro galvanized parts (zinc plating).  The coating can yield between 25 hours to about 200 hours depending upon the coating thickness and the chromate conversion bath used at the end of the process. I would not use these on outdoor furniture or on decks.

Organic coating (various paints) can get anywhere between 100 hours to 1000 hours of salt spray.

Note:  There are two ways that you can protect steel (iron) from rusting:

1.  Coating integrity.  An unbroken film of coating over the entire part.  This is the way paint protects.

2.  Galvanic anode.  If you remember your periodic chart of elements, then you will recall that sodium was very active and gold was very inactive and all the other elements fell in between.  If you place two elements of differing activity next to each other, the element with the higher activity will have to corrode entirely before any of the lower activity material starts to corrode.

So since iron (FE) has a periodic code number of 26 and zinc (ZN) has a periodic number of 31, the zinc would have to be “sacrificed” before the iron (steel) could start to rust.  So zinc plating and galvanizing provide both coating integrity and galvanic protection. 

While chrome (CR) has a lower number of 21, so the iron would be sacrificed to protect the chrome.  Chrome is not the perfect rust protection for harsh environments.

Cadmium has a much higher number (48) but because of environmental concerns and cost is rarely specified for consumer items.  The military does specific that finish though.

(I rambled on way more than I intended.  So I will stop here.)

 

[Cheese]

I prefer to use stainless fasteners for any outdoor projects. In the last 20 years their availability has increased ten fold. Typically I purchase 303/304/305 grade fasteners and on occasion 316 stainless. 316 SST used to be 2X the price of the other stainless grades but recently that's gone down to only a 20% premium. So SST screws for small items, SST nails for siding and SST bolts for larger projects. Buy once...cry once.

 

[Bob D.]

I have been ordering SS staples, brads, and other fasteners
from www.fastenerusa.com for use on outdoor items.

Prices are reasonable and delivery has been quick.

No complaints, I have bought over 20,000 brads and staples
from them so far.

 

[Packard]

I prefer to use stainless fasteners for any outdoor projects. In the last 20 years their availability has increased ten fold. Typically I purchase 303/304/305 grade fasteners and on occasion 316 stainless. 316 SST used to be 2X the price of the other stainless grades but recently that's gone down to only a 20% premium. So SST screws for small items, SST nails for siding and SST bolts for larger projects. Buy once...cry once.

For a lot of applications stainless steel is ideal.  But stainless steel cannot match the strength of high strength carbon steels, though run of the mill screws are not made from high strength steels. 

But the screws for Stimson fasteners are made from those grades of steel and stainless is not going to be an option.

Also note that stainless steel into aluminum can cause “galling”, where the two metals fuse together and in some cases (stainless and iron castings) they can actually weld together if heat is generated at the installation. 

The fact that galling occurs may not be an issue if the parts are not to be disassembled and reassembled.  If it is, there are thread lubricants that are anti-galling.
https://www.dicronite.com/anti-galling-coating/

The galling issue is for screws and bolts only.  Nails and staples would not be affected.

 

[Bob D.]

SS works fine for what I use it for.
Not building anything structural with SS staples or brads.
I use mainly with Cedar for outdoor projects.

 

[Packard]

Perfect for that.

 

[smorgasbord]

A pergola sounds like a good excuse to buy a DF-700 and some large Spiro dominos!
This is FOG, right?

 

[Packard]

A pergola sounds like a good excuse to buy a DF-700 and some large Spiro dominos!
This is FOG, right?

I wonder if you can install cedar shake shingles with a domino? [smile]

 

[Cheese]

Also note that stainless steel into aluminum can cause “galling”, where the two metals fuse together and in some cases (stainless and iron castings) they can actually weld together if heat is generated at the installation. 

The fact that galling occurs may not be an issue if the parts are not to be disassembled and reassembled.  If it is, there are thread lubricants that are anti-galling.
https://www.dicronite.com/anti-galling-coating/

The galling issue is for screws and bolts only.  Nails and staples would not be affected.

Actually stainless grade XYZ doesn’t like to be fastened to stainless grade XYZ. So if you use a 304 bolt, use a 316 or 410 nut to minimize galling.

However, I prefer to just use the same stainless grades together but coat the threads with Pure Nickel Nuclear grade Never-Seez. That stuff’s the best.

 

[Packard]

Many, many years ago I walked through the factory at Farberware (Bronx, NY—terrible and scary neighborhood).

The manufactured pots from stainless steel.  The used a drawing die to stamp out the form.  Those dies were made from phosphor-bronze. 

I asked about that.  Why not steel?

The answer was that there would be enough heat generated on the first or second part to weld bits of the stainless steel onto the iron/steel form.  The next part made would have deep score marks on it. 

I’ve seen that happen on stamping dies too, but generally a good water-soluble stamping oil will prevent that.

 

[Bob D.]

Pure Nickel Nuclear grade Never-Seez.

Yes, it it amazing stuff. A little bit goes a long way too. Get some on your gloves and before you know it everything you own is coated with Never-Seez.