Question about screws

[_Ryan_]

I am getting to the point in my build where I need to consider the type, coating, and length of screws on the build. Specifically I am curious about the screws to attach the sidewalls to the deck and roof spars.

I am building sandwich walls |1/8"| 3/4" |1/8"| so the screw heads will be covered by the outside skin (not to mention epoxy and spar varnish outside of that). Check my design thread for more pictures.

My question is what type of wood screws would you all recommend? What length? And does it matter if they are zinc, galvanized, or SS if they will never be exposed? Zinc is by far the easiest to come by around here.

Thanks!

[Thawley]

The "best" fastener for you depends on your joinery, the wood and grain direction you're screwing into. Are you using screws mostly for alignment to hold things in place as the glue sets up, or are the screws providing all the joint strength? Can't really tell from the drawings.

My general thoughts:
hardware store sheet metal screws are higher grade than commonly found wood screws
SS are usually higher grade than commonly found zinc or galv screws.
Common SS are not usually as strong as some of the specialty (hardened), deck or ProMaster type screws. If you're relying on the screws for all your joint strength (not ideal), I'd be looking there. http://www.mcfeelys.com/ has a great selection to get exactly what you need. I'm fond of the ProMaster type – either black oxide or the coated ones.

A simple dado and some glue will add a LOT of strength to that sidewall/deck relationship. Highly recommended if you haven't started your millwork yet. That takes the shear load off the joint and leaves the screws to just worry about tension – which is what screws do best.

[_Ryan_]

I will be using the screws mainly to hold the glue (probably PL Premium) for the joint as well as to aid in dry fitting the sidewalls. The screws will go through the 1/2" ply side into the edge of 3 1/2" poplar deck frame. Deck screws seem like a logical choice.

Is there a hard and fast rule on when to use SS vs zinc screws? I don't see any way for water to work it's way into the joint but I guess condensation could play a factor in the long term.

[Thawley]

Have to add another thought since I've been thinking about it most of the day, anyway...

IMHO, the most important joints in a TD like yours are where the cabinets/bulkheads meet the sidewalls. The roof spars and deck aren't triangulated. They hold vertical shear loads only and don't like torsion. It is the cabinets and/or bulkheads which give the cabin "box" it's torsional strength. Get those joints right so you've got ZERO torsional flex, and the deck and spars have a fairly easy job with the vertical loads. Leave enough flex in the box, though, and the spar and deck joints have to deal with torsion also. And THAT is when fasteners start to fail.

Your cabinetry is very similar to what I have in mind. Strong cabinets and strong cabinet-to-sidewall joints will make every other joint and fastener's job a lot easier.

[Thawley]

Is there a hard and fast rule on when to use SS vs zinc screws?

Depends on who you talk to. I'm more of a furniture/cabinet guy than a boat/RV builder. That said, is the cost difference of SS or coated fasteners worth risking a possible corrosion issue 10, 15 or 20 years down the road? If you're an RV builder trying to stay profitable, the answer's a lot different that a guy building something he might pass down to his kids.

I'm planning to use SS on all trim and everything exposed. Coated hardend steel on everything else. I've broken enough common zinc screws that I try to avoid them anymore. It's a strength issue for me – not about corrosion. I'm sure there are some truly superior zinc screws out there. But they seem to be rare in the big box store and hardware chains I use.

[aggie79]

+1 on the use of SS for exposed screws/fasteners and coated screws/fasteners where concealed.

[Lgboro]

I rotated biscuit joints and dowel pins to secure all my wood parts. All my aluminum trim I used stainless steel screws and stainless bolts to attach to frame. Made over 11,000 miles this spring/ summer without any issues.

[_Ryan_]

I appreciate all the good suggestions! As far as the strength of different types of screws I found an interesting article on the subject.

http://woodgears.ca/joint_strength/drywall_screws.html

[Thawley]

I appreciate all the good suggestions! As far as the strength of different types of screws I found an interesting article on the subject.
http://woodgears.ca/joint_strength/drywall_screws.html

I love his method, but wish he'd included several things more relevant for our purposes: deep-thread, production type wood screws (without self-countersinking heads), hardwood and/or top-grade plywood.

He makes note of the lower clamping forces of the self-countersinking screws, but just leaves it at that. Looks like his results do have one instance where the self-countersinking screw likely did NOT try to drill itself all the way thru the wood... It produced the highest score in the test. Would have been nice to see the same test in hardwood. Or without the self-countersinking heads...

That said, I would never have guessed common drywall screws would do as well as they did. Thanks for posting!

[Kody]

Very interesting info about the various types of screws that are most commonly used for our T/D's and other things we do. I'll toss in my 2 cents worth of what I know and it may help you to make a good decision in screw selection.
The green coloured screws are a specially treated steel screw for use with the green coloured treated pine used for outdoors. The treated pine will corrode a "normal" steel screw completely away due to the highly corrosive chemicals in the timber. No other screw is suitable for this style of treated timber. You can use a very heavily zinc coated screw but this too will eventually corrode. Never use stainless steel screws when fastening the treated pine, stainless steel is the worst material that could be chosen for treated pine.
The brown "decking screws" are similar to the green ones and are used for outdoors but may not last as long if used on treated pine.

Stainless steel screws should not be used where a lot of moisture or rain is part of the climate. They corrode to leave nothing holding the wood. The top of the screw will always look bright and shiny with not a sign of corrosion. The corrosion is created by blocking the oxygen getting to the shank of the screw. The water seeping into the wood will do this and then corrosion starts. Stainless steel is "rust proof" (?) because the oxygen in the air forms a thin coating of chrome/nickel oxide on the surface that resists the extreme corrosion as found on stainless steel bolts and screws in a moist or wet environment. If there is no oxygen, there will be no bolt or screw. The head is shiny because of the free oxygen in the air.

I have never heard of "zinc" screws so I can't comment other than to say that zinc is very brittle and it does corrode to leave a white powder, - Zinc Oxide.

Brass screws are very good but they are brittle and will snap if overloaded with a screwdriver, ask me how I know. Brass will also corrode and needs protection in a salty environment or better, not used at all .

If you want the ultimate strength and corrosion resistance from a screw, use screws and fasteners made from "Monel". This is an alloy of Copper (40%) and Nickel (60%). It may have some other elements in the mix to increase its strength. Monel has a nominal tensile strength equal to many of the high tensile steels.
An excellent fastener to use in conjunction with monel screws are monel nails. These are used quite extensively for boat building and have serrations on the shank. The serrations slope towards the head of the nail and are easy to drive in. Getting them out is another matter, especially when driven into a hard wood. They are perfect for use after the main (monel) screws have been driven in and it's essential that the panel be clamped down when or before the glue dries. They are much quicker to use than a screw of any sort. The only problem with monel screws is the initial cost but they are about the same as stainless screws (??). If you go the Stainless Steel route, make sure the Stainless Steel used for making the screw is "316" grade.

The type of head on the screw is more of a personal choice. The best results when using screws as a permanent faster is obtained by applying glue to the timber first. All joints used in your TD should be glued up before screwing the sheet or joint together.
Hope this helps a bit.

Kody

[eamarquardt]

Here is a picture of a SS nut that was on my boat. This is what happens to SS when oxygen is excluded:



It, in that state, wasn't entirely effective at holding the 3.5 tons of lead keel tight against the hull.

Cheers,

Gus

[VijayGupta]

I avoid drywall screws except for, well, hanging drywall or rough, temporary stuff. I've just seen too many of them break because of their brittle nature.

There are a lot of variables to consider
* materials being screwed together (e.g., softwood, hardwood, engineered wood)
* surrounding material (galvanic action of water, acids, alkaline, metals, etc.)
* drilling pilot holes or not
* type of stress - shear, tension, peel, sudden, gradual, continuous

[eamarquardt]

Top of the line stainless hardware, same stuff I used on my sailboat. Pay through the nose for it but when you're tacking with all that stress you know your hardware has your back.

Yup, in the Santa Barbara Channel (where I spent all of my time) a nice summer afternoon "breeze" is 35 knots or so. Locals call the channel "windy lane". Non locals call it "hurricane gulch". I've been out single handing in 50 knot winds and green water over the bow. Just me and Herr Auto (pilot). Loved it!!!!!!!

Cheers,

Gus

[NathanL]

Be VERY careful about making broad statements about what screw material is best without knowing what they are fastening. Monel is a great material and we use it a lot, however used against Aluminum it's asking for trouble. They are about as far apart on the similar metals chart as you can.

The closer the materails are on this chart the better. The further apart you are asking for corrosion problems.

Mercury
Monel
Nickel
Silicon Bronze
copper
Brass (Red)
Bronze
Brass (Yellow)
Bronze (Phosphoros)
Tin
Lead
Stainless Steel
Iron
Mild Steel
Aluminum
Cadmium
Galvanized Steel
Zinc
Magnesium

[Kody]

The very best and safest material for holding the keel onto the boat is silicon bronze or monel. Stainless Steel is never used as keel bolts here in Oz by any yachties that I know. Silicon bronze is expensive but you can trust it with your life as a bolt for attaching the keel.
I have seen SS bolts completely wasted away from corrosion (3/8" dia down to 1/8" dia.) but the hexagonal head and nut still looked perfect. It's bad news stuff if your hoping to use it for protection against corrosion in a wet area and especially in timber that is often underwater.

Kody