Screw Size Usage?

[Kent A Bathurst]

Just occurred to me that I don't really know the criteria for choosing between, say, a #6 wood screw and a #8. My on-hand inventory tends to influence my choice, but that seems rather inelegant. So:

I'm in a situation where I'm going to use your basic Philips flat head wood screw, 1-1/2" long. I have both #6 and #8 from which to choose. I will do the appropriate countersink with a tapered bit [like the Fuller gizmo], and wax the screws.

Is there any what you might call "science" that influences the choice between #6 x 1-1/2" and #8 x 1-1/2"?

In case length plays a role, then same question for 1" and 2".

[George Yetka]

Which one "Feels Right"? I feel 6 is more prone to snapping 8 is the point at which they very rarely break on me. I will use 6 on the shorter sizes in conjunction with glue and get tighten the joint "Enough" to set the glue. Aside from that make sure you pair the countersink appropriately. Beyond 1" I dont use #6. and beyond 1-1/2" I dont think I ever use #8

[Kent A Bathurst]

Beyond 1" I dont use #6. and beyond 1-1/2" I dont think I ever use #8

You're more conservative than I am. I'll take #6 up to 1-1/2" without a second thought, and #8 to 2". But that's why I asked - to get a sense of proper limits.

[George Yetka]

You're more conservative than I am. I'll take #6 up to 1-1/2" without a second thought, and #8 to 2". But that's why I asked - to get a sense of proper limits.

Kent I use screws to guide me. I buy the GRK r4 screws in 1-1/4 up to 2-1/2 they are #8 up to 2-1/2" and at 2-1/2 they have #9 as well I use 9 in the 2-1/2. For 1/2" to 1" I use Mcfeely black screws which are available 6-10. But I have no real knowledge of whats appropriate. I do know I dont like SS screw

[Kent A Bathurst]

Kent I use screws to guide me. I buy the GRK r4 screws in 1-1/4 up to 2-1/2 they are #8 up to 2-1/2" and at 2-1/2 they have #9 as well I use 9 in the 2-1/2. For 1/2" to 1" I use Mcfeely black screws which are available 6-10. But I have no real knowledge of whats appropriate. I do know I dont like SS screw

I get it - not familiar with the GRK, but they make part of the decision for you based on product offering. That's actually pretty cool.

Oddly enough, I'm building some stuff for outdoors, so SS screws are the game here.

Thanks for the input George.

[Tom Bain]

I get it - not familiar with the GRK, but they make part of the decision for you based on product offering. That's actually pretty cool.

Oddly enough, I'm building some stuff for outdoors, so SS screws are the game here.

Thanks for the input George.

FYI … You probably won’t even find a lot of options for #6 SS screws. Most SS are geared toward decks, siding and such so they tend to start at #7 or #8. However, you can get trim head SS screws if you want to make them less noticeable. I just repaired the soffit of my barn/workshop and used a ton of #7 x 1 5/8 trim heads for that.

[Edward Weber]

FYI … You probably won’t even find a lot of options for #6 SS screws. Most SS are geared toward decks, siding and such so they tend to start at #7 or #8. However, you can get trim head SS screws if you want to make them less noticeable. I just repaired the soffit of my barn/workshop and used a ton of #7 x 1 5/8 trim heads for that.

I get my SS screws and other supplies from Albany County Fasteners

https://www.albanycountyfasteners.com/

They have a wide selection, even in #6
https://www.albanycountyfasteners.co...%2Fspan%3E%236

[andy bessette]

In the work I do #8 sees a lot of use. But the job dictates appropriate screw size. And I regularly use #6 through 14, with the occasional #16.

[Kent A Bathurst]

I get my SS screws and other supplies from Albany County Fasteners

https://www.albanycountyfasteners.com/

They have a wide selection, even in #6
https://www.albanycountyfasteners.co...%2Fspan%3E%236

Thanks - always looking for a new source.

My SS screws are basic Phillips flat head wood screws. Bolt Depot and McMaster-Carr. Bolt Depot sells any qty, with price breaks. McM-C sells quantities like 25-50-100, depending on the item. In quantity, McM tends to win the "delivered ptice" game.

[Edward Weber]

Length is easy to factor. You typically want 2/3 the thickness in the secondary piece. If there is no limit, twice the length of ht primary piece.
Gauge usually depends on usage, will there be stress on the fastener. Is it being used as a structural fastener or simply holding a panel in place.
Also hardwood vs softwood may change the size of the pilot hole and thread pitch. Finer threads for hardwood, coarse threads for softwood.
You might like this
https://www.rockler.com/learn/wood-screw-buying-guide

[Kent A Bathurst]

Length is easy to factor. You typically want 2/3 the thickness in the secondary piece. If there is no limit, twice the length of ht primary piece.
Gauge usually depends on usage, will there be stress on the fastener. Is it being used as a structural fastener or simply holding a panel in place.
Also hardwood vs softwood may change the size of the pilot hole and thread pitch. Finer threads for hardwood, coarse threads for softwood.

The 2/3 rule was new to me - good, objective guideline - thanks. I think I've generally been under that - more towards just longer than 50/50

I don't tend to have only screws in places that will be subject to significant shear loading, so that issue of stress doesn't concern me too much.

I'm in-process on a couple of end tables where the design leaves short apron tenons into the leg mortises. So I go from the inside through the tenon with a couple screws to reinforce the glue joint. For Arts & Crafts, I might peg the joints, as part of my "amateurish" predilection toward "ridiculously over-decorated joinery".

[Malcolm McLeod]

A little engineering…??
Screws can fail in 3 ways: 1)shear, 2)pull out(tension), and 3)pull over(tension).
#1 - a side load breaks the shank.
#2 - the thread pulls out of the ‘base’ material.
#3 - the head pulls thru the ‘fastened’ material.

I think a quick Giggle for “screw shear strength” or “screw holding strength” should yield a table of recommended screw size. (You’ll have to estimate/calculate the expected loads.)

Or just grab something that feels ‘right’. And is already in the shop screw cabinet. (My preferred method.)

[Michael Burnside]

Malcolm brings up a good point. Are you picking a screw for something that is going to be structural, hold people, require sheer or pull strength? Or are you building a cabinet or other type of furniture? If it's the former, you better look up the code and make sure you're using the right screw for the job, or in fact, if screws are even allowed. Sometimes nails (or very specific types of screws) are required for their higher sheer strength. I would never assume or go by gut in those situations.

That said, if it's a cabinet or something that won't really see huge forces, hold people, etc., I'm with the "feels right" crew, usually by order of indoor/outdoor, length and diameter.

[Thomas McCurnin]

I'm with you--i reach for Number 6 90% of the time. Number 8 when there is some serious structural issues, and then move to 10-14 for door hinges and Number 10 construction screws with a square head for most serious outdoor stuff that needs to be solid.

[Edward Weber]

I can't/wont recommend a number or length of fastener like others, without first seeing it's application.
Yes there are some basic guidelines you can follow but "it depends" is always part of the equation.
Too many variables to consider.
This is just one selection of screws I have.
Screw Shelf (456 x 500).jpg