I've always taken a serendipity view of screw gauge when doing rough stuff as in "hey, I'd better use a bigger screw for this." I need to cobble together some two bys for a frame for a basement storage rack. I've got a box of number 9 2 1/2 inchers that ought to do the trick.
Question: Is there a formula for determining the minimum gauge for a screw if you know the load?
My favorite screw for that application is a "Spax":
That said, most engineers will tell you that for many construction/structural projects nails will provide more shear strength than screws, so it really depends upon the application.
I never thought about it. Just that so many screws today are absolute crap. If you shear off a screw while driving it into the wood, it is a bad sign. And it depends where you buy them too. I have had terrible luck with deck screws and lag screws from the local home depot. But the local ace hardware store gave me screws that were higher in quality, stronger and a few cents cheaper too.
If I got any kind of basic repair outside or in the garage these days, I just use their deck screws. It ain't art or furniture. But it is strong. They don't shear off and they don't rust outside.
The old standby rule about strength for fasteners is to estimate how many will do the job. Then put twice as many in there. I have always been accused of using too many screws anyway.
What you also need to ask is if there is a way to determine the quality of the screw you are using. I know of #6 screws that are stronger than #10's.
All things be in equal, McFeeleys.com has the specifically information that you are asking and IIRC their catalog has a chart of this also.
Those will do. If you are concerned, smear a little construction adhesive in there too. Up here in the Northeast, if we are framing a house, we cannot use screws. Must use nails or you will not pass the framing inspection.
RP
That seems a little strange - what's the rational given, if any?
I learned the hard way that any carpentry (as opposed to ww) work I do, someday I, or someone who comes after, will have to take it apart again. I use screws for all that now.
Nails generally have a greater shear strength than screws, thus the requirement in some areas are very specific as to the nails used, their size, makeup, and nailing patterns.
Typically hurricane straps add tremendous strength if you are building for wind storm resistance.
Using the right number and pattern of screws will achieve the same thing. You need a larger screw to give the same strength, generally speaking, because the root diameter if the screw is significantly less than the nominal diameter, and the strength of the screw (yield) is lowered by the stress rizers formed by the malformation of the metal at the thread root. Screws COULD be made that were almost as strong as the equivalent sized nail, but they would be way to expensive to be practical. An "old school" wood screw is stronger than today's "construction screw" or "deck screw" or, particularly, the "drywall screw".
The holding ability of a screw excedes that of a nail in most cases - but with a "deck screw" or "drywall screw" it also often excedes the yield strength of the screw itself. An "ardox" nail is a compromize - it's holding ability approaches that of a screw - with the overall strength of a nail, but without the removeability of a screw. A guy who used to work in construction with my dad used to drive screws with a hammer - he said the funny head and other features of the screw were just to make them easier to remove;).
Particularly in earthquake country where shear walls are commonly required by the building codes. The nailing patterns are very specific.
scott
A straight nail driven into end grain has about the same holding power (in tension) as yesterday's chewing gum.
If you google "strength of wood screws" you will find
Download the PDF and read it. You will find out all you ever wanted to know about wood screws, their strength and application. (and likely even MORE than you wanted to know). A good read.
When it is nailed from the top plate or sill into stud endgrain, it suffices since the roof holds them together - the nail is to keep the stud aligned, any forces will be perpendicular to the nail.
Look up "fasten master" structural screws. They are approved for a LOT of general construction use - and their "timberlok" screw can be used in place of hurricane straps to fasten trusses to double top sills - as an example - replacing the tiedown strap and 12 nails with ONE fastener.
Untill you get wind lift. Or siesmic activity. Which is why toe-nailing USED to be pretty well a requirement in the days of plain nails. Ardox nails help. TimberLok screws can be used in place of hurricane straps to connect trusses to sills. Fully code compliant, and you won't mistake them for a common screw or lag bolt - and you don't need to remove them or X-Ray to know how long the timberlok is (they are clearly marked on their black hex heads)
And put on earth quake straps on the foundation and sil board. That will work in other cases of strong wind. A force trying to flatten a house.
Another thing we did in earth quake country was to put in shear walls. Often the outer boards are 1x tongue and grove but they silip and slide in wind. On the inside put sheet ply It doesn't have to be all walls, but some of the side on all sides. Keep that side from laying over.
Mart> >> Used to be that framing had to be toe nailed...
You simply cannot make a blanket statement like the above without checking your local building code, as well as the Engineering specifications for the particular structure.
Without question, you will most assuredly find that your options in that regard are severely limited when it comes to structural members.
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