fenceroofscrewswood
I bought several of these Simpzon ez bases for our roof. Due to the climate here, I need to check and maintain the bases and the wood periodically.
For this reason, I shaved of some of the wood so that the wood posts can be easily removed from the base. Some I removed too much wood and move around a bit, but not too much.
Since I'll be using wood screws to secure the post to the base, what screws can I use to secure the base and stop the wood post from moving inside the base?
The pull-out strength of a wood screw is going to be more dependent on the wood into which it is installed than on the material of the screw. The geometry of the screw thread also plays a role. "Cheap" screws therefore aren't what you need to worry about, instead, focus on the screw size.
Pull-out happens when you have a shear failure of the wood. A couple of formulas have been proposed to make an estimate of holding power
The Forest Products Lab of the US Department of Agriculture proposes the following:
p=15700*G^2*D*L
where p is the pull out force in pounds; G is the specific gravity of the wood (density of the wood divided by the density of water, see table 5-3 here); D is the shank diameter of the screw in inches; L is the penetration of the threaded part of the screw in inches. (Assumes reasonable pilot hole, screw into side-grain, etc...I haven't communicated all the nuance here. If your application is important enough to be calculating the pullout load, read the whole relevant section of the source document.)
Others have proposed a simple formula (stated incorrectly after the link, corrected here by me) related directly to the shear strength of the wood:
p=pi*D*L*S
where p is again the pullout strength; pi=3.14; D is the diameter of the screw, L is the length of engaged threads, and S is the shear strength of the wood. (make sure your shear strength is in units compatible with your length, e.g. if your length is in inches, your shear strength should be in PSI).
This simpler formula idealizes things and assumes that all the load is carried by a cylindrical region of the wood that is the diameter and length of the embedded screw threads (the pi*D*L above is the area of this surface). If you use this formula, be sure to reduce the actual load by a generous safety factor of 2 or more.
Personally, I'd read the FPL report and use the first formula.
You have a few options available to you, based on the material which you find.
Provided that I understand how you want to assemble the structure you can do the following.
(see pics below)
Best Answer
When in Doubt, Consult the Manufacturer
Simpson Strongtie makes it simple, the only fasteners officially approved for use with the product happen to be the ones they make and sell.
From the Strongtie Site:
Could you use a generic equivalent fastener?
It's pointed out in the comments that Simpson only specifies THEIR screws, but a generic galvanized lag screw ought to work fine. I wouldn't lose a minute's sleep over it. I'd bet there's really nothing unusual about Simpson's screws shaft diameter, material, thread depth, corrosion resistance, etc.
(Goes without saying you have to know what you're doing to substitute; lots of people had decks fall down, sometimes with catastrophic results, because they thought roofing nails were close enough to joist hanger nails. They are not!)
Depending on the locale, code may require that specialty hardware like this be installed according to the manufacturer's instructions. Some products, like joist hanger nails, Simpson's literature specifies generic equivalents; but I didn't see any for these screws. Another hardware manufacturer, they might not make fasteners and might only specify generic hardware for a similar product.
So if you really want to go by the letter of the code, you have to use Simpson's screws. It's not a lot of money and it keeps things simple. I just consider the specialty fasteners part of the cost of the specialty hardware.