Predrilling into pressure treated wood. Engineering help

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ElevatedDeck

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My question has to do with screws, tension, and pre-drilling.

At Home Depot I bought 1/4" diameter hot dipped galvanized (HDG) wood screws for drilling into pressure treated wood. The exact make and model:
Simpson Strong-Tie
Strong-Drive Screw
Wood Screw
1/4" x 3" HDG
"Pre-Drilling Not Required"

UPC 7 07392 53960 5 (707392539605)

Simpson has a webpage with information on their screws:
SDS & SD Wood Screws

On that webpage they have a link to their engineering document:
Strong-Drive Screw Applications

GENERAL INFORMATION
Installation:
The Simpson Strong-Drive® wood screw has a hex washer head
for easy driving. The built-in reamer and type 17 tip allows for
installation without predrilling. Predrilling may be necessary
depending on the type and moisture content of wood. Refer to
the 2001 NDS Section 11.1.4 for drill diameter requirements.
Click to expand...

Bold/Italics added for emphasis. Now, why I'm confused:

The documentation they provide for the screws seems to contradict their selling point for the screws (no pre-drilling). Pressure treated wood obviously has a high moisture content and will shrink. How important is pre-drilling, and what type of bit do I pre-drill with for a 1/4" diameter screw?

I am worried I will lower the withdrawal load (tension) if I predrill. At the same time I don't want the wood splitting on me when it shrinks. What do you guys think? How do I get the maximum withdrawal load? What if I do 1/16 pre-drill? Does anyone know what the NDS (National Design Specification) says for pressure treated wood pre-drilling? What do you guys think?

Thanks :)
 
In general when using screws to hold two pieces of wood together you want the first piece of wood to allow the screw to turn freely. When it bites into the second piece it becomes a clamping effect. Unless you are clamping the pieces together first this is what you want to have happen. You don't say what part of the deck structure your using these 3" screws on so it is hard to say if they are correct for the application and if predrilling is needed.
Normally predrilling is not needed with a #8 screw except when close to the edge or end of a board. With a 1/4" screw you may find it different.
 
MinConst said:
Unless you are clamping the pieces together first this is what you want to have happen. You don't say what part of the deck structure your using these 3" screws on so it is hard to say if they are correct for the application and if predrilling is needed.
Click to expand...

Thank you for your reply. BTW I checked out your website and I thought it was informative and interesting.

To answer your question:

What I am doing is building a ramp for my dog. There is currently no way to do this that will comply with code. In Westchester County, NY no ramp may have a slope greater than 1:12 (12" of run for every 1" of rise). My deck is elevated, a little over 7 feet. This would require a ramp of over 85', which is just impossible. Also it's a ramp only for my dog.

So right now I have some pressure-treated 2x10x10's and also some 2x10x5's. I am going to take two 2x10x10's end to end. I will connect them together (sister) using the 2x10x5, so that 2 1/2 feet is on the first board and 2 1/2 feet is on the second board.
like this

-------======-------


I can clamp together the pieces I guess? The ramp is going to be twenty feet at 1:3. 1:3 works for my dog.

Do you think I should predrill? The pressure treated wood has been sitting in my garage since the very beginning of this year. What do you think of 1/16 predrill? Too little?

I still haven't figured out how to connect the 20' ramp to the deck.
 
w/tensile strength - they're different.

in order for your ramp to be legal, i'd use carriage bolts & a fitch plate making it ' temporary ' & removeable,,, then block the entry so passage's block'd other'n for the dog,,, far's ' attaching ' it to the deck, i wouldn't,,, just support the end w/4x4's.

the code's complying w/ ada requirements, NOT animal access,,, placing cross-cleats on the ramp might help.
 
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