# what size and length of screw to use?



## tito5 (Apr 5, 2011)

When screwing two boards together is there a general rule of thumb on what size and length of screw to use? I need to glue/screw two 3/4 in MDF boards together and I got 1 inch screws, but they don't look long enough to do the job, but I don't want to poke through to the other side either. which got me thinking....there has to be a rule of thumb on what size and length screw to use when screwing two pieces together


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## mdntrdr (Dec 22, 2009)

I would use 1-1/4" coarse thread DW screws, pre drilled and countersunk. :smile:


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## Leo G (Oct 16, 2006)

1" will work if you countersink them so you have 1/2" penetration in the other board.


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## HugeCow inc. (Jun 12, 2010)

is there a rule of thumb to go by though?


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## Leo G (Oct 16, 2006)

2/3rds the thickness of the board.


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## Old Skhool (Oct 31, 2009)

Ideally 2/3rds of the screw should be threaded into the piece that receives the threads. The piece that is being clamped by the screw head should have a pilot hole _slightly _larger than the o.d. of the screw thread so that the joint is pulled tightly together. You don't want the screw theading in both pieces, as that can actually hold the joint open. (Think about 2 nuts spaced an inch apart on a threaded rod, hold *both *nuts and turn the rod, they will travel down the rod, but stay 1 inch apart. And yeah, all you guys get your heads out of the gutter:laughing

In terms of size, 6,8, and 10 are the most common in cabinet work. In your case, I would use a 1 1/4" number 8. The would give some holding power, and as mentioned above a coarse thread designed for MDF would be appropriate. Only having about 1/2" does limit holding power, use glue if you can. A threaded insert may be preferable depending on your particular application.


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## tito5 (Apr 5, 2011)

for this project there will be glue involved too, I was just thinking about general rule of thumb for future projects and wondering what the general pop had to say.....


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## Brian_Hinther (Sep 13, 2009)

If you really want to get into it, McFeely's has a line of screws called ProMax Optimized Thread Length. They're sized in eighth-inch lengths for maximum penetration and have longer than usual unthreaded shanks to keep the boards from "jacking" without drilling a clearance hole. 

For instance, a screw for your use would be 1 3/8 inches long, with no thread on the top 13/16 inches of screw. To screw through one 2x4 into another you would need a 2 7/8 inch screw with no thread for 1 9/16 inches.

I find them really handy, especially for repetitive work like fences and decks.


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## papagym (May 12, 2011)

*Don't screw it*

Old Skhool got the screwing right, along with some others. But if you use glue along with some good clamping you don't have to use screws on MDF.:huh:
Glue alone will work on almost every joint.


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