What length screw?

[mike sato]

What length flat head screw to secure 3/4" thick wood to:

1. End grain.
2. Face or edge grain.

All wood is pine, not hard wood. Screw to be driven so top of head is flush with surface of wood. Shear force is not a consideration. Only withdrawing force is important. Specific project is not important as projects will vary.

I will be using Hillman bronze ceramic coated, star drive screws.

Same question for securing a 2x4 (1-1/2" x 3-1/2") to another piece of wood. What length screw for end or face grain?

[glenn bradley]

There is a rule of thumb that the anchor length (the length of threads in the board you are fastening to) should be twice that of the fastened material (the thickness of the material being fastened). So a 1-1/2" piece of material fastened to another piece of material should have 3" of threaded shaft into that material.

This varies with counter bore hole depth, material type, use-environment and purpose. You are asking about withdrawal pressure. Screws in to face grain can be shorter than screws into end grain. I have not seen a ratio-type rule of thumb that changes with grain direction but, a little common sense using the first mentioned rule will get you there. If you want to go scientific on the topic you will find fastener manufacturer's websites have data that can speak to that level.

[Bill Dufour]

it also depends to some extent on screw size and head shape. A miniature finish head screw may pull through the top board before the threads start to rip out. This is why mudsill bolt washers are the bigger square size and the regular round ones are not code approved for decades.
Bill D

[Bill Dufour]

In metal it is 1.5-2 times the bolt diameter if bolt and nut are the same material. Any more then that and the bolt shank will break before the threads fail anyway.

[Matt Day]

So a 1-1/2" piece of material fastened to another piece of material should have 3" of threaded shaft into that material

So a 2x4 can’t be anchored to another 2x4?

[mike sato]

Thanks to all who responded. What I'm trying to find out is if there is an accepted amount of additional screw length required for withdrawal resistance when screwing into end grain vs face grain. I won't be using wood dowels that are perpendicular to the screw. Nor using glue in the screw hole. Just the screw into end grain for simple jobs.

Thanks,
Mike

[Wayne Lomman]

There is not a set rule that applies. The variables are timber thickness, timber type, thread pitch, thread depth, screw gauge, pilot hole or no pilot hole etc.

If you are working to a building code, fastener size and type is specified. Look up your applicable code and it will point you in the right direction.

If you are doing something non structural, you have to use what works. Short heavy gauge will do the same job as long and light gauge. This is what you may have to consider if you have a thin substrate. If you are using common 8 gauge screws, make sure you have more of the screw in the second layer. Don't be tempted to use metal working screws in timber - the threads are too shallow and/or finely pitched. Cheers

[mike sato]

Thanks to all who responded. What I'm trying to find out is if there is an accepted amount of additional screw length required for withdrawal resistance when screwing into end grain vs face grain. I won't be using wood dowels that are perpendicular to the screw. Nor using glue in the screw hole. Just the screw into end grain for simple jobs.

There is not a set rule that applies.
Guess that's why I haven't been able to find what I'm looking for on the web.
Thanks,
Mike


The variables are timber thickness, timber type, thread pitch, thread depth, screw gauge, pilot hole or no pilot hole etc.

If you are working to a building code, fastener size and type is specified. Look up your applicable code and it will point you in the right direction.

If you are doing something non structural, you have to use what works. Short heavy gauge will do the same job as long and light gauge. This is what you may have to consider if you have a thin substrate. If you are using common 8 gauge screws, make sure you have more of the screw in the second layer. Don't be tempted to use metal working screws in timber - the threads are too shallow and/or finely pitched. Cheers

[Larry Frank]

For 3/4" boards, I use 1-1/2" and 3" for 1-1/2" boards.

For 3/4", I predrill with countersink. In pine, I drive slowly and do not over drive to avoid splitting.

[glenn bradley]

So a 2x4 can’t be anchored to another 2x4?

Absolutely. That's why 'rule of thumb' rules are so loose. If you wanted to follow the 2:1 idea you would counterbore one 2x4 on its 1-1/2" thickness to a depth of 3/4" and drive a ~2-1/4" screw into the 1-1/2" thickness of the 2x4 board behind it. You could also use a through hole in the top material and thread only into the anchor material which creates a whole different gripping scenario.

The point being that one formula often does not serve all situations. A recent wood magazine article project has 2 x 4’s being screwed into and grain with 2 1/2 inch number eight screws. There is such a variety of fasteners available to us today. I don't use a lot of screws in my work but even still, I probably have 2 dozen varieties.

[Mike Heidrick]

Use a type 17 point coarse screw. I countersink with a router and 60 degree V bit if I want it to look nice. No predrill needed for me. I use an impact to drive them. I have sank 15,000 kreg screws in the last 3 years. Use a coarse thread. The kreg screws I use are 2.5" 1.25" and 2". I try to avoid drilling into engrain In that situation I pocket hole and come through the end grain from the side so effectively joining from the other direction if possible. .

2x4 to 2x4 = 2.5
1x to 1x = 1.25"
1x to 2x4 = 2"

[mike sato]

For 3/4" boards, I use 1-1/2" and 3" for 1-1/2" boards.

For 3/4", I predrill with countersink. In pine, I drive slowly and do not over drive to avoid splitting.

Thank you. Guess those lengths are for screwing into face grain.

For 3/4" boards I'm thinking 1-3/4" screws for screwing into end grain. For 1-1/2" boards 3" screws for screwing into end grain. I always predrill the attaching piece to insure drawing it tight to the base piece. For pine, no countersink when attaching to face grain ..... countersink when attaching to end grain. For redwood, always countersink for face or end grain.

As previously mentioned, I'm using Hillman bronze ceramic coated, star drive screws.

[mike sato]

Use a type 17 point coarse screw. I countersink with a router and 60 degree V bit if I want it to look nice. No predrill needed for me. I use an impact to drive them. I have sank 15,000 kreg screws in the last 3 years. Use a coarse thread. The kreg screws I use are 2.5" 1.25" and 2". I try to avoid drilling into engrain In that situation I pocket hole and come through the end grain from the side so effectively joining from the other direction if possible. .

2x4 to 2x4 = 2.5
1x to 1x = 1.25"
1x to 2x4 = 2"

Are the Kreg screws rated for treated lumber or redwood?

Yes, Kreg pocket screw method is a great way to avoid fastening to end grain. I use it also. Little too involved for "simple" projects for me though.

Thanks!

[John Lanciani]

Are the Kreg screws rated for treated lumber or redwood?



The blue epoxy coated ones are.

[mike sato]

The blue epoxy coated ones are.

Good info to know. Thanks!

[Tom Bender]

You can do some tests to develop your own good practice.