Author Topic: Lag Screw & Wood Screw Pilot Hole Sizes & Info  (Read 101337 times)

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Offline MountainDon

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Lag Screw & Wood Screw Pilot Hole Sizes & Info
« on: April 29, 2009, 08:12:42 AM »
More than once in various topics I have admonished builders to use the correct size pilot hole when fastening with lag screws. Nobody's ever asked what the proper size is; I assume you've all looked it up. Anyhow here's a little list of proper size pilot holes for both hard wood and soft wood. Along with this is a notation to also be sure to drill a larger hole for the shank of the lag screw. (some flks call lag screws lag bolts; same thing, different name)

The shank size hole should be drilled completely through the piece of wood being attached to the larger piece of wood. The reason is if the screw threads are biting into the board to be attached, the screw threads can not pull the two pieces together. The total depth of the shank hole should be equivalent to the length of the shank portion of the lag screw.

The size of the pilot holes is somewhat dependent on the species of lumber and its age and because of that, some experimentation with the correct pilot holes for your project might be undertaken.

However, there is a rule of thumb that can be used.  The pilot hole for the shank should be the same as the diameter of the bolt and the pilot hole for the threaded portion should be 3/4 of the diameter for softwoods and slightly larger for larger diameter bolts used in hardwoods as shown in Table 1.

Some suggest the use of soap on the end of the threaded portion of the lag bolt in order to provide some lubrication to the threads.  This is definitely not a good idea, as soap will cause a steel lag bolt to rust prematurely, although it will have no effect on stainless steel or galvanized lag bolts.

If you would like to use a lubricant, bees wax is the best choice.  If you do not have bees wax readily available, use some vegetable oil.

FWIW, I prefer to drill the pilot hole full depth required first using a auger type wood bit.

Then I drill the pilot hole out to the shank size using a regular twist bit. I have a set of stop collars that can be fitted to the bit to easily set the drill depth. I also use masking tape wrapped around the bit as a visual assist when using the auger bits.


Here's a chart for regular wood screws, sizes 0 to 20, with common sizes in color...

To prevent brass screw heads from twisting off in hardwood use a steel screw of the same gauge to thread the wood, remove and then insert the brass screw. As noted above use wax rather than soap for a lubricant if you desire.

« Last Edit: April 29, 2009, 02:33:00 PM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline Don_P

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Re: Lag Screw Pilot Hole Sizes & Info
« Reply #1 on: April 29, 2009, 03:25:43 PM »
The guys that I grew up around on Dad's jobsites were from the DA generation, dragging a nail behind their ear or thru their hair when needed was all the lubricant needed  ;D A toilet bowl ring is excellent beeswax BTW, makes good grafting wax too.

The Wood Handbook goes into lags in chapter 7, it is a free download here;
It is the wood bible.

For soft softwoods a pilot hole of 40 to 70% of shank diameter is suggested, and actually you can get away with no pilot in the softer woods up to a 3/8 diameter without a penalty, this is why log homes typically run 3/8 lags or the new oly screws if they are a screw down style.Those hardened oly, ledgerlok and fastenmaster type screws are also excellent connectors, the steel is basically twice as strong as a common lag. With an impact I've broken lots of lags, at one point my wife had a zipper failure when trying to pull a twisted log down in a log wall. When the first 3/8x10" lag broke it snapped 4 more instantly. I've broken less than a handful of the hardened ones but... strength is also about diameter, she was using full diameter regular lags for their pulling power, I've spun out plenty of the smaller ones. For harder softwoods like Southern Pine or Douglas Fir a pilot hole is generally 60 to 75% of shank dia, in denser hardwoods it should be 65 to 85% of shank diameter. In a few woods like black locust I've been 90% to avoid snapping the lag. prebore is actually all about density as is the strength. The strength tables are set up by density as well, we then list the species along the tables according to their average density but in wood, if you don't know anything else about it, density is the thing to pay the most attention to.

For more check out course 201 here;

Offline John_M

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Re: Lag Screw & Wood Screw Pilot Hole Sizes & Info
« Reply #2 on: April 29, 2009, 04:02:03 PM »
Very helpful...but falls under the "now you tell me" category!!!  d* is short...enjoy the ride!!

Offline NM_Shooter

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Re: Lag Screw & Wood Screw Pilot Hole Sizes & Info
« Reply #3 on: April 29, 2009, 04:17:42 PM »
I brought new gate hinges up to our ranch last year, and brought the wrong kind. 

Try screwing 5/8" gate hinges into a 1/2" hole sometime through a 8" post.

I thought I was going to have a heart attack before that was done!
"Officium Vacuus Auctorita"

Offline lonelytree

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Re: Lag Screw & Wood Screw Pilot Hole Sizes & Info
« Reply #4 on: April 29, 2009, 04:47:09 PM »
I had to drill with bits to do my bracing. I used a spade type bit to flatten the 45 degree angle so the washer/head would seat flat, then a full length (8") hole then a shank hole. A bit of a pain, but they REALLY hold well (and I am not a fan of lag bolts). They pulled the bracing tight and you could run a truck into it afterward with no movement.


Offline glenn kangiser

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Re: Lag Screw & Wood Screw Pilot Hole Sizes & Info
« Reply #5 on: April 29, 2009, 07:50:00 PM »
I am on a job where I am running about 10 to 20,  3/4x6 lags into canopies or balconies - on second and 3rd floors.  I am drilling a 5/8 pilot hole as it is near equal to the small thread diameter and I am tightening them to 325 foot lbs, with a Makita 18v impact.  No strip outs as long as there is backing in the proper places. d*
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