Before using a lag bolt in a wood stud, the hole must be pre-drilled. This can also be called making a pilot hole. Due to the bolts large diameter predrilling is needed to avoid splitting the wooden stud and/or potentially damaging the bolt (usually seen as snapping the bolt head).
Below is the quick and easy chart for most situations.
|
Lag Bolt Size |
Pilot Hole Size |
|
1/4" |
5/32" |
|
5/16" |
3/16" |
|
3/8" |
15/64" |
|
1/2" |
5/16" |
|
5/8" |
13/32" |
|
3/4" |
1/2" |
Variables such as wood type, bolt length and bolt diameter all factor into ideal pilot hole sizes. The easiest of these to address is wood type. Wood types are grouped by the American Lumber Standard Committee and ranked depending on the hardness of the wood, the pilot hole is adjusted considerably. Most newer US houses have studs made of Spruce (group 3 - 4) or Douglas fir (group 2 - 3).
Consult an engineer for questions regarding specific applications.
|
Lag Bolt Size |
Pilot Hole Size |
||
|
Soft Wood* |
Medium Wood* |
Hard Wood* |
|
|
1/4" |
3/32" |
5/32" |
3/16" |
|
5/16" |
9/64" |
3/16" |
13/64" |
|
3/8" |
11/64" |
15/64" |
1/4" |
|
1/2" |
15/64" |
5/16" |
11/32" |
|
5/8" |
5/16" |
13/32" |
29/64" |
|
3/4" |
13/32" |
1/2" |
9/16" |
* Soft Wood (Group 3 and 4), Medium Wood (Group 2), Hard Wood (Group 1)