When a lag screw grabs the wood properly it makes a good crunching noise. When that happens you know it’s got a good bite on the joint. If you hear a tearing high pitched sound, then you know grain has separated. At this point you’re toast.
The key to avoiding one or the other is typicaly your pre-wrench decision. Drill the correct holes. Sounds easy enough, right? You want to drive a screw so you just drill a hole and insert the screw. Not so fast. Wood isn’t all created equal. The wood will react different than under pressure depending on its grain direction, the size of your fastener, and its density.
How to Drill Holes for Lag Screws
The chart above explain the separation between pilot hole and shank hole. This one thing will save more projects for you then nearly anything else in the box. When installing screws with lags, you need to make two holes for each lag screw. One hole pass all the way through top piece of wood. This one needs to be same size as smooth shank of the screw so it can slide through without dragging. And then there is the other hole which is in receiving member. This is where the threads grab.
This is where most folks go wrong. They either drill it too small and when the screw expands the fibers of the wood, it split the wood. Or they drill it too big, because now the threads don’t really have anything to bite into. So what happens? A loose connection fail under load. It’s more psychological than anything. You want that screw to feel tight, so you tend to drill smaller holes then needed. But tighter isn’t necessarily stronger.
In terms of how they handle mistakes, softwoods such as cedar or pine are easy to work with. A slightly smaller pilot hole works in softwood. Roughly six to seven- five percent of the shank diameter. Hardwoods on the other hand go a different way. Densities such as oak, maple, and hickory is solid. They don’t want to budge. Drill too small here and the screw become a wedge. That means the board cracks. In hardwoods you want a bigger pilot hole. Closer to eighty-five percent of the shank diameter. This allow the wood some wiggle room for the threads while still staying intact. It is a small adjustment, but it goes a long way.
Consider also the anatomy of the screw itself. What happens when there’s bad drilling? The head bears down on surface. The shank is the smooth part that gives clearance. And then there are coarse threads which do all the heavy lifting in the bottom board. Damage one of these elements and it ruins the entire assembly. To help distribute the load, we use a flat washer under the screw head. So not only does this prevent the screw from digging into the wood surface, but it’s a simple insurance policy against future structural failure.
You’re dealing with real weather and real weight when you’re out installing fence posts or deck ledgers. You can’t use a split to hang a deck. Using loose fasteners won’t cut it as a way to hang a fence. Maximum thread engagement is what we’re after without splitting the grain. Driving the screw in a lubricated thread (soap or wax) will reduces friction as well. This further reduce the chance of a split. It is an old carpenter trick that always seems to work.
Before reaching for the drill, consider the type of wood. Is it hard and brittle or soft and springy? Your drill bit size depend on this answer. Get that part correct and the rest is simply a matter of torque. Get it wrong, and you’re back to square one. The sound you’re listening for at the end is that solid crunch. All others are nothing but noise.
