When I have a visible split in a large slab tabletop, I’ll stabilize it with a wooden key, like I described here last week. But when it comes to the underside of a slab, I prefer to use a little pocket-hole jig to make a fast repair that is adjustable and easily removed if need be.
Keep in mind that I’m not trying to close the split – just keep it in check, so to speak.
The top here is oak, so I’m using the fine-thread screws from Kreg. For most splits, I use 1-1/4”-long screws. In this thick 2-1/2” top, I could use a longer screw without it blasting through the top, but 1-1/4” is usually fine.
The jig is a Kreg Mini, which is available at most home centers for $20 or less.
The process itself is simple: Clamp the jig so its front edge lines up with the split. By positioning it this way you will bore a clearance hole in one side of the split and a pilot hole on the other side of the chasm. That prevents the screw from “bridging” (where the threads engage on both sides of the joint) and makes the screw joint adjustable.
Bore the hole. Then move the jig to another location and put at least one hole on the other side of the split. This split is about 7” long and I used three screws across it.
Drive the screws. If you use a drill/driver, set the clutch so the screw doesn’t fully seat. You don’t want to close the split and distort the tabletop. Finish seating the screw by hand. I drive the screw until it kisses the bottom of the hole (yes, I’m allowed to write those words in that order).
If the split closes up over time (it can happen!), you can also tighten the screws to compensate. If something goes awry some day, you can remove the screws and take other action.
— Christopher Schwarz
P.S. And before you chap my hide with comments about pocket screws, they have been around a lot longer than your great, great grandpa <blows raspberry>.