Pocket Hole Joinery: Screw Sizes, Strength and Limits

A pocket hole is an angled, counterbored hole drilled through one workpiece so a screw can enter at a shallow angle and drive into the face or edge of the other. A jig sets the angle and the depth, a stepped drill bit makes the pocket and the pilot in one plunge, and the joint is done in under a minute with no waiting for glue.

It is fast and it holds parts in alignment while you work, which is why it dominates face frames, shop cabinets, jigs and flat-pack furniture. But it is still a screw driven partly into end grain or into a narrow edge, so it is worth knowing where it stops being the right answer.

Matthias Wandel of Woodgears broke chair-scale joints on a test stand, loading a rail 20 cm out from the post. His pocket-hole joints in spruce failed at 115, 110 and 140 lb, with the 2 inch #7 screws pulling out of the post. Screwing the same rail into a maple post raised it to 140 lb, with the rail splitting around the pocket holes. In maple with 2 inch #8 screws the joint reached 220 lb before the pocket holes split apart.

For comparison in the same rig: mortise and tenon joints averaged 172 lb in spruce and reached 300 and 270 lb in maple, and dowel joints averaged 135 lb in spruce and 230 to 245 lb in maple.

The number is only half the story. Wandel reports that all the screwed joints opened up by several degrees long before ultimate failure - past the point he considered acceptable for furniture - and concludes that for furniture where strength is critical, screw joints should be avoided, while they are fine for sawhorses, scaffolding and packing crates where gradual yielding is actually useful.

Kreg's own selection guide puts thickness first and is explicit that most wood sold in home centres is named by nominal dimensions that are larger than the actual board, so you must measure the real thickness before choosing a screw length and setting the jig collar.

Thread type follows the wood.

• Coarse thread for softwoods and sheet goods: pine, cedar, fir, spruce, poplar, aspen, basswood, butternut, plywood, particle board, MDF and melamine. Soft, low-density fibres need deep, aggressive threads to bite.
• Fine thread for hardwoods: oak, maple, cherry, birch, ash, walnut, mahogany, hickory. Coarse threads tear dense fibres and split the board; fine threads are less aggressive but there are more of them, which keeps the holding power.

Then the environment: zinc-coated screws for indoor work, and a corrosion-resistant screw for anything that will live outside.

• Face frames - two rails and two stiles, flat on a bench, no clamping pressure to fight. This is the application the system was designed around.
• Shop cabinets, jigs, workbench bases - where speed and the ability to take it apart beat the last 40% of strength.
• Attaching a tabletop from underneath - into an apron, with elongated holes or clips where wood movement needs room.
• Plywood casework - coarse thread, and the screws land in the face of the plies rather than in the weak edge.
• Panel backs and box bottoms - Wandel's own carve-out: it is fine to use screws to attach a panel to the back of a dresser or the bottom of a box.

Where it does not belong: chair frames, table frames and anything else that takes repeated racking. His test found the joint opens before it fails, and a chair that has opened up is already broken as far as the person sitting in it is concerned.

Measure the actual board thickness, set the jig collar and the stop collar on the bit to match, and drill a test hole in scrap from the same stock. Clamp the two parts together before driving the screw - a pocket screw pulls the parts sideways as it seats, and without a clamp the joint slides out of alignment.

Add glue on the mating faces anyway. The screw is a clamp that stays in place, and where the mating surface is long grain to long grain the glue is doing more of the work than the screw. The Wood Handbook's point about end grain applies here too: end-grain surfaces are many times more porous than radial or tangential surfaces, and adhesive overpenetrates them, so do not count on a glued end-grain contact to add much.

Finally, keep the pockets off show faces. Plugs are available but they never disappear completely, especially under stain, because the plug end grain takes colour differently from the face around it.