WORKBENCH GUIDE
Screws and pilot holes
Pilot hole size, wood density and grain direction decide how well a screw holds. Rules from the Wood Handbook, with worked numbers.
Shank, root and pilot hole
A wood screw has a smooth shank and a threaded part whose core is called the root. The Wood Handbook notes that the root diameter of most wood screws averages about two-thirds of the shank diameter, and its withdrawal equation assumes pilot (lead) holes of about 70% of the root diameter in softwoods and about 90% in hardwoods.
The table applies those ratios to common screw gauges, with the shank diameters of Wood Handbook Table 8-9. Real screws vary by maker and thread type, so measure the root of the screw you use when the fit matters. Self-drilling screws, with a drill-shaped tip, are designed to make pre-drilling unnecessary.
| Screw gauge | Shank | Root (approx.) | Pilot, softwood (70%) | Pilot, hardwood (90%) |
|---|---|---|---|---|
| #4 | 2.8 mm | 1.9 mm | 1.3 mm | 1.7 mm |
| #6 | 3.5 mm | 2.3 mm | 1.6 mm | 2.1 mm |
| #8 | 4.2 mm | 2.8 mm | 1.9 mm | 2.5 mm |
| #10 | 4.8 mm | 3.2 mm | 2.3 mm | 2.9 mm |
Density and holding power
Withdrawal resistance from side grain rises with the square of specific gravity. The Wood Handbook’s test equation is p = 108.25 × G² × D × L, in newtons, with D the shank diameter and L the threaded penetration in millimetres. Doubling the density roughly quadruples the holding power, which is why a screw that strips in light pine holds firmly in oak.
These are ultimate loads from short tests, not design values. Very long screws add little in dense hardwoods, because the screw itself fails in tension first, and screws driven into end grain hold on average only 75% of their side-grain load, with more erratic results.
Particleboard
In particleboard the usual failure is a stripped thread. The Wood Handbook gives the maximum torque for an 8-gauge screw penetrating 15.9 mm as 3.16 + 0.0096 × density, in N·m: about 8.9 N·m at 600 kg/m³ and 10.8 N·m at 800 kg/m³. Pilot holes between 50% and 90% of the root diameter made no significant difference to withdrawal.
Modest tightening is the best compromise between withdrawal resistance and stripping: raising the setting torque from 60% to 90% of the maximum gained only about 3% in withdrawal load.
Practical points
Lubricating screws with soap eases driving in dense woods and has little effect on withdrawal, according to the Handbook. Pre-drilling also reduces splitting: for nails, lead holes of about 90% of the nail diameter slightly increase withdrawal resistance and prevent or reduce splitting, particularly in dense species.
A #8 screw (shank 4.17 mm) with 25 mm of thread in loblolly pine (G = 0.51): 108.25 × 0.51² × 4.17 × 25 ≈ 2,935 N. In white oak (G = 0.68): about 5,218 N, roughly 1.8 times as much.