Which hinge do I need for a box lid or a drop-down flap?

A door hangs vertically and the hinge carries its weight in shear. A lid or a flap swings through gravity, so the hinge takes a bending load that changes as it opens, and the whole assembly is at its worst when the lid is fully open — which is when someone's hands are inside. That is why lid hardware is really two decisions rather than one.

Safety

Step by step

1. Decide how far it needs to open.A lid that stops at vertical, a lid that lies flat back, a flap that drops to horizontal, a flap that opens downwards past horizontal. The answer eliminates most hinge types immediately.
2. Butt hinges for a light lid stopping at or near vertical.Simple, cheap, strong enough, and recessed into both parts or surface mounted on the inside. The knuckle position sets the opening angle.
3. Piano or continuous hinges for long or heavy lids.The load is spread along the entire edge rather than concentrated at two or three points, which is what stops a long lid twisting and what stops a thin plywood lid tearing at the hinges.
4. Flush hinges where there is no room to recess.One leaf folds inside the other so the hinge closes to the thickness of a single leaf. Weaker than a butt hinge and fine for light boxes and small doors.
5. Cranked or flap hinges for drop-down flaps.Table and flap hinges are shaped to let a flap fold flat against the carcase or open to a level surface without the knuckle fouling. They exist because a butt hinge will not do it.
6. Friction and soft-close hinges where a lid must stay where it is put.A friction hinge holds the lid at any position by damped resistance, which removes the need for a stay entirely. This is what a decent tool chest or ottoman uses.
7. Decide what holds it open before you buy the hinge.A lid stay, a gas strut, a sliding stay for a flap, or a friction hinge. On anything heavy this is not an accessory — it is the safety component.
8. Size the hinge to the lid, not to the box.Hinge manufacturers rate by weight and leaf length. A long light lid and a short heavy one are different problems and need different hardware.
9. Think about the stress at the fixings.A lid pulls its hinge screws outwards, especially near the open position. In thin plywood or MDF that is exactly where screws pull out, so use longer screws, a thicker rail behind, or a continuous hinge to spread the load.

Tips

• A gas strut has to be specified by force as well as length, and the force depends on the lid's weight and where the strut is mounted. Manufacturers publish sizing charts — use them rather than guessing.
• On a toy box or a blanket box, a lid support that holds at any angle is the safe choice rather than a stay that only holds fully open.
• For a lid that has to lie completely flat, the hinge knuckle has to sit above the surface. Surface mounting it on the outside is the simple answer and looks deliberate on a workshop or utility piece.

Common mistakes

• Fitting a heavy lid on two small butt hinges — The load is concentrated at two points on thin material, the screws work loose, and the lid begins to sag and bind. A continuous hinge spreads it.
• Leaving the stay until later — The stay's mounting points affect where the hinge goes and how the lid is built. Deciding it afterwards frequently means there is nowhere sensible to mount it.