Why does my glued wood joint keep coming apart?

Wood glue is not the weak point. A correctly made joint breaks in the timber beside the glue line rather than in the glue, so a joint that separates cleanly is telling you something about how it was made.

What you'll need

• PVA or aliphatic wood glue
• Clamps
• A damp cloth

Step by step

1. Check where it broke.A clean separation with glue on one face only means the joint never bonded. Torn wood fibres on both faces means the glue held and the timber failed, which is a design problem rather than a glue problem.
2. Stop relying on end grain.End grain sucks glue into the open fibres like a straw, leaving a starved joint. A butt joint between two end grains is the weakest thing in woodworking and needs a mechanical element — a dowel, a domino, a screw or a proper joint.
3. Size end grain if you must glue it.Brush on a thin coat of glue, let it soak in for a few minutes, then apply fresh glue and assemble. It fills the pores so the second coat stays on the surface where it can bond.
4. Fit the joint dry first.Glue is not a gap filler. PVA has almost no strength across a gap, so a joint that needs forcing or that shows daylight will fail regardless of technique.
5. Use enough glue, on both faces.A continuous film with a small bead squeezing out along the whole joint. A dry patch is a hole in the joint.
6. Clamp firmly and evenly.PVA needs pressure to form a thin, even glue line. Too little pressure leaves a thick line which is weaker; clamps every 150 to 200mm along a long joint keeps it even.
7. Respect the open time.Standard PVA gives roughly five to ten minutes from spreading to clamping. Beyond that the surface skins and will not bond. Complicated glue-ups need a slow-set glue, not faster working.
8. Leave it clamped long enough.About an hour for handling, but a full day before stressing the joint. Cold workshops slow this substantially — most PVAs are unreliable below about 10°C.
9. Match the glue to the conditions.Interior PVA for indoor work, a D3 or D4 exterior PVA for anything that gets damp, and polyurethane or epoxy where the joint will get properly wet.

Tips

• If the broken surfaces are clean, it never bonded. If wood fibres tore out, the glue was fine.
• Dry fit everything before you open the glue. You cannot fix a fit problem once it is wet.
• Cold slows PVA badly. Below about 10°C, warm the workshop or wait.

Common mistakes

• Treating glue as a gap filler — PVA has very little strength across a gap. A sloppy joint clamped hard just squeezes the glue out and leaves two surfaces held by a thin, brittle skin.
• Gluing end grain to end grain and expecting it to hold — The open fibres draw the glue away from the surface, so the joint is starved before it even sets. It needs a dowel, biscuit or proper joint to give face-grain contact.

If it doesn't work

Questions people ask