Adhesive is a binder that joins two surfaces without a mechanical fastener. A glued joint gets its strength from two things at once: adhesion — the grip on the surface — and cohesion — the internal strength of the glue layer itself.
In construction, adhesive has partly displaced screwing and nailing in many places: timber structures, sheet materials, floors, tiling, facade panels and even masonry laid in thin-bed mortar.
The main types
- PVA (white wood glue). The standard for interior joinery. It cures by losing water, so at least one of the parts must be able to absorb moisture. Cheap, odourless and easy to work with, but it creeps under sustained load and is not heat resistant.
- Polyurethane adhesive (PU). Cures using moisture from the timber and the air, foaming as it does. Waterproof and suited to exterior work and to combinations of timber with stone or metal. The foam fills gaps but has no strength — a good fit is still required. Remove squeezed-out foam only once it has hardened.
- Epoxy (two-part). The strongest option and the only one that bridges serious gaps without shrinkage. Used for structural repairs, chemical anchors and bonding metal and stone. Expensive and sensitive to mixing ratio and temperature.
- MS polymer / hybrid construction adhesive. The cartridge in the gun. Stays slightly elastic, grips almost anything including damp substrates, and can be overpainted. Excellent for skirtings, thresholds, facade panels and anything that must be allowed to move — not for structural work.
- Contact adhesive. Applied to both parts, left to flash off, then pressed together. The bond is instant and final; there is no correcting it. For HPL surfaces and sheet flooring.
The D classes for wood glue
Wood glue is classified for moisture resistance under the European standard EN 204:
- D1 — dry interior climate, no moisture exposure.
- D2 — interior, occasional short exposure to moisture or high humidity.
- D3 — damp interiors and exterior work not directly exposed to weather; kitchen and bathroom work.
- D4 — prolonged moisture exposure and exterior use.
Loadbearing structures such as glulam beams are subject to separate, heavier requirements; D4 is not automatically sufficient there.
What goes wrong in practice
- Too much adhesive. A thick glue line is weaker than a thin one. PVA and PU want a thin, closed film on one of the parts, not a bead that squeezes out.
- No clamping pressure. Almost every glued joint needs pressure until the adhesive has cured. Clamps, screws or weight — but do not let go after five minutes.
- Open time exceeded. Every adhesive has a window between application and assembly. Miss it and a skin has already formed, and it will not bond. Prepare the work before you open the bottle.
- Grease, dust or a loose substrate. Adhesive grips what is underneath, not what is lying on top. Vacuum off dust, degrease, remove flaking paint first.
- Applied too cold. Most adhesives need at least about 5 °C, and perform clearly better around room temperature. In a cold shed in January, PVA cures badly.
Checks before and during gluing
- Apply a thin glue line: PVA and PU want a thin, closed film on one of the parts, not a bead that squeezes out.
- Keep clamping pressure until cured: Almost every glued joint needs pressure until the adhesive has cured.
- Assemble within the open time: If the window is missed, a skin has already formed and it will not bond.
- Clean the substrate: Vacuum off dust, degrease and remove flaking paint first.
- Check the temperature: Most adhesives need at least about 5 °C and perform clearly better around room temperature.
Related terms
- Wood glue
- Epoxy adhesive
- Glued truss
- Construction adhesive
- Tile adhesive
- Sealant
- Adhesion
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