What is plywood underlayment?

Plywood underlayment, in the Dutch sense of the word, is a smooth structural plywood panel, often with spruce face veneers and tongue and groove edges, used as a load-bearing floor deck, roof sheathing or wall sheathing. In Dutch construction ‘underlayment’ almost always means this panel.

One word, several meanings

The term can cause confusion when reading plans or product lists from different countries:

• Netherlands: ‘underlayment’ is structural plywood, commonly 1220 x 2440 mm and most often 18 mm thick. It is laid directly on the joists, so it acts as the subfloor itself.
• United States: ‘roofing underlayment’ is the felt or synthetic membrane laid under shingles. ‘Floor underlayment’ can be a thin plywood or hardboard layer laid over the subfloor to give a smooth base for vinyl or tile, or a foam or cork layer under laminate.
• United Kingdom: ‘underlay’ usually means the cushioning layer under carpet or laminate.

So when a Dutch plan says ‘underlayment 18 mm’, read it as tongue and groove structural plywood, not as a membrane or foam layer.

Plywood, OSB or chipboard?

• OSB is made of pressed wood strands. Usually cheaper, but with a rougher surface.
• Chipboard (particleboard) is more sensitive to moisture and holds screws less well.
• Plywood is stiffer for its thickness, flatter and more dimensionally stable, but usually more expensive.

Application

• Floor deck on timber joists, instead of individual floorboards. Faster to lay and a flat base for any finish.
• Roof sheathing on rafters or purlins.
• Wall sheathing in timber frame construction, sheds and lofts.

Tips and points of attention

• Stagger the joints: do not line up end joints, and land them on a joist unless the short edges are also tongued and grooved.
• Glue and screw: glue the tongue and groove and screw into every joist.
• Expansion gap: leave a small gap around the walls, hidden later by the skirting.
• Span: check the manufacturer’s data for the joist spacing that suits the chosen thickness.
• Moisture: for exterior or damp use, choose exterior-grade bonding (bond class 3 to EN 314-2) and let panels acclimatise before laying.

Related terms

• Plywood
• OSB board
• Chipboard
• Roof sheathing
• Tongue and groove
• Spruce wood

Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.