Materials
Plywood, MDF and chipboard fail in three different ways
Sheet materials look interchangeable in a merchant yard and are not. Each is made differently, and each has one situation where it is the obvious answer and one where it is a mistake.
The options around sheet materials are set out side by side below, with the conditions that genuinely favour one over the other.
The difference in one place
- Plywood holds screws in its edge; MDF and chipboard do so poorly.
- MDF machines to a clean edge that takes paint, which no plywood edge does.
- All three swell irreversibly when wet, but they swell at different speeds.
Three different constructions
Plywood is thin veneers glued in alternating grain directions, which is what gives it strength in two axes and resistance to splitting. Medium density fibreboard is wood broken down to fibre and re-bonded under pressure, producing a uniform material with no grain at all.
Chipboard is larger particles bonded together, cheaper to make and correspondingly weaker and coarser in the edge. Those constructions explain every practical difference between them, including how they hold fixings and how they behave when they get wet. Oriented strand board sits between plywood and chipboard, using large flakes deliberately aligned, and is structural rather than decorative.
How each one holds a fixing
A screw into the face of any of the three holds reasonably; the differences appear when a screw goes into the edge. Plywood edges grip because the screw crosses multiple glued veneers, which is why plywood carcasses can be assembled edge to edge. MDF edges split when screwed without pilot holes, because the fibre matrix has no long grain to resist the wedge of the thread.
Chipboard edges are the weakest of all, and a stripped hole in chipboard cannot be recut in the same place. This is the reason cabinet makers use plywood where a carcass will carry load and MDF where it will be painted and supported.
Edges, faces and finish
MDF has no voids and machines to a crisp edge, which is why it is the standard material for painted mouldings, panelling and shaped work. Plywood edges show the laminations, which some people treat as a feature and others cover with a solid timber lipping. Plywood faces can contain small voids and patches under the veneer, and a cheap board will telegraph those through a painted finish.
Live with it a while and chipboard is almost always used with a factory-applied foil or melamine face, because the raw surface takes finish poorly. Once a foil face is chipped it cannot be repaired invisibly, which is why painted MDF is easier to maintain over many years.
Water is the common enemy
All three absorb water and swell, and the swelling is permanent because the fibres are crushed rather than compressed when they expand. Chipboard fails fastest, going from a firm board to a soft crumbling mass in a comparatively short exposure. MDF swells at the edges first, which is why a painted skirting behind a leaking appliance bulges along its bottom edge.
Over years, plywood tolerates dampness better, and boards bonded with weather-resistant adhesive are made specifically for exposure.
Sealing cut edges before installation is the single most useful precaution, since the edge is where water enters every one of these materials.
Weight, sag and span
MDF is dense and heavy, and a wide unsupported shelf in it will sag under books within a year or two. Plywood is lighter for its stiffness and holds a span better, which is why it is the usual choice for shelving that must stay flat.
Adding a timber lipping to the front edge of any shelf dramatically increases its stiffness, often more than increasing thickness would. Sag increases sharply with span, so shortening a shelf helps far more than most people expect. Chipboard shelving of the kind supplied in flat-pack units is designed for a specific load and span and should be treated as such.
Budgets and buildings differ, and old buildings differ most.
Dust, health and handling
Cutting and sanding these boards produces very fine dust containing the resins used to bond them, and that dust should not be breathed. Appropriate extraction and respiratory protection are standard practice for this work, and guidance on it varies by country. Formaldehyde emissions from bonded boards are regulated in many markets, with graded standards for low-emission products.
The quieter point is that sheets are heavy and awkward, and a full sheet is a two-person lift for most people regardless of strength. Where any doubt exists about health precautions or ventilation during work, the sensible course is to ask a qualified professional.
Side by side
| Consideration | What it means in practice |
|---|---|
| Three different constructions | Plywood holds screws in its edge; MDF and chipboard do so poorly. |
| How each one holds a fixing | MDF machines to a clean edge that takes paint, which no plywood edge does. |
| Edges, faces and finish | All three swell irreversibly when wet, but they swell at different speeds. |
The takeaway
Choose plywood for strength and edges, MDF for paint and shaped work, chipboard for cheap faced boxes, and seal every cut edge.
A room that is used will always beat a room that is finished.
Questions readers ask
Which board is best for kitchen carcasses?
Plywood if you want edge fixings, weight capacity and better damp tolerance; faced chipboard is what most factory units use because it is cheaper and dimensionally stable.
Can MDF be used in a bathroom?
Only sealed thoroughly on every face and edge, and ideally in a moisture-resistant grade. Standard MDF swells permanently once water reaches an exposed edge.
