Which type of MDF do I need?

MDF is not one product, and the differences between the versions are real. The green board people reach for in a bathroom is the one most often misunderstood, so it has a guide of its own; the rest are straightforward once you know they exist.

Step by step

1. Use standard MDF for ordinary interior work.Shelving, cabinet carcasses, panelling, painted joinery, jigs and templates. It is smooth on both faces, has no grain, machines to a crisp profile and takes paint beautifully once the edges are sealed. Its weakness is water and it has no useful strength when wet.
2. Use moisture-resistant MDF where humidity is the issue.Marked MDF.MR and usually dyed green. It is designed for humid environments — kitchens, utility rooms, bathroom cabinetry away from splashing — and it resists swelling from moist air far better than standard board. It is not waterproof and it will still swell if water sits on it.
3. Use high-density MDF where thin has to be strong.Marked MDF.H. Denser, harder, heavier and more expensive, and it holds a screw and a machined detail far better. Used for thin panels, laser-cut and routed work, and where a small section has to take a fixing.
4. Use flexible MDF for curves.Sold as flexible or bendy MDF, with a series of grooves routed across one face so the sheet will bend in one direction. Used for curved cabinet fronts, bar fronts and radiused panelling, normally laminated in two layers over a former so that the finished curve is rigid.
5. Use flame-retardant MDF only where specified.Usually dyed red or blue. It is treated to achieve a fire classification and is specified for particular commercial and public building situations. In a domestic setting it is an unusual requirement and expensive, so do not buy it speculatively.
6. Check the emission class on anything going in a bedroom.Boards sold in Britain are normally E1 class for formaldehyde emission. Lower-emission grades exist and are marked. It is worth a look on anything going into a nursery or a bedroom.
7. Seal the edges whatever type you buy.Every MDF edge is porous and drinks paint, and it is where water gets in. Sealing the edge is not optional on any version, including the moisture-resistant one.
8. Choose plywood instead where there is any real water risk.If the answer to "could water sit on this" is yes — a bathroom floor, a bath panel, a sink surround, anything outdoors — MDF of any grade is the wrong material. Exterior-grade plywood is the right one.

Tips

• Moisture-resistant MDF is only a little dearer than standard. In a kitchen or a utility room it is worth the small premium as insurance against the humidity, even where you do not expect wetting.
• MDF with a factory primer already on it is available and saves a real amount of work on painted joinery. It still needs its cut edges sealing.
• Veneered and melamine-faced MDF gives you a finished face without any painting, at the cost of having to edge the panel and not being able to sand it.

Common mistakes

• Using moisture-resistant MDF for a bath panel or a shower surround — It resists humidity, not water. Water running down a panel and sitting at the bottom edge will swell it just like standard board, only slightly slower. Use exterior-grade plywood or a proper waterproof board.
• Buying flame-retardant MDF because it sounds safer — It is a specified product for particular building situations, several times the price, and it makes no difference in a normal domestic cupboard. Buy it because a specification says so, not because it sounds prudent.
• Screwing into an MDF edge without a pilot hole — The edge is the weakest part of the board and it splits or mushrooms. Always pilot, keep well back from the corner, and prefer not to fix into the edge at all where you can avoid it.

Alternatives

• Plywood: Stronger, lighter, far better with moisture and holds screws in the edge. Less smooth, more expensive, and the edge shows plies. The right answer for anything structural or damp.
• Melamine-faced chipboard: Cheaper, comes with a finished white or woodgrain face, needs no painting. Heavier, weaker, edges must be banded, and it is very unforgiving of water.
• Veneered MDF: MDF stability with a real wood face. Excellent for visible cabinetry, at the cost of needing a lipping or banding and being unsanded once through the veneer.

Questions people ask