Tapered edge or square edge plasterboard — which do I need?

This decision is made at the merchant and it commits you to a finishing method. Almost every problem people have with visible plasterboard joints traces back to buying the wrong edge for the finish they intended, and it is not obvious in the rack because the difference is a shallow taper you have to look along the edge to see.

Step by step

1. Look along the long edge to tell them apart.Tapered edge board thins gradually over the last few centimetres of each long edge, so two boards butted together form a shallow trough across the joint. Square edge board is the same thickness right to the edge. Sight along the edge in the rack and it is obvious.
2. Buy tapered edge for a taped and jointed finish.This is now the normal domestic finish: paper or mesh tape bedded in jointing compound, built up in coats, sanded, then decorated straight onto the board. The taper exists precisely so the tape and compound sit in a recess and end up flush.
3. Buy square edge for a full skim.If a plasterer is going to skim the entire surface in a couple of millimetres of finish plaster, the joint recess is irrelevant because the whole wall gets covered. Square edge is the traditional partner for skimming.
4. Understand that you can skim over tapered board too.It is very commonly done and works perfectly well — the recess simply gets filled by the skim along with everything else. What does not work is the reverse: taping and jointing square edge board without a recess to fill.
5. Remember the ends are always square.Every board has square-cut ends whatever the long edges are. So a butt joint where two board ends meet behaves like the square edge case, has no recess, and needs the compound feathered out very wide — often 300mm or more each side — to disappear. This is why board layouts avoid end joints where possible and stagger them where not.
6. Treat any edge you cut as a square edge.The moment you rip a tapered board down its length, the new edge has no taper. Plan the layout so cut edges land in internal corners, at the floor or against a ceiling, where a corner bead or a trim covers them, rather than in the middle of a wall.
7. Check what you are buying if you are ordering online.Product listings often say only "plasterboard 12.5mm" and the edge type is buried in the specification. It matters enough to check, because a pallet of the wrong edge is not a small problem.

Tips

• Some boards are tapered on the long edges and have a slight bevel or radius elsewhere; manufacturers vary. If you are matching into existing board, look at what is already there.
• Where a cut edge has to meet a tapered edge in the middle of a wall, you can create an artificial recess by running a plane or a rasp along the cut edge at a shallow angle. It is fiddly but it works.
• Tapered edge board is often no more expensive than square edge, so if you are undecided about the finish, tapered keeps both options open.

Common mistakes

• Taping and jointing square edge board — There is nowhere for the tape and compound to go, so the joint stands proud of the face. It can only be hidden by feathering the compound out very wide, which means sanding a large area, or by skimming the whole wall.
• Putting cut edges in the middle of a wall — A cut edge has no taper, so that joint is as hard to finish as a butt joint. Plan the layout so cut edges go into corners and against the floor and ceiling.
• Buying square edge because it was in stock and hoping — The finishing method is decided by the board. If tapered is not available, either wait, or accept that the wall will need skimming rather than taping.

Questions people ask