How do I trim a door without splintering it?

Almost all splintering happens on the exit side of the blade and at the very ends of the cut. Both are entirely preventable with a knife line and the right cutting direction.

Safety

What you'll need

• Trestles or a solid bench
• A sharp marking knife
• A straight edge and clamps
• A circular saw with a fine blade, or a sharp handsaw
• A block plane
• Masking tape

Step by step

1. Establish how much you can take.A solid-core or engineered door usually allows 6–10mm off each edge before you cut into the lock block or the lipping. A hollow-core door allows only the depth of its lipping, typically 25–40mm at the bottom and less at the sides.
2. Take the door off and lay it flat.Trestles, good face upward, fully supported so the offcut does not break away as the cut finishes.
3. Mark from the opening, not from the door.Measure the opening at several points, subtract the clearance you want — 2mm at the sides and top, 6mm over a hard floor — and transfer that to the door.
4. Share the cut between both edges where you can.Taking 3mm off each side rather than 6mm off one keeps the stiles looking even and keeps the lock in the right place.
5. Score the line with a knife.Firmly, on both faces and around the edges. This severs the veneer fibres so the saw cannot lift them, and it is the single step that decides whether the cut looks professional.
6. Add masking tape over the line.It supports the surface and lets the saw's sole plate slide. Cut through the tape and peel it towards the cut afterwards.
7. Cut on the waste side.Leave the knife line visible. A circular saw cuts upward through the underside, so keep the good face up; a handsaw cuts on the down stroke, so keep the good face up for that too.
8. Clamp a straight edge as a guide.Set back from the line by the exact distance between the saw blade and the edge of its sole plate. Freehand across a metre of door is not accurate enough.
9. Finish with a plane.Leave half a millimetre and take it off with a sharp block plane, working from each end towards the middle so you never plane off the end grain and chip the corner.
10. Reinstate the lipping if you cut through it.On a hollow-core door, glue a new hardwood lipping into the exposed edge. Without it there is nothing to screw a hinge or a latch into.
11. Seal every cut edge.Primer, paint or varnish. A bare door edge takes up moisture faster than any other part and is the usual reason a trimmed door starts binding again.

Tips

• Cut with the door face up and the saw's baseplate on masking tape and you will get a clean edge with an ordinary blade.
• A track saw or a clamped straight edge is worth borrowing for this. The cut is a metre or two long and any wander shows as a tapered gap.
• Keep the offcut. It is a perfect sample for matching paint or veneer later.

Common mistakes

• Trimming a fire door — Fire doors are certified with a stated trimming allowance, usually a few millimetres, and cutting further removes the intumescent seal groove or the lock block and voids the certification. It also destroys the 3mm gap tolerance the door relies on.
• Taking the whole amount off one edge — On a door with a lock block on one side and hinges on the other, removing 10mm from the wrong edge takes you into the block or leaves too little timber for the hinge screws.

If it doesn't work

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