How do I undercut a door frame so the flooring runs under it?

Scribing flooring around a door frame is fiddly, never quite neat, and leaves no expansion gap. Undercutting the frame instead takes ten minutes per door and gives a result that looks like the frame was built around the floor.

What you'll need

• An oscillating multi-tool with a flush blade, or a flooring jamb saw
• An offcut of the flooring and the underlay
• A pencil
• A chisel
• A vacuum

Step by step

1. Do it before laying the floor.The undercut is much easier with the subfloor clear, and the offcut gauge sits at exactly the right height.
2. Make a height gauge.A piece of the underlay with a piece of the flooring on top, exactly as it will be laid. That stack is the cut height.
3. Add a fraction of clearance.A slip of card or thin ply under the gauge gives 1mm of clearance so the board slides under without scraping. It is hidden by the frame.
4. Rest the saw on the gauge.A multi-tool with a flush-cut blade held flat on the offcut, or a purpose-made jamb saw which is designed to do exactly this.
5. Cut the architrave and the frame.Both, all the way through. Cutting only the architrave leaves the frame proud and the board still cannot pass.
6. Cut both sides of the doorway.And the door stop if it runs to the floor. Anything at floor level that the board would meet has to be cut.
7. Keep the blade flat.Any tilt and the cut is not parallel with the floor, so the board will not slide fully under at one end. Pressure downwards on the gauge, not on the saw.
8. Clear the offcut.A chisel gets the small triangular waste out from the corner, where the blade cannot reach.
9. Vacuum before laying.The sawdust falls exactly where the board has to slide, and it is the reason a board suddenly will not go home.
10. Check the fit with an offcut.Slide a piece under before committing. It should go in with light hand pressure and leave 10mm of gap behind it.
11. Leave the gap under the frame.The board should stop 10mm short under the architrave, not butt against the frame. The architrave hides it completely.
12. Do the same at a hearth or a fitted unit.The same technique applies to anything with a projecting foot or plinth at floor level.

Tips

• A pull saw or a dedicated jamb saw costs very little and produces a flatter, cleaner cut than a multi-tool at floor level.
• Undercut the frame for the deepest floor build-up you might ever use, not just the current one. It costs nothing now and saves the job later.
• If you are laying tiles rather than a floating floor, undercut for the tile plus the adhesive bed, and test the fit with an actual tile on a bed of adhesive-thickness card.

Common mistakes

• Scribing the flooring around the frame instead — It takes far longer, never looks as neat, and leaves the board butted hard against the frame with no expansion gap — which is one of the classic pinch points that makes a whole floating floor lift.
• Cutting the architrave but not the frame behind it — The frame or lining behind the architrave usually projects further into the opening. The board hits it, will not go home, and the cut has to be redone with the flooring already laid.

If it doesn't work

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