How to make a corner in trunking

To take trunking round a corner you cut it on the diagonal — both pieces the same, then butt the cut faces together. No wedge is notched out the way it is in steel tray: the plastic is simply sawn across. The only question is where to put the marks.

Where the offset comes from

The cut runs from one edge of the trunking to the other, so the wider the trunking the longer the diagonal. For a 90 degree turn the offset equals the width: on 40 mm trunking it is 40 mm, on 25 mm it is 25 mm. That is where the familiar rule of thumb comes from — measure a width, draw the diagonal. Take a shallower angle though and the rule breaks down: the same 40 mm trunking at 45 degrees needs 17 mm, not 20. That is what the calculator works out.

Marking out without a mitre box

A mitre box holds the angle for you, but often there is none on site. Then you mark it out on the profile: a mark on one edge, a second on the opposite edge offset by the figure, and the line carried across the base and both walls — round every face. Cut strictly to that line with a fine-tooth hacksaw. The point of marking every face is to stop the blade wandering: you can see where it should go all the way through the cut.

Internal and external corners

An internal corner is where the trunking runs into the corner of a room and the two walls close towards each other. An external one is where it wraps round a pier, a column or a reveal and the corner points back at you. The offset is the same figure for both, but the cut slopes the opposite way: on an internal corner the long side stays on the edge furthest from the corner, on an external one it stays on the nearest. That is behind the commonest mistake — both pieces cut the same hand, and the joint opens up in a wedge. Checking it is easy: hold both pieces to the wall dry, before the lid and before any screws. If the cut faces meet with no gap, fix it down.

How to cut trunking straight

A cut that wanders shows even on 16 mm trunking, and on a wide profile it also stops the lid seating. This is what keeps it straight. Carry the line round all four faces, not just the base. Clamp the trunking — in a vice through a block of wood, or at least knee it down onto a board; unsupported, the wall pulls the blade over. Start the cut on the wall rather than the base: the blade follows the drawn line and settles into the plane by itself. Make the first few strokes on the pull, with no pressure, so the kerf gets established. Take the last millimetres slowly, or the plastic breaks out and leaves a lip. Then dress the end with a file, or just the back of the blade — there is always a whisker of swarf that makes the joint look dirty.

What to cut trunking with

A fine-tooth hacksaw is the best of it: it takes PVC cleanly and does not tear the edge. A wood blade leaves a ragged end and can split a thin profile. An angle grinder with a cutting disc melts PVC — the mark never comes out and the smell stays in the room for hours. Plenty of people cut 16×16 or 20×10 with heavy side cutters or a cable cutter in one squeeze, but the line still gets drawn first: without a mark the tool slides off along the chamfer. The lid cuts well with a knife against a straight edge, scored from both sides and snapped.

Turning sideways and turning in height

Where trunking runs along a wall and goes round a corner, the offset is worked from its width. Where it goes up or down — dropping from the wall to skirting, say — it comes from the height. You take whichever dimension lies in the plane of the turn; the working is the same either way.

The lid

The lid is cut the same way and separately, once the base is fitted. It is easier to mark it in place: hold it against the finished joint and draw along the actual cut. That beats transferring measurements — the lid clips on with a slight tension and a couple of millimetres show.

When a moulded corner is the better buy

Moulded fittings are made for common trunking sizes: internal corners, external corners and tee pieces. They clip on and look neater than any cut, which matters on show in a flat or an office. Cutting on the diagonal earns its place when the fitting is not to hand, the angle is non-standard, or the trunking is too large for fittings to be made for it at all.

Cut offset in millimetres: how far the second mark sits from the first. For a turn up or down, use the height of the trunking instead of the width.