Studio Video Guide

How to Set Out and Fit a 15 mm Shadow Gap

A shadow gap is a deliberate void where two surfaces meet, and it reads as a crisp line only if it is set out before either surface is built. Fifteen millimetres is the studio default: wide enough to stay dark from a standing eye height, narrow enough that it never collects a visible edge of dust.

Time needed
About four hours
Number of steps
5 steps
Setting out the gap before the plasterboard goes up. A studio lead marking the datum line, fixing the shadow gap bead to it, and checking the reveal depth with a gauge before the board is cut.

What You Need

Tools

  • A laser level with a datum function
  • A 15 mm shadow gap bead, aluminium
  • A reveal depth gauge
  • A fine-tooth board saw

Supplies

  • Plasterboard, 12.5 mm, square-edge rather than tapered
  • Setting compound and jointing tape
  • Matt emulsion in the ceiling colour, for the gap interior

  1. 01Establish one datum for the whole room

    Set a laser datum at a metre above the finished floor level and mark it on every wall before anything is fixed. A shadow gap follows the datum, not the ceiling, and a ceiling that falls 8 mm across a room will take the gap with it if the line is taken from above.

    • One datum, marked on all four walls before any fixing
    • Measure down from the datum, never up from the floor
    • Record the worst deviation — it sets the minimum board trim

  2. 02Fix the bead to the line, not to the substrate

    The bead is what makes the gap straight, so it is fixed to the marked line and packed out wherever the substrate falls away from it. Fixing it flat to an uneven wall transfers every undulation into the one element on the wall the eye will follow.

    • Pack behind the bead rather than bending it to the wall
    • Fixings at 300 mm centres, closer at the ends
    • Butt joints in the bead land away from sightlines

  3. 03Paint the gap interior before the board goes on

    The inside face of the gap is unreachable once the board is fixed and is the part that has to read as pure shadow. Paint it in the ceiling colour, matt, and let it dry fully — a satin finish in there picks up the very light the detail exists to exclude.

    • Matt only; any sheen defeats the detail
    • Two coats, both before the board is cut in
    • Mask the bead face so it stays clean for the setting coat

  4. 04Cut the board short and let the bead hold the edge

    The board stops short of the bead by two to three millimetres. Cutting it tight jams the edge against the aluminium, and the joint compound then has nowhere to key, which is where the hairline cracks along a shadow gap come from a year later.

    • A 2 to 3 mm gap between board edge and bead
    • Square-edge board, not tapered, at the gap
    • Tape and set the board joint, never the bead joint

  5. 05Check the reveal along its full length before setting

    Run the depth gauge along the whole gap and correct anything more than a millimetre out while the compound is still workable. A shadow gap is judged by its consistency rather than its dimension, and a run that varies between 14 and 16 mm reads as a mistake in a way a consistent 18 mm never would.

    • Gauge at 500 mm intervals along the full run
    • Correct anything over 1 mm out before the compound sets
    • Sight down the run from each end, at standing height