How to specify
a shadow gap
Everyone asks for one. Almost nobody gives it a number, a tolerance and a reference surface — which is why it drifts from 5 mm to 12 mm up the same wall.
Drawn from jobs run in 304 and 316L on our own factory floor in Foshan, not from a style guide.
- Reading time
- ~6 min
- Typical gap
- 5 mm ±0.5
- Usual failure
- No reference surface
- Visible from
- 1.5 m
What the gap is doing
A shadow gap is a deliberate recess where two materials meet, so the junction reads as a line rather than as a sealant bead. It exists because the eye prefers a line to a smudge.
It only works if it is constant. A gap that varies by 3 mm stops reading as a designed line and starts reading as a bad fit — and the eye finds that immediately, even when it cannot say why.
Give it a number and a tolerance
"5 mm shadow gap" is not a specification. "5 mm ±0.5 mm, constant over the full length, referenced to the finished plaster face" is.
The reference surface is the part people forget. If the gap is dimensioned from the structural wall and the wall is plastered, the plaster thickness becomes part of your gap.
Why it drifts
- The wall is out of plumb, so a gap parallel to the metal is not constant against the plaster
- The plaster thickness varies, so a gap set from structure varies
- The metal edge is cut to nominal, not to measured
- Two trades each assume the other will absorb the difference
Any one of these moves the gap by a few millimetres. Together they produce the taper that gets filled with sealant on the last day.
How to stop it drifting
Measure the finished substrate, not the structural one. Cut the metal to the measured face. Put the gap and its tolerance on the shop drawing at 1:5, with the reference surface named.
A shadow gap is a joint between two trades, not a detail on one drawing. Whoever owns the reference surface owns the gap.
Got a drawing
that contradicts
one of these notes?
Send it. We will tell you what we would do with it, whether you order or not. The 48-hour question list is free.