Why a Repair Shows in Raking Light

Why this matters

You finish a drywall patch at ten in the morning with the overhead lights on, it disappears, and you leave. At four in the afternoon the sun comes in low through the window at the end of the hall, and the customer can see a raised square from across the house. Nothing about the patch changed. The light did. Raking light, meaning light that skims along a surface at a low angle, turns height differences too small to see head-on into shadows and bright bands you cannot miss. A tech who understands the geometry can predict where a repair will show, build the patch so it survives, and check it before leaving instead of hearing about it the next day.

What raking light does, in one picture

 A RIDGE AT A PATCH EDGE, wall seen edge-on

 LOW LIGHT, about 5 deg to the wall face
   light
     \___
         \___      ridge
             \___ /\
 wall ___________/  \______________________
                 ^   |<-shadow->|
          lit face   shadow length is about
          brighter   11x the ridge height

 HEAD-ON LIGHT, about 90 deg to the wall face
            |    |    |    |    |
 wall ___________/\________________________
                 no shadow cast past the
                 ridge; it reads as flat

A bump on a wall does two things under light: it casts a shadow behind it, and its slopes face the light at a slightly different angle than the flat wall around them. Head-on light makes both effects tiny. Light at a grazing angle makes both large. The rest of this card puts numbers on how large.

The shadow: why a hair-thin ridge throws a long line

A ridge of height h lit at an angle a measured from the wall surface casts a shadow of length h divided by tan(a).

• At 45 degrees, tan(45) = 1, so the shadow is as long as the ridge is tall.
• At 5 degrees, tan(5) is about 0.0875, so the shadow is about 1 / 0.0875 = 11.4 times the ridge height.

A 1/32 in ridge, about 0.031 in, at a patch edge:

• At 45 degrees: shadow about 0.031 in, invisible from across a room.
• At 5 degrees: 0.031 / 0.0875 = about 0.36 in, a dark line more than a third of an inch wide running the whole length of the edge.

Checked the other way: 0.36 in of shadow times tan(5) is 0.36 x 0.0875 = 0.031 in, the ridge height. So the same ridge goes from nothing to a pencil-wide line purely by moving the light toward the wall.

The shading: why a hump shows even with no hard edge

A patch with no ridge at all can still show, because its feathered edges are slopes. A surface tilted toward a grazing light gets brighter; a surface tilted away gets darker. For a flat, matte paint, which scatters light about evenly in all directions, the brightness of a surface is roughly proportional to the sine of the angle between the light and the surface. That is an approximation that holds best for flat sheens; with eggshell and above, a glare component on top makes the difference larger, not smaller.

Take a patch built up 1/16 in, 0.0625 in, above the wall, lit at 3 degrees to the wall, and compare two feathering widths:

Feathered over 2 in. The edge slope is 0.0625 / 2 = 0.031, about 1.8 degrees.

• Slope facing the light: sin(3 + 1.8) / sin(3) = sin(4.8) / sin(3) = 0.084 / 0.052 = about 1.6, so 60 percent brighter than the wall.
• Slope facing away: sin(3 - 1.8) / sin(3) = sin(1.2) / sin(3) = 0.021 / 0.052 = about 0.40, so 60 percent darker.

Feathered over 8 in. The edge slope is 0.0625 / 8 = 0.0078, about 0.45 degrees.

• Facing the light: sin(3.45) / sin(3) = 0.060 / 0.052 = about 1.15, 15 percent brighter.
• Facing away: sin(2.55) / sin(3) = 0.044 / 0.052 = about 0.85, 15 percent darker.

The same 2 in feather, lit nearly head-on at 80 degrees to the wall: sin(81.8) / sin(80) = 0.990 / 0.985 = about 1.005, half a percent. Nobody sees half a percent.

Three things fall out of those numbers. Spreading the same build-up over four times the width cuts the shading from about 60 percent to about 15 percent either way, which is why finishers feather wide rather than sand a patch thin. The light angle matters more than anything you do to the patch: the identical edge reads as a 60 percent band at 3 degrees and half a percent at 80. And a patch that passes head-on tells you nothing about how it will look in raking light, because head-on light hides exactly the defects raking light reveals.

What the geometry says about building a patch

• Aim for flush, not proud. Every bit of height you leave above the wall becomes slope at the edges. Recess the tape by keeping the patch piece flush with or slightly below the surface, so the compound fills to flat rather than building a hump.
• Width is cheaper than thinness. Sanding a hump thin risks cutting through to tape and paper. Widening the feather does the same job for shading with no risk, at the cost of more compound and one more coat.
• The sharp edge is worse than the gentle hump. A tape edge or a knife line throws a hard shadow line in any grazing light, and the lower the light the longer the line; a smooth crown only shades. Kill edges first.
• Check the surface, not the paint. Primer and paint add no flatness. Whatever the work light shows on bare compound, it will show through two coats.

Where raking light happens in a house

You can usually find it on the walk-through, before you cut anything:

• A wall beside a window, especially a window in the adjacent wall that sends light along the surface rather than at it.
• A long hallway lit from one end, by a window, a door or a single fixture.
• Recessed can lights within a foot or two of a wall, which wash the wall from above at a steep angle to the ceiling and a shallow angle to the wall.
• A ceiling beside a window or a sliding door, lit across its whole width by low sun.
• A backsplash or wall under an under-cabinet light.
• A stair wall under a skylight.

Then think about time of day. East windows rake in the morning, west windows in late afternoon, and in winter the sun is lower all day, so a wall that never rakes in June may rake every afternoon in December. The customer lives in the room at all of those hours. You saw it at one.

Worked example: a patch in a hallway

An electrician has cut a 9 in by 9 in opening to add a switch box location in a hallway wall, then patched it. The hallway is 20 ft long with a window at its far end, and the patch is about 6 ft from the window on the long wall, so late-day light runs nearly along the wall surface. Smooth finish, flat paint.

First check, before primer. The electrician holds a work light flat against the wall a few feet from the patch, aimed along the surface, which reproduces the window's raking angle. Two things show:

• A ridge along the top edge of the patch where the tape edge was not feathered fully, estimated at about 1/32 in. At the window's low angle, roughly 5 degrees, the shadow math above says it will throw a line about 0.36 in wide along the full 9 in edge.
• The build-up over the whole patch, about 1/16 in, feathered only about 2 in past the patch edges. As the sun drops toward a 3 degree rake later in the afternoon, the shading math says the side of the patch nearest the window will read about 60 percent brighter than the wall and the far side about 60 percent darker.

The fix, priced. The ridge is sanded down, with a damp sanding sponge or a dust-extracting sander because dry-sanded compound is a fine dust you breathe (respiratory protection under the shop's written program if sanding dry, and in an older house, compound identified first per Textured Ceilings and Old Joint Compound), and the feather widened to about 8 in on every side. That turns the 9 in patch into a finished area of 9 + 8 + 8 = 25 in on each dimension, so the compound now covers a 25 in square instead of a 13 in square (9 + 2 + 2). The extra coat and sanding add about 0.75 hours and one dry time, which in this case means returning the next morning before priming. At the wider feather the shading drops to about 15 percent brighter and darker, and with the ridge gone there is no shadow line.

Second check, after the final coat. The work light goes back on, from both directions along the wall and from above. No line, and a broad, soft gradient that is hard to see without knowing where to look. That is a blended result in the vocabulary of Invisible, Blended or Acceptable: The Three Grades of Finish Repair, and it is the grade the work order promised for a smooth wall with flat paint touched up on the patch only.

What would have happened unchecked. The original patch looked fine head-on, and head-on is all the electrician saw that morning. The window would have shown the 0.36 in line and the bright-dark pair at about 60 percent that afternoon, and the second visit would have been a callback instead of a planned trip.

What changes the answer. On an eggshell or satin wall, glare from a bright patch edge makes the band stronger than these flat-paint numbers, and a wider feather alone may not reach blended. On a wall that was never finished flat to begin with, as GA-214's finish levels describe, the patch can be perfect and the wall around it still shows its own waves in the same light. And on a textured surface the geometry changes entirely: the texture's own shadows dominate, and what shows is the difference in pattern, which is a texture-matching problem rather than a flatness one. See related: How to Patch a Textured Ceiling After an Access Cut.

How to verify a patch against raking light

Check with a work light held flat to the surface, aimed along it, from at least two directions and from above or below for a ceiling, at the skim stage, before primer, and again after paint. Use the room's own raking source if it exists: open the blinds, turn on the can light nearest the wall. Run a straightedge across the patch; a rocking straightedge finds a hump the light will find later. If the patch passes the work-light check, write that on the work order along with the light source you checked against. If the room's worst light comes at a time you will not be there, say that too, in the same line.

References

• Gypsum Association, GA-214, Recommended Levels of Gypsum Board Finish
• See related: How to Patch Drywall After an Access Cut
• See related: Invisible, Blended or Acceptable: The Three Grades of Finish Repair
• See related: Why Paint Touch-Ups Flash
• See related: The Patch That Showed in Raking Light