diff --git a/global_defs.scad b/global_defs.scad
index f7f635a..d9960ba 100644
--- a/global_defs.scad
+++ b/global_defs.scad
@@ -29,6 +29,7 @@ rr_green = [0, 146/255, 0];                                               // Rep
 crimson  = [220/255, 20/255, 60/255];
 
 layer_height    = is_undef($layer_height)    ? 0.25   : $layer_height;    // layer height when printing
+layer_height0   = is_undef($layer_height0)   ? layer_height : $layer_height0; // height of first layer if different
 extrusion_width = is_undef($extrusion_width) ? 0.5    : $extrusion_width; // filament width when printing
 nozzle          = is_undef($nozzle)          ? 0.45   : $nozzle;          // 3D printer nozzle
 cnc_bit_r       = is_undef($cnc_bit_r)       ? 1.2    : $cnc_bit_r;       // minimum tool radius when milling 2D objects
@@ -51,10 +52,7 @@ eps = 1/128;     // small fudge factor to stop CSG barfing on coincident faces.
 $fa = 6;
 $fs = extrusion_width / 2;
 
-function round_to_layer(z) = ceil(z / layer_height) * layer_height;         //! Round up to a multiple of layer_height.
-
 // Some additional named colours
-function grey(n) = [0.01, 0.01, 0.01] * n;                                  //! Generate a shade of grey to pass to color().
 silver           = [0.75, 0.75, 0.75];
 gold             = [255, 215,   0] / 255;
 brass            = [255, 220, 100] / 255;
diff --git a/readme.md b/readme.md
index 80b9415..15b854a 100644
--- a/readme.md
+++ b/readme.md
@@ -6918,6 +6918,7 @@ Original version by Doug Moen on the OpenSCAD forum
 <a name="Global"></a>
 ## Global
 Global constants, functions and modules. This file is used directly or indirectly in every scad file.
+See [global_defs.scad](../../global_defs.scad) for a list of global constants.
 
 [utils/core/global.scad](utils/core/global.scad) Implementation.
 
@@ -6930,6 +6931,7 @@ Global constants, functions and modules. This file is used directly or indirectl
 | `cm(x)` | cm to mm conversion |
 | `echoit(x)` | Echo expression and return it, useful for debugging |
 | `foot(x)` | Foot to mm conversion |
+| `grey(n)` | Generate a shade of grey to pass to color(). |
 | `in(list, x)` | Returns true if `x` is an element in the `list` |
 | `inch(x)` | Inch to mm conversion (For fractional inches, 'inch(1 + 7/8)' will work as expected.) |
 | `limit(x, min, max)` | Force x in range min <= x <= max |
@@ -6938,6 +6940,7 @@ Global constants, functions and modules. This file is used directly or indirectl
 | `no_point(str)` | Replace decimal point in string with 'p' |
 | `r2sides(r)` | Replicates the OpenSCAD logic to calculate the number of sides from the radius |
 | `r2sides4n(r)` | Round up the number of sides to a multiple of 4 to ensure points land on all axes |
+| `round_to_layer(z)` | Round up to a layer boundary using `layer_height0` for the first layer and `layer_height` for subsequent layers. |
 | `slice(list, start = 0, end = undef)` | Slice a list or string with Python type semantics |
 | `sqr(x)` | Returns the square of `x` |
 | `yard(x)` | Yard to mm conversion |
diff --git a/utils/core/global.scad b/utils/core/global.scad
index f0d9949..63499d4 100644
--- a/utils/core/global.scad
+++ b/utils/core/global.scad
@@ -19,6 +19,7 @@
 
 //
 //! Global constants, functions and modules. This file is used directly or indirectly in every scad file.
+//! See [global_defs.scad](../../global_defs.scad) for a list of global constants.
 //
 include <../../global_defs.scad>
 
@@ -38,6 +39,13 @@ function r2sides(r) = $fn ? $fn : ceil(max(min(360/ $fa, r * 2 * PI / $fs), 5));
 function r2sides4n(r) = floor((r2sides(r) + 3) / 4) * 4;                            //! Round up the number of sides to a multiple of 4 to ensure points land on all axes
 function limit(x, min, max) = max(min(x, max), min);                                //! Force x in range min <= x <= max
 
+function round_to_layer(z) =  //! Round up to a layer boundary using `layer_height0` for the first layer and `layer_height` for subsequent layers.
+    z <= 0 ? 0 :
+    z <= layer_height0 ? layer_height0 :
+    ceil((z -layer_height0) / layer_height) * layer_height + layer_height0;
+
+function grey(n) = [0.01, 0.01, 0.01] * n;                                          //! Generate a shade of grey to pass to color().
+
 module translate_z(z)                                                               //! Shortcut for Z only translations
     translate([0, 0, z]) children();