diff --git a/src/p_map.c b/src/p_map.c
index 93c885241..c6044c6f9 100644
--- a/src/p_map.c
+++ b/src/p_map.c
@@ -58,6 +58,8 @@ mobj_t *tmfloorthing; // the thing corresponding to tmfloorz or NULL if tmfloorz
 mobj_t *tmhitthing; // the solid thing you bumped into (for collisions)
 ffloor_t *tmfloorrover, *tmceilingrover;
 pslope_t *tmfloorslope, *tmceilingslope;
+static fixed_t tmfloordiff;
+static fixed_t tmceilingdiff;
 
 // keep track of the line that lowers the ceiling,
 // so missiles don't explode against sky hack walls
@@ -1704,6 +1706,7 @@ static boolean PIT_CheckLine(line_t *ld)
 		ceilingline = ld;
 		tmceilingrover = openceilingrover;
 		tmceilingslope = opentopslope;
+		tmceilingdiff = openceilingdiff;
 	}
 
 	if (openbottom > tmfloorz)
@@ -1711,6 +1714,7 @@ static boolean PIT_CheckLine(line_t *ld)
 		tmfloorz = openbottom;
 		tmfloorrover = openfloorrover;
 		tmfloorslope = openbottomslope;
+		tmfloordiff = openfloordiff;
 	}
 
 	if (highceiling > tmdrpoffceilz)
@@ -1800,6 +1804,9 @@ boolean P_CheckPosition(mobj_t *thing, fixed_t x, fixed_t y)
 	tmfloorslope = newsubsec->sector->f_slope;
 	tmceilingslope = newsubsec->sector->c_slope;
 
+	tmfloordiff = 0;
+	tmceilingdiff = 0;
+
 	// Check list of fake floors and see if tmfloorz/tmceilingz need to be altered.
 	if (newsubsec->sector->ffloors)
 	{
@@ -2517,7 +2524,7 @@ boolean P_TryMove(mobj_t *thing, fixed_t x, fixed_t y, boolean allowdropoff)
 						thing->ceilingrover = tmceilingrover;
 						thing->eflags |= MFE_JUSTSTEPPEDDOWN;
 					}
-					else if (tmceilingz < thingtop && thingtop - tmceilingz <= maxstep)
+					else if (tmceilingz < thingtop && tmceilingdiff <= maxstep)
 					{
 						thing->z = (thing->ceilingz = thingtop = tmceilingz) - thing->height;
 						thing->ceilingrover = tmceilingrover;
@@ -2530,7 +2537,7 @@ boolean P_TryMove(mobj_t *thing, fixed_t x, fixed_t y, boolean allowdropoff)
 					thing->floorrover = tmfloorrover;
 					thing->eflags |= MFE_JUSTSTEPPEDDOWN;
 				}
-				else if (tmfloorz > thing->z && tmfloorz - thing->z <= maxstep)
+				else if (tmfloorz > thing->z && tmfloordiff <= maxstep)
 				{
 					thing->z = thing->floorz = tmfloorz;
 					thing->floorrover = tmfloorrover;
diff --git a/src/p_maputl.c b/src/p_maputl.c
index de6feb143..a0ef680fb 100644
--- a/src/p_maputl.c
+++ b/src/p_maputl.c
@@ -279,6 +279,8 @@ fixed_t P_InterceptVector(divline_t *v2, divline_t *v1)
 fixed_t opentop, openbottom, openrange, lowfloor, highceiling;
 pslope_t *opentopslope, *openbottomslope;
 ffloor_t *openfloorrover, *openceilingrover;
+fixed_t openfloordiff;
+fixed_t openceilingdiff;
 
 // P_CameraLineOpening
 // P_LineOpening, but for camera
@@ -424,6 +426,7 @@ void P_CameraLineOpening(line_t *linedef)
 void P_LineOpening(line_t *linedef, mobj_t *mobj)
 {
 	sector_t *front, *back;
+	vertex_t cross;
 
 	if (linedef->sidenum[1] == 0xffff)
 	{
@@ -432,6 +435,8 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 		return;
 	}
 
+	P_ClosestPointOnLine(tmx, tmy, linedef, &cross);
+
 	// Treat polyobjects kind of like 3D Floors
 	if (linedef->polyobj && (linedef->polyobj->flags & POF_TESTHEIGHT))
 	{
@@ -456,6 +461,8 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 		highceiling = INT32_MIN;
 		lowfloor = INT32_MAX;
 		opentopslope = openbottomslope = NULL;
+		openfloordiff = 0;
+		openceilingdiff = 0;
 	}
 	else
 	{ // Set open and high/low values here
@@ -492,6 +499,18 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 			lowfloor = frontheight;
 			openbottomslope = back->f_slope;
 		}
+
+		openfloordiff =
+			abs(
+					P_GetSectorFloorZAt(front, cross.x, cross.y) -
+					P_GetSectorFloorZAt(back,  cross.x, cross.y)
+			);
+
+		openceilingdiff =
+			abs(
+					P_GetSectorCeilingZAt(front, cross.x, cross.y) -
+					P_GetSectorCeilingZAt(back,  cross.x, cross.y)
+			);
 	}
 
 	if (mobj)
@@ -548,10 +567,16 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 
 				if (delta1 > delta2) { // Below
 					if (opentop > texbottom)
+					{
+						openceilingdiff = ( opentop - texbottom );
 						opentop = texbottom;
+					}
 				} else { // Above
 					if (openbottom < textop)
+					{
+						openfloordiff = ( textop - openbottom );
 						openbottom = textop;
+					}
 				}
 			}
 		}
@@ -579,14 +604,24 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj)
 				delta2 = abs(thingtop - (polybottom + ((polytop - polybottom)/2)));
 
 				if (polybottom < opentop && delta1 >= delta2)
+				{
+					openceilingdiff = ( opentop - polybottom );
 					opentop = polybottom;
+				}
 				else if (polybottom < highceiling && delta1 >= delta2)
+				{
 					highceiling = polybottom;
+				}
 
 				if (polytop > openbottom && delta1 < delta2)
+				{
+					openfloordiff = ( polytop - openbottom );
 					openbottom = polytop;
+				}
 				else if (polytop > lowfloor && delta1 < delta2)
+				{
 					lowfloor = polytop;
+				}
 			}
 			// otherwise don't do anything special, pretend there's nothing else there
 		}
diff --git a/src/p_maputl.h b/src/p_maputl.h
index 1c071c2cd..1a885c586 100644
--- a/src/p_maputl.h
+++ b/src/p_maputl.h
@@ -58,6 +58,8 @@ void P_HitSpecialLines(mobj_t *thing, fixed_t x, fixed_t y, fixed_t momx, fixed_
 extern fixed_t opentop, openbottom, openrange, lowfloor, highceiling;
 extern pslope_t *opentopslope, *openbottomslope;
 extern ffloor_t *openfloorrover, *openceilingrover;
+extern fixed_t openfloordiff;
+extern fixed_t openceilingdiff;
 
 void P_LineOpening(line_t *plinedef, mobj_t *mobj);