diff --git a/src/g_shared/p_3dfloors.cpp b/src/g_shared/p_3dfloors.cpp
index 1d3e2d4c5..9ef599a69 100644
--- a/src/g_shared/p_3dfloors.cpp
+++ b/src/g_shared/p_3dfloors.cpp
@@ -692,6 +692,7 @@ void P_LineOpening_XFloors (FLineOpening &open, AActor * thing, const line_t *li
 			double    lowestfloor[2] = {
 				linedef->frontsector->floorplane.ZatPoint(x, y), 
 				linedef->backsector->floorplane.ZatPoint(x, y) };
+			bool lowestfloorset[2] = { false, false };
 			FTextureID highestfloorpic;
 			int highestfloorterrain = -1;
 			FTextureID lowestceilingpic;
@@ -741,7 +742,11 @@ void P_LineOpening_XFloors (FLineOpening &open, AActor * thing, const line_t *li
 							highestfloorplanes[j] = rover->top.plane;
 						}
 					}
-					if(ff_top > lowestfloor[j] && ff_top <= thing->Z() + thing->MaxStepHeight) lowestfloor[j] = ff_top;
+					if (ff_top > lowestfloor[j] && ff_top <= thing->Z() + thing->MaxStepHeight)
+					{
+						lowestfloor[j] = ff_top;
+						lowestfloorset[j] = true;
+					}
 				}
 			}
 			
@@ -771,8 +776,14 @@ void P_LineOpening_XFloors (FLineOpening &open, AActor * thing, const line_t *li
 				open.topsec = lowestceilingsec;
 				open.topffloor = lowestceilingffloor;
 			}
-			
-			open.lowfloor = MIN(lowestfloor[0], lowestfloor[1]);
+
+			// Don't overwrite still valid info from portals here.
+			if ((open.lowfloorthroughportal & 1) && lowestfloorset[0]) open.lowfloorthroughportal &= ~1;
+			if ((open.lowfloorthroughportal & 2) && lowestfloorset[1]) open.lowfloorthroughportal &= ~2;
+
+			double low1 = (open.lowfloorthroughportal & 1) ? open.lowfloor : lowestfloor[0];
+			double low2 = (open.lowfloorthroughportal & 2) ? open.lowfloor : lowestfloor[1];
+			open.lowfloor = MIN(low1, low2);
 		}
     }
 }
diff --git a/src/g_shared/p_checkposition.h b/src/g_shared/p_checkposition.h
index c475e282d..36f3e735b 100644
--- a/src/g_shared/p_checkposition.h
+++ b/src/g_shared/p_checkposition.h
@@ -36,6 +36,7 @@ struct FCheckPosition
 	TMap<AActor*, bool> LastRipped;
 	bool			DoRipping;
 	bool			portalstep;
+	bool			dropoffisportal;
 	int				portalgroup;
 
 	int				PushTime;
diff --git a/src/p_map.cpp b/src/p_map.cpp
index b33e8b133..06bd5dce1 100644
--- a/src/p_map.cpp
+++ b/src/p_map.cpp
@@ -280,6 +280,7 @@ void P_GetFloorCeilingZ(FCheckPosition &tmf, int flags)
 
 	tmf.ceilingz = NextHighestCeilingAt(sec, tmf.pos.X, tmf.pos.Y, tmf.pos.Z, tmf.pos.Z + tmf.thing->Height, flags, &tmf.ceilingsector, &ffc);
 	tmf.floorz = tmf.dropoffz = NextLowestFloorAt(sec, tmf.pos.X, tmf.pos.Y, tmf.pos.Z, flags, tmf.thing->MaxStepHeight, &tmf.floorsector, &fff);
+	tmf.dropoffisportal = tmf.floorsector != sec;
 
 	if (fff)
 	{
@@ -1018,7 +1019,12 @@ bool PIT_CheckLine(FMultiBlockLinesIterator &mit, FMultiBlockLinesIterator::Chec
 
 			if (open.lowfloor < tm.dropoffz)
 			{
-				tm.dropoffz = open.lowfloor;
+				// Do not alter the dropoff if the previous portal layer got it solely from its own data.
+				if (!(cres.portalflags & FFCF_NOCEILING) || tm.dropoffisportal)
+				{
+					tm.dropoffz = open.lowfloor;
+					tm.dropoffisportal = open.lowfloorthroughportal;
+				}
 			}
 		}
 	}
@@ -1680,6 +1686,7 @@ bool P_CheckPosition(AActor *thing, const DVector2 &pos, FCheckPosition &tm, boo
 		else
 		{
 			tm.floorz = tm.dropoffz = newsec->floorplane.ZatPoint(pos);
+			tm.dropoffisportal = false;
 			tm.floorpic = newsec->GetTexture(sector_t::floor);
 			tm.ceilingz = newsec->ceilingplane.ZatPoint(pos);
 			tm.ceilingpic = newsec->GetTexture(sector_t::ceiling);
@@ -1704,6 +1711,7 @@ bool P_CheckPosition(AActor *thing, const DVector2 &pos, FCheckPosition &tm, boo
 			if (ff_top > tm.floorz && fabs(delta1) < fabs(delta2))
 			{
 				tm.floorz = tm.dropoffz = ff_top;
+				tm.dropoffisportal = false;
 				tm.floorpic = *rover->top.texture;
 				tm.floorterrain = rover->model->GetTerrain(rover->top.isceiling);
 			}
diff --git a/src/p_maputl.cpp b/src/p_maputl.cpp
index 16f6a72ad..a9cd38355 100644
--- a/src/p_maputl.cpp
+++ b/src/p_maputl.cpp
@@ -177,12 +177,17 @@ void P_LineOpening (FLineOpening &open, AActor *actor, const line_t *linedef, co
 
 		bool usefront;
 
+		// Store portal state to avoid registering false dropoffs.
+		open.lowfloorthroughportal = 0;
+		if (ff == LINEOPEN_MIN) open.lowfloorthroughportal |= 1;
+		if (bf == LINEOPEN_MIN) open.lowfloorthroughportal |= 2;
+
 		// [RH] fudge a bit for actors that are moving across lines
 		// bordering a slope/non-slope that meet on the floor. Note
 		// that imprecisions in the plane equation mean there is a
 		// good chance that even if a slope and non-slope look like
 		// they line up, they won't be perfectly aligned.
-		if (ff == -FLT_MIN || bf == -FLT_MIN || ref == NULL || fabs (ff-bf) > 1./256)
+		if (ff == LINEOPEN_MIN || bf == LINEOPEN_MIN || ref == NULL || fabs (ff-bf) > 1./256)
 		{
 			usefront = (ff > bf);
 		}
@@ -196,6 +201,7 @@ void P_LineOpening (FLineOpening &open, AActor *actor, const line_t *linedef, co
 				usefront = !P_PointOnLineSide (*ref, linedef);
 		}
 
+		open.lowfloorthroughportal = false;
 		if (usefront)
 		{
 			open.bottom = ff;
diff --git a/src/p_maputl.h b/src/p_maputl.h
index 5a65c27d3..e86d363cf 100644
--- a/src/p_maputl.h
+++ b/src/p_maputl.h
@@ -111,6 +111,7 @@ struct FLineOpening
 	int				floorterrain;
 	bool			touchmidtex;
 	bool			abovemidtex;
+	uint8_t			lowfloorthroughportal;
 	F3DFloor		*topffloor;
 	F3DFloor		*bottomffloor;
 };