diff --git a/src/p_map.cpp b/src/p_map.cpp
index 578375358a..d1878162f9 100644
--- a/src/p_map.cpp
+++ b/src/p_map.cpp
@@ -162,11 +162,22 @@ static inline fixed_t GetCoefficientClosestPointInLine24(line_t *ld, fixedvec2 p
 
 static inline fixedvec2 FindRefPoint(line_t *ld, fixedvec2 pos)
 {
-	if (!((((ld->frontsector->floorplane.a | ld->frontsector->floorplane.b) |
-		(ld->backsector->floorplane.a | ld->backsector->floorplane.b) |
-		(ld->frontsector->ceilingplane.a | ld->frontsector->ceilingplane.b) |
-		(ld->backsector->ceilingplane.a | ld->backsector->ceilingplane.b)) == 0)
-		&& ld->backsector->e->XFloor.ffloors.Size() == 0 && ld->frontsector->e->XFloor.ffloors.Size() == 0))
+	// If there's any chance of slopes getting in the way we need to get a proper refpoint, otherwise we can save the work.
+	// Slopes can get in here when:
+	// - the actual sector planes are sloped
+	// - there's 3D floors in this sector
+	// - there's a crossable floor portal (for which the dropoff needs to be calculated within P_LineOpening, and the lower sector can easily have slopes)
+	if (
+		(((ld->frontsector->floorplane.a | ld->frontsector->floorplane.b) |
+			(ld->backsector->floorplane.a | ld->backsector->floorplane.b) |
+			(ld->frontsector->ceilingplane.a | ld->frontsector->ceilingplane.b) |
+			(ld->backsector->ceilingplane.a | ld->backsector->ceilingplane.b)) != 0)
+		||
+			ld->backsector->e->XFloor.ffloors.Size() == 0 
+		||
+			ld->frontsector->e->XFloor.ffloors.Size() == 0
+		|| 
+			!ld->frontsector->PortalBlocksMovement(sector_t::floor))
 	{
 		fixed_t r = GetCoefficientClosestPointInLine24(ld, pos);
 		if (r <= 0)
@@ -972,7 +983,9 @@ bool PIT_CheckLine(FMultiBlockLinesIterator &mit, FMultiBlockLinesIterator::Chec
 		}
 
 		if (open.lowfloor < tm.dropoffz)
+		{
 			tm.dropoffz = open.lowfloor;
+		}
 	}
 
 	// if contacted a special line, add it to the list
diff --git a/src/p_maputl.cpp b/src/p_maputl.cpp
index 057c4329c3..9d98f4bf55 100644
--- a/src/p_maputl.cpp
+++ b/src/p_maputl.cpp
@@ -150,7 +150,7 @@ void P_LineOpening (FLineOpening &open, AActor *actor, const line_t *linedef,
 	if (!(flags & FFCF_ONLY3DFLOORS))
 	{
 		sector_t *front, *back;
-		fixed_t fc, ff, bc, bf;
+		fixed_t fc = 0, ff = 0, bc = 0, bf = 0;
 
 		if (linedef->backsector == NULL)
 		{
@@ -162,29 +162,20 @@ void P_LineOpening (FLineOpening &open, AActor *actor, const line_t *linedef,
 		front = linedef->frontsector;
 		back = linedef->backsector;
 
-		if (!(flags & FFCF_NOPORTALS) && !linedef->frontsector->PortalBlocksMovement(sector_t::ceiling) && 
-			linedef->backsector->SkyBoxes[sector_t::ceiling] &&
-			linedef->frontsector->SkyBoxes[sector_t::ceiling]->Sector->PortalGroup == linedef->backsector->SkyBoxes[sector_t::ceiling]->Sector->PortalGroup)
+		if (!(flags & FFCF_NOPORTALS))
 		{
-			fc = bc = FIXED_MAX;
-		}
-		else
-		{
-			fc = front->ceilingplane.ZatPoint(x, y);
-			bc = back->ceilingplane.ZatPoint(x, y);
-		}
-		if (!(flags & FFCF_NOPORTALS) && !linedef->frontsector->PortalBlocksMovement(sector_t::floor) &&
-			linedef->backsector->SkyBoxes[sector_t::floor] &&
-			linedef->frontsector->SkyBoxes[sector_t::floor]->Sector->PortalGroup == linedef->backsector->SkyBoxes[sector_t::floor]->Sector->PortalGroup)
-		{
-			ff = bf = FIXED_MIN;
-		}
-		else
-		{
-			ff = front->floorplane.ZatPoint(x, y);
-			bf = back->floorplane.ZatPoint(x, y);
+			if (!linedef->frontsector->PortalBlocksMovement(sector_t::ceiling)) fc = FIXED_MAX;
+			if (!linedef->backsector->PortalBlocksMovement(sector_t::ceiling)) bc = FIXED_MAX;
+			if (!linedef->frontsector->PortalBlocksMovement(sector_t::floor)) ff = FIXED_MIN;
+			if (!linedef->backsector->PortalBlocksMovement(sector_t::floor)) bf = FIXED_MIN;
 		}
 
+		if (fc == 0) fc = front->ceilingplane.ZatPoint(x, y);
+		if (bc == 0) bc = back->ceilingplane.ZatPoint(x, y);
+		if (ff == 0) ff = front->floorplane.ZatPoint(x, y);
+		if (bf == 0) bf = back->floorplane.ZatPoint(x, y);
+
+
 		/*Printf ("]]]]]] %d %d\n", ff, bf);*/
 
 		open.topsec = fc < bc? front : back;
@@ -198,8 +189,7 @@ void P_LineOpening (FLineOpening &open, AActor *actor, const line_t *linedef,
 		// 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 (refx == FIXED_MIN ||
-			abs (ff-bf) > 256)
+		if (ff == FIXED_MIN || bf == FIXED_MIN || (refx == FIXED_MIN || abs (ff-bf) > 256))
 		{
 			usefront = (ff > bf);
 		}
@@ -219,7 +209,13 @@ void P_LineOpening (FLineOpening &open, AActor *actor, const line_t *linedef,
 			open.bottomsec = front;
 			open.floorpic = front->GetTexture(sector_t::floor);
 			open.floorterrain = front->GetTerrain(sector_t::floor);
-			open.lowfloor = bf;
+			if (bf != FIXED_MIN) open.lowfloor = bf;
+			else
+			{
+				// We must check through the portal for the actual dropoff.
+				// If there's no lines in the lower sections we'd never get a usable value otherwise.
+				open.lowfloor = back->NextLowestFloorAt(refx, refy, back->SkyBoxes[sector_t::floor]->threshold-1);
+			}
 		}
 		else
 		{
@@ -227,7 +223,13 @@ void P_LineOpening (FLineOpening &open, AActor *actor, const line_t *linedef,
 			open.bottomsec = back;
 			open.floorpic = back->GetTexture(sector_t::floor);
 			open.floorterrain = back->GetTerrain(sector_t::floor);
-			open.lowfloor = ff;
+			if (ff != FIXED_MIN) open.lowfloor = ff;
+			else
+			{
+				// We must check through the portal for the actual dropoff.
+				// If there's no lines in the lower sections we'd never get a usable value otherwise.
+				open.lowfloor = front->NextLowestFloorAt(refx, refy, front->SkyBoxes[sector_t::floor]->threshold - 1);
+			}
 		}
 		open.frontfloorplane = front->floorplane;
 		open.backfloorplane = back->floorplane;