diff --git a/src/p_maputl.c b/src/p_maputl.c
index 14958c941..10aa4bb3d 100644
--- a/src/p_maputl.c
+++ b/src/p_maputl.c
@@ -754,24 +754,30 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj, opening_t *open)
 
 				if (delta1 > delta2)
 				{
-					if (polybottom < open->ceiling)
+					if (open->fofType != LO_FOF_FLOORS)
 					{
-						open->ceiling = polybottom;
-					}
-					else if (polybottom < open->highceiling)
-					{
-						open->highceiling = polybottom;
+						if (polybottom < open->ceiling)
+						{
+							open->ceiling = polybottom;
+						}
+						else if (polybottom < open->highceiling)
+						{
+							open->highceiling = polybottom;
+						}
 					}
 				}
 				else
 				{
-					if (polytop > open->floor)
+					if (open->fofType != LO_FOF_CEILINGS)
 					{
-						open->floor = polytop;
-					}
-					else if (polytop > open->lowfloor)
-					{
-						open->lowfloor = polytop;
+						if (polytop > open->floor)
+						{
+							open->floor = polytop;
+						}
+						else if (polytop > open->lowfloor)
+						{
+							open->lowfloor = polytop;
+						}
 					}
 				}
 			}
@@ -811,15 +817,28 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj, opening_t *open)
 						|| (rover->fofflags & FOF_BLOCKOTHERS && !mobj->player)))
 						continue;
 
-					topheight = P_GetFOFTopZ(mobj, front, rover, tm.x, tm.y, linedef);
-					bottomheight = P_GetFOFBottomZ(mobj, front, rover, tm.x, tm.y, linedef);
+					if (open->fofType != LO_FOF_ANY)
+					{
+						topheight = P_VeryTopOfFOF(rover);
+						bottomheight = P_VeryBottomOfFOF(rover);
+					}
+					else
+					{
+						topheight = P_GetFOFTopZ(mobj, front, rover, tm.x, tm.y, linedef);
+						bottomheight = P_GetFOFBottomZ(mobj, front, rover, tm.x, tm.y, linedef);
+					}
 
 					midheight = bottomheight + (topheight - bottomheight) / 2;
 					delta1 = abs(mobj->z - midheight);
 					delta2 = abs(thingtop - midheight);
 
-					if (delta1 >= delta2)
+					if (delta1 > delta2)
 					{
+						if (open->fofType == LO_FOF_FLOORS)
+						{
+							continue;
+						}
+
 						// thing is below FOF
 						if ((rover->fofflags & FOF_INTANGIBLEFLATS) != FOF_PLATFORM)
 						{
@@ -832,6 +851,11 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj, opening_t *open)
 					}
 					else
 					{
+						if (open->fofType == LO_FOF_CEILINGS)
+						{
+							continue;
+						}
+
 						// thing is above FOF
 						if ((rover->fofflags & FOF_INTANGIBLEFLATS) != FOF_REVERSEPLATFORM)
 						{
@@ -858,15 +882,28 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj, opening_t *open)
 						|| (rover->fofflags & FOF_BLOCKOTHERS && !mobj->player)))
 						continue;
 
-					topheight = P_GetFOFTopZ(mobj, back, rover, tm.x, tm.y, linedef);
-					bottomheight = P_GetFOFBottomZ(mobj, back, rover, tm.x, tm.y, linedef);
+					if (open->fofType != LO_FOF_ANY)
+					{
+						topheight = P_VeryTopOfFOF(rover);
+						bottomheight = P_VeryBottomOfFOF(rover);
+					}
+					else
+					{
+						topheight = P_GetFOFTopZ(mobj, back, rover, tm.x, tm.y, linedef);
+						bottomheight = P_GetFOFBottomZ(mobj, back, rover, tm.x, tm.y, linedef);
+					}
 
 					midheight = bottomheight + (topheight - bottomheight) / 2;
 					delta1 = abs(mobj->z - midheight);
 					delta2 = abs(thingtop - midheight);
 
-					if (delta1 >= delta2)
+					if (delta1 > delta2)
 					{
+						if (open->fofType == LO_FOF_FLOORS)
+						{
+							continue;
+						}
+
 						// thing is below FOF
 						if ((rover->fofflags & FOF_INTANGIBLEFLATS) != FOF_PLATFORM)
 						{
@@ -879,6 +916,11 @@ void P_LineOpening(line_t *linedef, mobj_t *mobj, opening_t *open)
 					}
 					else
 					{
+						if (open->fofType == LO_FOF_CEILINGS)
+						{
+							continue;
+						}
+
 						// thing is above FOF
 						if ((rover->fofflags & FOF_INTANGIBLEFLATS) != FOF_REVERSEPLATFORM)
 						{
diff --git a/src/p_maputl.h b/src/p_maputl.h
index 4656ffdd4..0d6d5a346 100644
--- a/src/p_maputl.h
+++ b/src/p_maputl.h
@@ -70,8 +70,13 @@ struct opening_t
 	fixed_t ceilingstep, ceilingdrop;
 	fixed_t floorstep, floordrop;
 	INT32 ceilingpic, floorpic;
+	UINT8 fofType; // LO_FOF_ types for forcing FOF collide
 };
 
+#define LO_FOF_ANY		(0)
+#define LO_FOF_FLOORS	(1)
+#define LO_FOF_CEILINGS	(2)
+
 void P_LineOpening(line_t *plinedef, mobj_t *mobj, opening_t *open);
 
 typedef enum
diff --git a/src/p_sight.c b/src/p_sight.c
index 761b4b47b..20d94484d 100644
--- a/src/p_sight.c
+++ b/src/p_sight.c
@@ -384,6 +384,7 @@ static boolean P_CanBotTraverse(seg_t *seg, divline_t *divl, register los_t *los
 	tm.y = los->strace.y + FixedMul(los->strace.dy, frac);
 
 	// set openrange, opentop, openbottom
+	open.fofType = (flip ? LO_FOF_CEILINGS : LO_FOF_FLOORS);
 	P_LineOpening(line, los->t1, &open);
 	maxstep = P_GetThingStepUp(los->t1, tm.x, tm.y);
 
@@ -451,10 +452,11 @@ static boolean P_CanWaypointTraverse(seg_t *seg, divline_t *divl, register los_t
 	tm.y = los->strace.y + FixedMul(los->strace.dy, frac);
 
 	// set openrange, opentop, openbottom
+	open.fofType = (flip ? LO_FOF_CEILINGS : LO_FOF_FLOORS);
 	P_LineOpening(line, los->t1, &open);
 	maxstep = P_GetThingStepUp(los->t1, tm.x, tm.y);
 
-#if 1
+#if 0
 	if (los->t2->type == MT_WAYPOINT)
 	{
 		waypoint_t *wp = K_SearchWaypointHeapForMobj(los->t2);