diff --git a/src/p_sight.c b/src/p_sight.c
index 579b94e56..1b3ff0cf9 100644
--- a/src/p_sight.c
+++ b/src/p_sight.c
@@ -14,6 +14,7 @@
 #include "doomdef.h"
 #include "doomstat.h"
 #include "p_local.h"
+#include "p_slopes.h"
 #include "r_main.h"
 #include "r_state.h"
 
@@ -216,6 +217,10 @@ static boolean P_CrossSubsector(size_t num, register los_t *los)
 		const sector_t *front, *back;
 		const vertex_t *v1,*v2;
 		fixed_t frac;
+		fixed_t frontf, backf, frontc, backc;
+#ifdef ESLOPE
+		fixed_t fracx, fracy;
+#endif
 
 		// already checked other side?
 		if (line->validcount == validcount)
@@ -250,37 +255,51 @@ static boolean P_CrossSubsector(size_t num, register los_t *los)
 		if (!(line->flags & ML_TWOSIDED))
 			return false;
 
+		// calculate fractional intercept (how far along we are divided by how far we are from t2)
+		frac = P_InterceptVector2(&los->strace, &divl);
+
+		front = seg->frontsector;
+		back  = seg->backsector;
+#ifdef ESLOPE
+		// calculate position at intercept
+		fracx = los->strace.x + FixedMul(los->strace.dx, frac);
+		fracy = los->strace.y + FixedMul(los->strace.dy, frac);
+		// calculate sector heights
+		frontf = (front->f_slope) ? P_GetZAt(front->f_slope, fracx, fracy) : front->floorheight;
+		frontc = (front->c_slope) ? P_GetZAt(front->c_slope, fracx, fracy) : front->ceilingheight;
+		backf  = (back->f_slope)  ? P_GetZAt(back->f_slope, fracx, fracy)  : back->floorheight;
+		backc  = (back->c_slope)  ? P_GetZAt(back->c_slope, fracx, fracy)  : back->ceilingheight;
+#else
+		frontf = front->floorheight;
+		frontc = front->ceilingheight;
+		backf  = back->floorheight;
+		backc  = back->ceilingheight;
+#endif
 		// crosses a two sided line
 		// no wall to block sight with?
-		if ((front = seg->frontsector)->floorheight ==
-			(back = seg->backsector)->floorheight   &&
-			front->ceilingheight == back->ceilingheight
-			&& !front->ffloors && !back->ffloors)
+		if (frontf == backf && frontc == backc
+		&& !front->ffloors & !back->ffloors) // (and no FOFs)
 			continue;
 
 		// possible occluder
 		// because of ceiling height differences
-		popentop = front->ceilingheight < back->ceilingheight ?
-			front->ceilingheight : back->ceilingheight ;
+		popentop = min(frontc, backc);
 
 		// because of floor height differences
-		popenbottom = front->floorheight > back->floorheight ?
-			front->floorheight : back->floorheight ;
+		popenbottom = max(frontf, backf);
 
 		// quick test for totally closed doors
 		if (popenbottom >= popentop)
 			return false;
 
-		frac = P_InterceptVector2(&los->strace, &divl);
-
-		if (front->floorheight != back->floorheight)
+		if (frontf != backf)
 		{
 			fixed_t slope = FixedDiv(popenbottom - los->sightzstart , frac);
 			if (slope > los->bottomslope)
 				los->bottomslope = slope;
 		}
 
-		if (front->ceilingheight != back->ceilingheight)
+		if (frontc != backc)
 		{
 			fixed_t slope = FixedDiv(popentop - los->sightzstart , frac);
 			if (slope < los->topslope)
@@ -295,6 +314,7 @@ static boolean P_CrossSubsector(size_t num, register los_t *los)
 		{
 			ffloor_t *rover;
 			fixed_t topslope, bottomslope;
+			fixed_t topz, bottomz;
 			// check front sector's FOFs first
 			for (rover = front->ffloors; rover; rover = rover->next)
 			{
@@ -303,8 +323,16 @@ static boolean P_CrossSubsector(size_t num, register los_t *los)
 				{
 					continue;
 				}
-				topslope = FixedDiv(*rover->topheight - los->sightzstart , frac);
-				bottomslope = FixedDiv(*rover->bottomheight - los->sightzstart , frac);
+
+#ifdef ESLOPE
+				topz    = (*rover->t_slope) ? P_GetZAt(*rover->t_slope, fracx, fracy) : *rover->topheight;
+				bottomz = (*rover->b_slope) ? P_GetZAt(*rover->b_slope, fracx, fracy) : *rover->bottomheight;
+#else
+				topz    = *rover->topheight;
+				bottomz = *rover->bottomheight;
+#endif
+				topslope    = FixedDiv(topz - los->sightzstart , frac);
+				bottomslope = FixedDiv(bottomz - los->sightzstart , frac);
 				if (topslope >= los->topslope && bottomslope <= los->bottomslope)
 					return false; // view completely blocked
 			}
@@ -316,8 +344,16 @@ static boolean P_CrossSubsector(size_t num, register los_t *los)
 				{
 					continue;
 				}
-				topslope = FixedDiv(*rover->topheight - los->sightzstart , frac);
-				bottomslope = FixedDiv(*rover->bottomheight - los->sightzstart , frac);
+
+#ifdef ESLOPE
+				topz    = (*rover->t_slope) ? P_GetZAt(*rover->t_slope, fracx, fracy) : *rover->topheight;
+				bottomz = (*rover->b_slope) ? P_GetZAt(*rover->b_slope, fracx, fracy) : *rover->bottomheight;
+#else
+				topz    = *rover->topheight;
+				bottomz = *rover->bottomheight;
+#endif
+				topslope    = FixedDiv(topz - los->sightzstart , frac);
+				bottomslope = FixedDiv(bottomz - los->sightzstart , frac);
 				if (topslope >= los->topslope && bottomslope <= los->bottomslope)
 					return false; // view completely blocked
 			}