diff --git a/src/r_bsp.cpp b/src/r_bsp.cpp
index 689977911..c7f6d8497 100644
--- a/src/r_bsp.cpp
+++ b/src/r_bsp.cpp
@@ -1185,7 +1185,7 @@ void R_Subsector (subsector_t *sub)
 					frontsector->GetTexture(sector_t::floor),
 					floorlightlevel + r_actualextralight,				// killough 3/16/98
 					frontsector->GetAlpha(sector_t::floor),
-					!!(frontsector->GetFlags(sector_t::floor) & PLANEF_ADDITIVE),
+					!!(fakeFloor->flags && FF_ADDITIVETRANS),
 					frontsector->GetXOffset(position),		// killough 3/7/98
 					frontsector->GetYOffset(position),		// killough 3/7/98
 					frontsector->GetXScale(position),
@@ -1250,7 +1250,7 @@ void R_Subsector (subsector_t *sub)
 					frontsector->GetTexture(sector_t::ceiling),
 					ceilinglightlevel + r_actualextralight,				// killough 4/11/98
 					frontsector->GetAlpha(sector_t::ceiling),
-					!!(frontsector->GetFlags(sector_t::ceiling) & PLANEF_ADDITIVE),
+					!!(fakeFloor->flags && FF_ADDITIVETRANS),
 					frontsector->GetXOffset(position),		// killough 3/7/98
 					frontsector->GetYOffset(position),		// killough 3/7/98
 					frontsector->GetXScale(position),
diff --git a/src/r_draw.cpp b/src/r_draw.cpp
index cc9bc9b26..60a0e46e4 100644
--- a/src/r_draw.cpp
+++ b/src/r_draw.cpp
@@ -79,6 +79,8 @@ void (*R_DrawSpan)(void);
 void (*R_DrawSpanMasked)(void);
 void (*R_DrawSpanTranslucent)(void);
 void (*R_DrawSpanMaskedTranslucent)(void);
+void (*R_DrawSpanAddClamp)(void);
+void (*R_DrawSpanMaskedAddClamp)(void);
 void (STACK_ARGS *rt_map4cols)(int,int,int);
 
 //
@@ -1324,6 +1326,152 @@ void R_DrawSpanMaskedTranslucentP_C (void)
 	}
 }
 
+void R_DrawSpanAddClampP_C (void)
+{
+	dsfixed_t			xfrac;
+	dsfixed_t			yfrac;
+	dsfixed_t			xstep;
+	dsfixed_t			ystep;
+	BYTE*				dest;
+	const BYTE*			source = ds_source;
+	const BYTE*			colormap = ds_colormap;
+	int 				count;
+	int 				spot;
+	DWORD *fg2rgb = dc_srcblend;
+	DWORD *bg2rgb = dc_destblend;
+
+	xfrac = ds_xfrac;
+	yfrac = ds_yfrac;
+
+	dest = ylookup[ds_y] + ds_x1 + dc_destorg;
+
+	count = ds_x2 - ds_x1 + 1;
+
+	xstep = ds_xstep;
+	ystep = ds_ystep;
+
+	if (ds_xbits == 6 && ds_ybits == 6)
+	{
+		// 64x64 is the most common case by far, so special case it.
+		do
+		{
+			spot = ((xfrac>>(32-6-6))&(63*64)) + (yfrac>>(32-6));
+			DWORD a = fg2rgb[colormap[source[spot]]] + bg2rgb[*dest];
+			DWORD b = a;
+
+			a |= 0x01f07c1f;
+			b &= 0x40100400;
+			a &= 0x3fffffff;
+			b = b - (b >> 5);
+			a |= b;
+			*dest++ = RGB32k[0][0][a & (a>>15)];
+			xfrac += xstep;
+			yfrac += ystep;
+		} while (--count);
+	}
+	else
+	{
+		BYTE yshift = 32 - ds_ybits;
+		BYTE xshift = yshift - ds_xbits;
+		int xmask = ((1 << ds_xbits) - 1) << ds_ybits;
+		do
+		{
+			spot = ((xfrac >> xshift) & xmask) + (yfrac >> yshift);
+			DWORD a = fg2rgb[colormap[source[spot]]] + bg2rgb[*dest];
+			DWORD b = a;
+
+			a |= 0x01f07c1f;
+			b &= 0x40100400;
+			a &= 0x3fffffff;
+			b = b - (b >> 5);
+			a |= b;
+			*dest++ = RGB32k[0][0][a & (a>>15)];
+			xfrac += xstep;
+			yfrac += ystep;
+		} while (--count);
+	}
+}
+
+void R_DrawSpanMaskedAddClampP_C (void)
+{
+	dsfixed_t			xfrac;
+	dsfixed_t			yfrac;
+	dsfixed_t			xstep;
+	dsfixed_t			ystep;
+	BYTE*				dest;
+	const BYTE*			source = ds_source;
+	const BYTE*			colormap = ds_colormap;
+	int 				count;
+	int 				spot;
+	DWORD *fg2rgb = dc_srcblend;
+	DWORD *bg2rgb = dc_destblend;
+
+	xfrac = ds_xfrac;
+	yfrac = ds_yfrac;
+
+	dest = ylookup[ds_y] + ds_x1 + dc_destorg;
+
+	count = ds_x2 - ds_x1 + 1;
+
+	xstep = ds_xstep;
+	ystep = ds_ystep;
+
+	if (ds_xbits == 6 && ds_ybits == 6)
+	{
+		// 64x64 is the most common case by far, so special case it.
+		do
+		{
+			BYTE texdata;
+
+			spot = ((xfrac>>(32-6-6))&(63*64)) + (yfrac>>(32-6));
+			texdata = source[spot];
+			if (texdata != 0)
+			{
+				DWORD a = fg2rgb[colormap[texdata]] + bg2rgb[*dest];
+				DWORD b = a;
+
+				a |= 0x01f07c1f;
+				b &= 0x40100400;
+				a &= 0x3fffffff;
+				b = b - (b >> 5);
+				a |= b;
+				*dest = RGB32k[0][0][a & (a>>15)];
+			}
+			dest++;
+			xfrac += xstep;
+			yfrac += ystep;
+		} while (--count);
+	}
+	else
+	{
+		BYTE yshift = 32 - ds_ybits;
+		BYTE xshift = yshift - ds_xbits;
+		int xmask = ((1 << ds_xbits) - 1) << ds_ybits;
+		do
+		{
+			BYTE texdata;
+		
+			spot = ((xfrac >> xshift) & xmask) + (yfrac >> yshift);
+			texdata = source[spot];
+			if (texdata != 0)
+			{
+				DWORD a = fg2rgb[colormap[texdata]] + bg2rgb[*dest];
+				DWORD b = a;
+
+				a |= 0x01f07c1f;
+				b &= 0x40100400;
+				a &= 0x3fffffff;
+				b = b - (b >> 5);
+				a |= b;
+				*dest = RGB32k[0][0][a & (a>>15)];
+			}
+			dest++;
+			xfrac += xstep;
+			yfrac += ystep;
+		} while (--count);
+	}
+}
+
 // [RH] Just fill a span with a color
 void R_FillSpan (void)
 {
@@ -2040,6 +2188,8 @@ void R_InitColumnDrawers ()
 #endif
 	R_DrawSpanTranslucent		= R_DrawSpanTranslucentP_C;
 	R_DrawSpanMaskedTranslucent = R_DrawSpanMaskedTranslucentP_C;
+	R_DrawSpanAddClamp			= R_DrawSpanAddClampP_C;
+	R_DrawSpanMaskedAddClamp	= R_DrawSpanMaskedAddClampP_C;
 }
 
 // [RH] Choose column drawers in a single place
diff --git a/src/r_draw.h b/src/r_draw.h
index ace6bde0b..c8a7f2d47 100644
--- a/src/r_draw.h
+++ b/src/r_draw.h
@@ -106,6 +106,12 @@ extern void (*R_DrawSpanTranslucent)(void);
 // Span drawing for masked, translucent textures.
 extern void (*R_DrawSpanMaskedTranslucent)(void);
 
+// Span drawing for translucent, additive textures.
+extern void (*R_DrawSpanAddClamp)(void);
+
+// Span drawing for masked, translucent, additive textures.
+extern void (*R_DrawSpanMaskedAddClamp)(void);
+
 // [RH] Span blit into an interleaved intermediate buffer
 extern void (*R_DrawColumnHoriz)(void);
 void R_DrawMaskedColumnHoriz (const BYTE *column, const FTexture::Span *spans);
diff --git a/src/r_plane.cpp b/src/r_plane.cpp
index 0652121f7..0e8237cb6 100644
--- a/src/r_plane.cpp
+++ b/src/r_plane.cpp
@@ -596,7 +596,6 @@ visplane_t *R_FindPlane (const secplane_t &height, FTextureID picnum, int lightl
 		else sky = 0;	// not skyflatnum so it can't be a sky
 		skybox = NULL;
 		alpha = FRACUNIT;
-		additive = false;
 	}
 		
 	// New visplane algorithm uses hash table -- killough
@@ -1047,7 +1046,7 @@ void R_DrawHeightPlanes(fixed_t height)
 				viewy = pl->viewy;
 				viewangle = pl->viewangle;
 				MirrorFlags = pl->MirrorFlags;
-				R_DrawSinglePlane (pl, pl->sky & 0x7FFFFFFF, false, true);
+				R_DrawSinglePlane (pl, pl->sky & 0x7FFFFFFF, pl->Additive, true);
 			}
 		}
 	}
@@ -1528,16 +1527,24 @@ void R_DrawNormalPlane (visplane_t *pl, fixed_t alpha, bool additive, bool maske
 	else
 		plane_shade = true;
 
-	// Additive not supported yet because the drawer function doesn't look like it can handle it.
 	if (spanfunc != R_FillSpan)
 	{
 		if (masked)
 		{
-			if (alpha < OPAQUE)
+			if (alpha < OPAQUE || additive)
 			{
-				spanfunc = R_DrawSpanMaskedTranslucent;
-				dc_srcblend = Col2RGB8[alpha>>10];
-				dc_destblend = Col2RGB8[(OPAQUE-alpha)>>10];
+				if (!additive)
+				{
+					spanfunc = R_DrawSpanMaskedTranslucent;
+					dc_srcblend = Col2RGB8[alpha>>10];
+					dc_destblend = Col2RGB8[(OPAQUE-alpha)>>10];
+				}
+				else
+				{
+					spanfunc = R_DrawSpanMaskedAddClamp;
+					dc_srcblend = Col2RGB8_LessPrecision[alpha>>10];
+					dc_destblend = Col2RGB8_LessPrecision[FRACUNIT>>10];
+				}
 			}
 			else
 			{
@@ -1546,11 +1553,20 @@ void R_DrawNormalPlane (visplane_t *pl, fixed_t alpha, bool additive, bool maske
 		}
 		else
 		{
-			if (alpha < OPAQUE)
+			if (alpha < OPAQUE || additive)
 			{
-				spanfunc = R_DrawSpanTranslucent;
-				dc_srcblend = Col2RGB8[alpha>>10];
-				dc_destblend = Col2RGB8[(OPAQUE-alpha)>>10];
+				if (!additive)
+				{
+					spanfunc = R_DrawSpanTranslucent;
+					dc_srcblend = Col2RGB8[alpha>>10];
+					dc_destblend = Col2RGB8[(OPAQUE-alpha)>>10];
+				}
+				else
+				{
+					spanfunc = R_DrawSpanAddClamp;
+					dc_srcblend = Col2RGB8_LessPrecision[alpha>>10];
+					dc_destblend = Col2RGB8_LessPrecision[FRACUNIT>>10];
+				}
 			}
 			else
 			{