diff --git a/source/core/maptypes.h b/source/core/maptypes.h
index 557e03be7..5519d49ec 100644
--- a/source/core/maptypes.h
+++ b/source/core/maptypes.h
@@ -421,6 +421,7 @@ struct sectortype
 			short number;  // usually used for matching number
 			bool u_defined;
 			uint8_t flags2;
+			uint8_t interpolate; // this one is valid even without u_defined.
 		};
 
 	};
diff --git a/source/core/savegamehelp.cpp b/source/core/savegamehelp.cpp
index f367e7bfd..71953737f 100644
--- a/source/core/savegamehelp.cpp
+++ b/source/core/savegamehelp.cpp
@@ -549,7 +549,8 @@ FSerializer &Serialize(FSerializer &arc, const char *key, sectortype &c, sectort
 				("damage", c.damage, def->damage)
 				("number", c.number, def->number)
 				("u_defined", c.u_defined, def->u_defined)
-				("flags2", c.flags2, def->flags2);
+				("flags2", c.flags2, def->flags2)
+				("interpolate", c.interpolate, def->interpolate);
 		}
 
 		arc.EndObject();
diff --git a/source/games/sw/src/interpso.cpp b/source/games/sw/src/interpso.cpp
index 85e539f13..1ff68d8bc 100644
--- a/source/games/sw/src/interpso.cpp
+++ b/source/games/sw/src/interpso.cpp
@@ -159,10 +159,12 @@ static void setvalue(so_interp::interp_data& element, double value)
         wall[index].moved();
         break;
     case soi_ceil:
-        sector[index].setceilingz(value);
+        if (!(sector[index].interpolate & 2)) 
+            sector[index].setceilingz(value);
         break;
     case soi_floor:
-        sector[index].setfloorz(value);
+        if (!(sector[index].interpolate & 1))
+            sector[index].setfloorz(value);
         break;
     case soi_sox:
         SectorObject[index].pmid.X = value;
diff --git a/source/games/sw/src/sector.cpp b/source/games/sw/src/sector.cpp
index a0117dbe9..a61dc2fd3 100644
--- a/source/games/sw/src/sector.cpp
+++ b/source/games/sw/src/sector.cpp
@@ -499,24 +499,30 @@ void SectorSetup(void)
 
             // set the first on up
             swf = &SineWaveFloor[NextSineWave][swf_ndx];
-            if (tag != TAG_SINE_WAVE_CEILING) StartInterpolation(sectp, Interp_Sect_Floorz);
-            if (tag != TAG_SINE_WAVE_FLOOR) StartInterpolation(sectp, Interp_Sect_Ceilingz);
 
             swf->flags = 0;
 
             switch (num)
             {
             case 0:
+                StartInterpolation(sectp, Interp_Sect_Floorz);
+                sectp->interpolate |= 1;
                 swf->flags |= (SINE_FLOOR);
                 if ((sectp->floorstat & CSTAT_SECTOR_SLOPE))
                 {
+                    StartInterpolation(sectp, Interp_Sect_Floorheinum);
                     swf->flags |= (SINE_SLOPED);
                 }
                 break;
             case 1:
+                StartInterpolation(sectp, Interp_Sect_Ceilingz);
+                sectp->interpolate |= 2;
                 swf->flags |= (SINE_CEILING);
                 break;
             case 2:
+                StartInterpolation(sectp, Interp_Sect_Floorz);
+                StartInterpolation(sectp, Interp_Sect_Ceilingz);
+                sectp->interpolate |= 3;
                 swf->flags |= (SINE_FLOOR | SINE_CEILING);
                 break;
             }
@@ -2550,6 +2556,7 @@ void DoSineWaveFloor(void)
     int wave;
     int flags;
 
+    Printf("---------------------------\n");
     for (wave = 0; wave < MAX_SINE_WAVE; wave++)
     {
         for (swf = &SineWaveFloor[wave][0], flags = swf->flags; swf->sectp != nullptr && swf < &SineWaveFloor[wave][SIZ(SineWaveFloor[wave])]; swf++)
@@ -2560,6 +2567,7 @@ void DoSineWaveFloor(void)
             if ((flags & SINE_FLOOR))
             {
                 double newz = swf->floorOrigz + swf->Range * BobVal(swf->sintable_ndx);
+                Printf("sector %d: orig = %2.5f, range = %2.5f, new = %2.5f, index = %d\n", int(swf->sectp - sector.Data()), swf->floorOrigz, swf->Range, newz, swf->sintable_ndx);
                 swf->sectp->setfloorz(newz);
             }