diff --git a/src/info.c b/src/info.c
index 9d65c8721..ed5de0125 100644
--- a/src/info.c
+++ b/src/info.c
@@ -2484,22 +2484,22 @@ state_t states[NUMSTATES] =
 
 	{SPR_NULL, 0, 1, {A_RockSpawn}, 0, 0, S_ROCKSPAWN}, // S_ROCKSPAWN
 
-	{SPR_ROIA, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEA}, // S_ROCKCRUMBLEA
-	{SPR_ROIB, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEB}, // S_ROCKCRUMBLEB
-	{SPR_ROIC, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEC}, // S_ROCKCRUMBLEC
-	{SPR_ROID, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLED}, // S_ROCKCRUMBLED
-	{SPR_ROIE, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEE}, // S_ROCKCRUMBLEE
-	{SPR_ROIF, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEF}, // S_ROCKCRUMBLEF
-	{SPR_ROIG, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEG}, // S_ROCKCRUMBLEG
-	{SPR_ROIH, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEH}, // S_ROCKCRUMBLEH
-	{SPR_ROII, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEI}, // S_ROCKCRUMBLEI
-	{SPR_ROIJ, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEJ}, // S_ROCKCRUMBLEJ
-	{SPR_ROIK, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEK}, // S_ROCKCRUMBLEK
-	{SPR_ROIL, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEL}, // S_ROCKCRUMBLEL
-	{SPR_ROIM, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEM}, // S_ROCKCRUMBLEM
-	{SPR_ROIN, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEN}, // S_ROCKCRUMBLEN
-	{SPR_ROIO, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEO}, // S_ROCKCRUMBLEO
-	{SPR_ROIP, FF_ANIMATE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEP}, // S_ROCKCRUMBLEP
+	{SPR_ROIA, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEA}, // S_ROCKCRUMBLEA
+	{SPR_ROIB, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEB}, // S_ROCKCRUMBLEB
+	{SPR_ROIC, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEC}, // S_ROCKCRUMBLEC
+	{SPR_ROID, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLED}, // S_ROCKCRUMBLED
+	{SPR_ROIE, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEE}, // S_ROCKCRUMBLEE
+	{SPR_ROIF, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEF}, // S_ROCKCRUMBLEF
+	{SPR_ROIG, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEG}, // S_ROCKCRUMBLEG
+	{SPR_ROIH, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEH}, // S_ROCKCRUMBLEH
+	{SPR_ROII, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEI}, // S_ROCKCRUMBLEI
+	{SPR_ROIJ, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEJ}, // S_ROCKCRUMBLEJ
+	{SPR_ROIK, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEK}, // S_ROCKCRUMBLEK
+	{SPR_ROIL, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEL}, // S_ROCKCRUMBLEL
+	{SPR_ROIM, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEM}, // S_ROCKCRUMBLEM
+	{SPR_ROIN, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEN}, // S_ROCKCRUMBLEN
+	{SPR_ROIO, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEO}, // S_ROCKCRUMBLEO
+	{SPR_ROIP, FF_ANIMATE|FF_MIDDLESTARTCHANCE, -1, {NULL}, 4, 2, S_ROCKCRUMBLEP}, // S_ROCKCRUMBLEP
 
 	{SPR_SRBA, 0, 5, {A_Look}, 0, 0, S_SRB1_CRAWLA1}, // S_SRB1_CRAWLA1
 	{SPR_SRBA, 0, 3, {A_Chase}, 0, 0, S_SRB1_CRAWLA3}, // S_SRB1_CRAWLA2
diff --git a/src/p_floor.c b/src/p_floor.c
index 5a1b9e78d..5ee08beea 100644
--- a/src/p_floor.c
+++ b/src/p_floor.c
@@ -15,6 +15,7 @@
 #include "doomstat.h"
 #include "m_random.h"
 #include "p_local.h"
+#include "p_slopes.h"
 #include "r_state.h"
 #include "s_sound.h"
 #include "z_zone.h"
@@ -2893,7 +2894,7 @@ void EV_CrumbleChain(sector_t *sec, ffloor_t *rover)
 	size_t topmostvertex = 0, bottommostvertex = 0;
 	fixed_t leftx, rightx;
 	fixed_t topy, bottomy;
-	fixed_t topz;
+	fixed_t topz, bottomz;
 	fixed_t widthfactor, heightfactor;
 	fixed_t a, b, c;
 	mobjtype_t type = MT_ROCKCRUMBLE1;
@@ -2950,10 +2951,15 @@ void EV_CrumbleChain(sector_t *sec, ffloor_t *rover)
 	rightx = sec->lines[rightmostvertex]->v1->x;
 	topy = sec->lines[topmostvertex]->v1->y-(spacing>>1);
 	bottomy = sec->lines[bottommostvertex]->v1->y;
-	topz = *rover->topheight-(spacing>>1);
 
-	widthfactor = (rightx + topy - leftx - bottomy)>>3;
-	heightfactor = (topz - *rover->bottomheight)>>2;
+	topz = *rover->topheight-(spacing>>1);
+	bottomz = *rover->bottomheight;
+
+	if (flags & ML_EFFECT1)
+	{
+		widthfactor = (rightx + topy - leftx - bottomy)>>3;
+		heightfactor = (topz - *rover->bottomheight)>>2;
+	}
 
 	for (a = leftx; a < rightx; a += spacing)
 	{
@@ -2962,13 +2968,14 @@ void EV_CrumbleChain(sector_t *sec, ffloor_t *rover)
 			if (R_PointInSubsector(a, b)->sector == sec)
 			{
 				mobj_t *spawned = NULL;
-				for (c = topz; c > *rover->bottomheight; c -= spacing)
+				if (*rover->t_slope)
+					topz = P_GetZAt(*rover->t_slope, a, b) - (spacing>>1);
+				if (*rover->b_slope)
+					bottomz = P_GetZAt(*rover->b_slope, a, b);
+
+				for (c = topz; c > bottomz; c -= spacing)
 				{
 					spawned = P_SpawnMobj(a, b, c, type);
-
-					if (spawned->frame & FF_ANIMATE)
-						spawned->frame += P_RandomKey(spawned->state->var1);
-
 					spawned->angle += P_RandomKey(36)*ANG10; // irrelevant for default objects but might make sense for some custom ones
 
 					if (flags & ML_EFFECT1)