diff --git a/include/qbsp/solidbsp.hh b/include/qbsp/solidbsp.hh
index ff6b06f8..414a05ca 100644
--- a/include/qbsp/solidbsp.hh
+++ b/include/qbsp/solidbsp.hh
@@ -22,7 +22,9 @@
 #ifndef QBSP_SOLIDBSP_HH
 #define QBSP_SOLIDBSP_HH
 
-extern int splitnodes;
+#include <atomic>
+
+extern std::atomic<int> splitnodes;
 
 void DetailToSolid(node_t *node);
 int Contents_Priority(int contents);
diff --git a/qbsp/portals.cc b/qbsp/portals.cc
index 35e80930..6dd477f2 100644
--- a/qbsp/portals.cc
+++ b/qbsp/portals.cc
@@ -667,7 +667,7 @@ CutNodePortals_r(node_t *node, portal_state_t *state)
 
     /* Display progress */
     state->iNodesDone++;
-    Message(msgPercent, state->iNodesDone, splitnodes);
+    Message(msgPercent, state->iNodesDone, splitnodes.load());
 
     CutNodePortals_r(front, state);
     CutNodePortals_r(back, state);
diff --git a/qbsp/solidbsp.cc b/qbsp/solidbsp.cc
index c9bad28f..e482eec9 100644
--- a/qbsp/solidbsp.cc
+++ b/qbsp/solidbsp.cc
@@ -23,14 +23,16 @@
 
 #include <qbsp/qbsp.hh>
 
+#include <atomic>
+
 #include "tbb/task_group.h"
 
-int splitnodes;
+std::atomic<int> splitnodes;
 
-static int leaffaces;
-static int nodefaces;
-static int c_solid, c_empty, c_water, c_detail, c_detail_illusionary, c_detail_fence;
-static int c_illusionary_visblocker;
+static std::atomic<int> leaffaces;
+static std::atomic<int> nodefaces;
+static std::atomic<int> c_solid, c_empty, c_water, c_detail, c_detail_illusionary, c_detail_fence;
+static std::atomic<int> c_illusionary_visblocker;
 static bool usemidsplit;
 
 /**
@@ -462,6 +464,8 @@ SelectPartition
 
 Selects a surface from a linked list of surfaces to split the group on
 returns NULL if the surface list can not be divided any more (a leaf)
+
+Called in parallel.
 ==================
 */
 static surface_t *
@@ -782,7 +786,9 @@ int Contents_Priority(int contents)
 LinkConvexFaces
 
 Determines the contents of the leaf and creates the final list of
-original faces that have some fragment inside this leaf
+original faces that have some fragment inside this leaf.
+
+Called in parallel.
 ==================
 */
 static void
@@ -879,7 +885,9 @@ First subdivides surface->faces.
 Then, duplicates the list of subdivided faces and returns it.
 
 For each surface->faces, ->original is set to the respective duplicate that 
-is returned here (why?)
+is returned here (why?).
+
+Called in parallel.
 ==================
 */
 static face_t *
@@ -913,6 +921,8 @@ LinkNodeFaces(surface_t *surface)
 /*
 ==================
 PartitionSurfaces
+
+Called in parallel.
 ==================
 */
 static void
@@ -929,7 +939,7 @@ PartitionSurfaces(surface_t *surfaces, node_t *node)
     }
 
     splitnodes++;
-    Message(msgPercent, splitnodes, csgmergefaces);
+    Message(msgPercent, splitnodes.load(), csgmergefaces);
 
     node->faces = LinkNodeFaces(split);
     node->children[0] = (node_t *)AllocMem(NODE, 1, true);
@@ -1041,16 +1051,16 @@ SolidBSP(const mapentity_t *entity, surface_t *surfhead, bool midsplit)
 
     PartitionSurfaces(surfhead, headnode);
 
-    Message(msgStat, "%8d split nodes", splitnodes);
-    Message(msgStat, "%8d solid leafs", c_solid);
-    Message(msgStat, "%8d empty leafs", c_empty);
-    Message(msgStat, "%8d water leafs", c_water);
-    Message(msgStat, "%8d detail leafs", c_detail);
-    Message(msgStat, "%8d detail illusionary leafs", c_detail_illusionary);
-    Message(msgStat, "%8d detail fence leafs", c_detail_fence);
-    Message(msgStat, "%8d illusionary visblocker leafs", c_illusionary_visblocker);
-    Message(msgStat, "%8d leaffaces", leaffaces);
-    Message(msgStat, "%8d nodefaces", nodefaces);
+    Message(msgStat, "%8d split nodes", splitnodes.load());
+    Message(msgStat, "%8d solid leafs", c_solid.load());
+    Message(msgStat, "%8d empty leafs", c_empty.load());
+    Message(msgStat, "%8d water leafs", c_water.load());
+    Message(msgStat, "%8d detail leafs", c_detail.load());
+    Message(msgStat, "%8d detail illusionary leafs", c_detail_illusionary.load());
+    Message(msgStat, "%8d detail fence leafs", c_detail_fence.load());
+    Message(msgStat, "%8d illusionary visblocker leafs", c_illusionary_visblocker.load());
+    Message(msgStat, "%8d leaffaces", leaffaces.load());
+    Message(msgStat, "%8d nodefaces", nodefaces.load());
 
     return headnode;
 }
diff --git a/qbsp/surfaces.cc b/qbsp/surfaces.cc
index 7dba31d2..d822db5e 100644
--- a/qbsp/surfaces.cc
+++ b/qbsp/surfaces.cc
@@ -409,7 +409,7 @@ MakeFaceEdges_r(mapentity_t *entity, node_t *node, int progress)
         FindFaceEdges(entity, f);
     }
 
-    Message(msgPercent, ++progress, splitnodes);
+    Message(msgPercent, ++progress, splitnodes.load());
     progress = MakeFaceEdges_r(entity, node->children[0], progress);
     progress = MakeFaceEdges_r(entity, node->children[1], progress);
 
diff --git a/qbsp/util.cc b/qbsp/util.cc
index 618be54b..8ae9c33c 100644
--- a/qbsp/util.cc
+++ b/qbsp/util.cc
@@ -206,6 +206,8 @@ FreeAllMem(void)
 /* Keep track of output state */
 static bool fInPercent = false;
 
+std::mutex messageLock;
+
 /*
 =================
 Message
@@ -216,6 +218,8 @@ Generic output of warnings, stats, etc
 void
 Message(int msgType, ...)
 {
+    std::unique_lock<std::mutex> lck { messageLock };
+
     va_list argptr;
     char szBuffer[512];
     char *szFmt;