diff --git a/src/BuildTiles/Parallel/client.cxx b/src/BuildTiles/Parallel/client.cxx
index a5dfbc01..a800394f 100644
--- a/src/BuildTiles/Parallel/client.cxx
+++ b/src/BuildTiles/Parallel/client.cxx
@@ -17,12 +17,14 @@
 #include <netinet/in.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <dirent.h>
+#include <errno.h>
 
 #include <utmp.h>
 
 #include <stdio.h>
 #include <stdlib.h>		// atoi()
-#include <string.h>		// bcopy()
+#include <string.h>		// bcopy(), sterror()
 #include <strings.h>		// bcopy() on Irix
 
 #include <simgear/compiler.h>
@@ -34,6 +36,7 @@
 #include <simgear/bucket/newbucket.hxx>
 
 SG_USING_STD(cout);
+SG_USING_STD(cerr);
 SG_USING_STD(endl);
 
 
@@ -177,14 +180,12 @@ long int get_next_task( const string& host, int port, long int last_tile ) {
     return -1;
 }
 
-static bool file_exists( const string& file ) {
-    struct stat buf;
-
-    if ( stat( file.c_str(), &buf ) == 0 ) {
-	return true;
-    } else {
-	return false;
-    }
+bool endswith(const char* s, const char* suffix) {
+        size_t slen=strlen(s);
+        size_t sufflen=strlen(suffix);
+        if (slen<sufflen)
+                return false;
+        return (strncmp(s+slen-sufflen,suffix,sufflen)==0);
 }
 
 // check if the tile really has to be generated
@@ -194,11 +195,72 @@ static bool must_generate( const SGBucket& b ) {
     
     string btg_file = output_base + "/" + b.gen_base_path()
 	+ "/" + b.gen_index_str() + ".btg.gz";
-    if ( file_exists( btg_file ) ) {
-	return false;
+    string stg_file = output_base + "/" + b.gen_base_path()
+	+ "/" + b.gen_index_str() + ".stg";
+    struct stat btg_stat, stg_stat;
+    bool have_btg, have_stg;
+    
+    have_btg= ( stat( btg_file.c_str(), &btg_stat ) == 0 );
+    have_stg = ( stat( stg_file.c_str(), &stg_stat ) == 0 );
+    
+    if ( !have_btg ) {
+            cout << "Output file " << btg_file << " was not found\n";
     }
-
-    return true;
+    
+    if ( !have_stg ) {
+            cout << "Output file " << stg_file << " was not found\n";
+    }
+    
+    /* Now check the load dirs for any source data and
+     * whether any of that is newer than the btg-file or the stg-file
+     */
+    const string& prefix=b.gen_index_str()+".";
+    size_t prefix_len=prefix.size();
+    for (int i = 0; i < (int)load_dirs.size(); i++) {
+            string path=load_dirs[i]+"/"+b.gen_base_path();
+            DIR *loaddir=opendir(path.c_str());
+            if (!loaddir) {
+                    if (errno!=ENOENT)
+                            cout << " Could not open load directory " << path << ":" << strerror(errno) << "\n";
+                    continue;
+            }
+            
+            struct dirent* de;
+            struct stat src_stat;
+            
+            while ((de=readdir(loaddir))) {
+                    if (strncmp(de->d_name,prefix.c_str(),prefix_len))
+                            continue;
+                    string file=path+"/"+de->d_name;
+                    if ( stat(file.c_str(), &src_stat) != 0) {
+                            /* huh!? */
+                            cerr << " Could not stat file " << file << ":" << strerror(errno) << "\n";
+                            continue;
+                    }
+                    if ( have_btg && src_stat.st_mtime>btg_stat.st_mtime ) {
+                            cout << " File " << file << " is newer than btg-file => rebuild\n";
+                            return true;
+                    }
+                    if ( have_stg && src_stat.st_mtime>stg_stat.st_mtime ) {
+                            cout << " File " << file << " is newer than stg-file => rebuild\n";
+                            return true;
+                    }
+                    /* Ignore elevation data, as it is not used if we have no
+                     * landmass data. So in addition to elevation data we need at
+                     * least one polygon file.
+                     */
+                    if ( endswith( de->d_name, ".arr.gz" ) || endswith( de->d_name, ".fit.gz" ) )
+                            continue;
+                    if ( !(have_stg && have_btg) ) {
+                            cout << " There is source-data (" << file << ") for tile " << b.gen_index_str() << " but .btg or .stg is missing => build\n";
+                            return true;
+                    }
+            }
+            
+            closedir(loaddir);
+    }
+    
+    return false;
 }
 
 
@@ -301,6 +363,10 @@ int main(int argc, char *argv[]) {
       cout << "Running in rude mode" << endl;
     else
       cout << "Running in polite mode" << endl;
+    if (do_overwrite)
+      cout << "Will overwrite existing buckets" << endl;
+    else
+      cout << "Will not overwrite up-to-date existing buckets" << endl;
     for (int i = arg_pos; i < argc; i++) {
       string dir;
       dir = work_base + "/";
@@ -334,6 +400,7 @@ int main(int argc, char *argv[]) {
 	if ( result ) {
 	    last_tile = tile;
 	} else {
+            cout << "Build of tile " << tile << " failed\n";
 	    last_tile = -tile;
 	}