diff --git a/space_map.h b/space_map.h
index 05c709e..a0a5549 100644
--- a/space_map.h
+++ b/space_map.h
@@ -54,9 +54,8 @@ namespace persistent_data {
 	public:
 		typedef boost::shared_ptr<persistent_space_map> ptr;
 
-		// FIXME: these two should be const
-		virtual size_t root_size() = 0;
-		virtual void copy_root(void *dest, size_t len) = 0;
+		virtual size_t root_size() const = 0;
+		virtual void copy_root(void *dest, size_t len) const = 0;
 	};
 
 	class checked_space_map : public persistent_space_map {
diff --git a/space_map_disk.cc b/space_map_disk.cc
index ff349fd..287a97c 100644
--- a/space_map_disk.cc
+++ b/space_map_disk.cc
@@ -381,11 +381,11 @@ namespace {
 			}
 		}
 
-		virtual size_t root_size() {
+		virtual size_t root_size() const {
 			return sizeof(sm_root_disk);
 		}
 
-		virtual void copy_root(void *dest, size_t len) {
+		virtual void copy_root(void *dest, size_t len) const {
 			sm_root_disk d;
 			sm_root v;
 
diff --git a/space_map_recursive.cc b/space_map_recursive.cc
index b757f95..7973a42 100644
--- a/space_map_recursive.cc
+++ b/space_map_recursive.cc
@@ -120,13 +120,13 @@ namespace {
 			sm_->iterate(it);
 		}
 
-		virtual size_t root_size() {
+		virtual size_t root_size() const {
 			cant_recurse("root_size");
 			recursing_const_lock lock(*this);
 			return sm_->root_size();
 		}
 
-		virtual void copy_root(void *dest, size_t len) {
+		virtual void copy_root(void *dest, size_t len) const {
 			cant_recurse("copy_root");
 			recursing_const_lock lock(*this);
 			return sm_->copy_root(dest, len);