diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index 5d7c187..2985541 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -13,7 +13,10 @@ module Rscons
     attr_accessor :echo
 
     # String or +nil+
-    attr_accessor :build_root
+    attr_reader :build_root
+    def build_root=(build_root)
+      @build_root = build_root.gsub('\\', '/')
+    end
 
     # Create an Environment object.
     # @param options [Hash]
@@ -106,7 +109,7 @@ module Rscons
         end
       end
       if @build_root and not found_match
-        unless source_fname.absolute_path?
+        unless source_fname.absolute_path? or build_fname.start_with?("#{@build_root}/")
           build_fname = "#{@build_root}/#{build_fname}"
         end
       end
diff --git a/spec/rscons/environment_spec.rb b/spec/rscons/environment_spec.rb
index 6b63e73..0c31b8f 100644
--- a/spec/rscons/environment_spec.rb
+++ b/spec/rscons/environment_spec.rb
@@ -61,6 +61,18 @@ module Rscons
           env.get_build_fname("src\\dir\\other.d", ".a").should == "src/dir/other.a"
           env.get_build_fname("source.cc", ".o").should == "source.o"
         end
+
+        context "with a build_root" do
+          it "uses the build_root unless the path is absolute" do
+            env = Environment.new
+            env.build_root = "build/proj"
+            env.get_build_fname("src/dir/file.c", ".o").should == "build/proj/src/dir/file.o"
+            env.get_build_fname("/some/lib.c", ".a").should == "/some/lib.a"
+            env.get_build_fname("C:\\abspath\\mod.cc", ".o").should == "C:/abspath/mod.o"
+            env.get_build_fname("build\\proj\\generated.c", ".o").should == "build/proj/generated.o"
+            env.get_build_fname("build/proj.XX", ".yy").should == "build/proj/build/proj.yy"
+          end
+        end
       end
 
       context "with build directories" do
@@ -73,6 +85,21 @@ module Rscons
           env.get_build_fname("libs/otherlib/otherlib.cc", ".o").should == "build/libs/otherlib/otherlib.o"
           env.get_build_fname("other_directory/o.d", ".a").should == "other_directory/o.a"
         end
+
+        context "with a build_root" do
+          it "uses the build_root unless a build directory matches or the path is absolute" do
+            env = Environment.new
+            env.build_dir("src", "bld")
+            env.build_dir(%r{^libs/([^/]+)}, 'build/libs/\1')
+            env.build_root = "bldit"
+
+            env.get_build_fname("src/input.cc", ".o").should == "bld/input.o"
+            env.get_build_fname("libs/lib1/some/file.c", ".o").should == "build/libs/lib1/some/file.o"
+            env.get_build_fname("libs/otherlib/otherlib.cc", ".o").should == "build/libs/otherlib/otherlib.o"
+            env.get_build_fname("other_directory/o.d", ".a").should == "bldit/other_directory/o.a"
+            env.get_build_fname("bldit/some/mod.d", ".a").should == "bldit/some/mod.a"
+          end
+        end
       end
     end