diff --git a/api/src/main/groovy/com/jessebrault/ssg/buildscript/Build.groovy b/api/src/main/groovy/com/jessebrault/ssg/buildscript/Build.groovy
index 55bbd0c..43bd3d5 100644
--- a/api/src/main/groovy/com/jessebrault/ssg/buildscript/Build.groovy
+++ b/api/src/main/groovy/com/jessebrault/ssg/buildscript/Build.groovy
@@ -63,7 +63,7 @@ final class Build {
 
     static Build concat(Build b0, Build b1) {
         new Build(
-                b1.name.blank ? b0.name : b1.name,
+                b0.name.blank ? b1.name : b0.name,
                 OutputDirFunctions.concat(b0.outputDirFunction, b1.outputDirFunction),
                 SiteSpec.concat(b0.siteSpec, b1.siteSpec),
                 b0.globals + b1.globals,
diff --git a/api/src/main/groovy/com/jessebrault/ssg/buildscript/OutputDirFunctions.groovy b/api/src/main/groovy/com/jessebrault/ssg/buildscript/OutputDirFunctions.groovy
index 01fd39b..cf820dd 100644
--- a/api/src/main/groovy/com/jessebrault/ssg/buildscript/OutputDirFunctions.groovy
+++ b/api/src/main/groovy/com/jessebrault/ssg/buildscript/OutputDirFunctions.groovy
@@ -12,7 +12,7 @@ final class OutputDirFunctions {
             Function<Build, OutputDir> f0,
             Function<Build, OutputDir> f1
     ) {
-        f1 == OutputDirFunctions.DEFAULT ? f0 : f1
+        f0 == OutputDirFunctions.DEFAULT ? f1 : f0
     }
 
     static Function<Build, OutputDir> of(File dir) {
diff --git a/api/src/main/groovy/com/jessebrault/ssg/task/TaskFactorySpec.groovy b/api/src/main/groovy/com/jessebrault/ssg/task/TaskFactorySpec.groovy
index eeaeca2..d765f1d 100644
--- a/api/src/main/groovy/com/jessebrault/ssg/task/TaskFactorySpec.groovy
+++ b/api/src/main/groovy/com/jessebrault/ssg/task/TaskFactorySpec.groovy
@@ -4,6 +4,7 @@ import groovy.transform.EqualsAndHashCode
 import groovy.transform.NullCheck
 import groovy.transform.TupleConstructor
 
+import java.util.function.BiFunction
 import java.util.function.Consumer
 import java.util.function.Supplier
 
diff --git a/api/src/test/groovy/com/jessebrault/ssg/buildscript/BuildTests.groovy b/api/src/test/groovy/com/jessebrault/ssg/buildscript/BuildTests.groovy
new file mode 100644
index 0000000..a114e54
--- /dev/null
+++ b/api/src/test/groovy/com/jessebrault/ssg/buildscript/BuildTests.groovy
@@ -0,0 +1,142 @@
+package com.jessebrault.ssg.buildscript
+
+import com.jessebrault.ssg.SiteSpec
+import com.jessebrault.ssg.task.TaskFactory
+import com.jessebrault.ssg.task.TaskFactorySpec
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.extension.ExtendWith
+import org.mockito.Mock
+import org.mockito.junit.jupiter.MockitoExtension
+
+import java.util.function.Function
+import java.util.function.Supplier
+
+import static org.junit.jupiter.api.Assertions.assertEquals
+
+@ExtendWith(MockitoExtension)
+final class BuildTests {
+
+    @ExtendWith(MockitoExtension)
+    static final class AllBuildsTests {
+
+        @Test
+        void twoEmptiesEqual() {
+            def ab0 = Build.AllBuilds.getEmpty()
+            def ab1 = Build.AllBuilds.getEmpty()
+            assertEquals(ab0, ab1)
+        }
+
+        @Test
+        void siteSpecsAdded() {
+            def ab0 = new Build.AllBuilds(
+                    new SiteSpec('test', ''),
+                    [:],
+                    []
+            )
+            def ab1 = new Build.AllBuilds(
+                    new SiteSpec('', 'test'),
+                    [:],
+                    []
+            )
+            def sum = ab0 + ab1
+            assertEquals(new SiteSpec('test', 'test'), sum.siteSpec)
+        }
+
+        @Test
+        void globalsAdded() {
+            def ab0 = new Build.AllBuilds(
+                    SiteSpec.getBlank(),
+                    [a: 0],
+                    []
+            )
+            def ab1 = new Build.AllBuilds(
+                    SiteSpec.getBlank(),
+                    [b: 1],
+                    []
+            )
+            def sum = ab0 + ab1
+            assertEquals([a: 0, b: 1], sum.globals)
+        }
+
+        @Test
+        void taskFactorySpecsAdded(@Mock Supplier<TaskFactory> taskFactorySupplier) {
+            def spec0 = new TaskFactorySpec<>(taskFactorySupplier, [])
+            def spec1 = new TaskFactorySpec<>(taskFactorySupplier, [])
+            def ab0 = new Build.AllBuilds(
+                    SiteSpec.getBlank(),
+                    [:],
+                    [spec0]
+            )
+            def ab1 = new Build.AllBuilds(
+                    SiteSpec.getBlank(),
+                    [:],
+                    [spec1]
+            )
+            def sum = ab0 + ab1
+            assertEquals([spec0, spec1], sum.taskFactorySpecs)
+        }
+
+    }
+
+    @Test
+    void twoEmptiesEqual() {
+        def b0 = Build.getEmpty()
+        def b1 = Build.getEmpty()
+        assertEquals(b0, b1)
+    }
+
+    @Test
+    void ifName0BlankTakeName1() {
+        def b0 = Build.get(name: '')
+        def b1 = Build.get(name: 'test')
+        def sum = b0 + b1
+        assertEquals('test', sum.name)
+    }
+
+    @Test
+    void ifName0NotBlankTakeName0() {
+        def b0 = Build.get(name: 'b0')
+        def b1 = Build.get(name: 'b1')
+        def sum = b0 + b1
+        assertEquals('b0', sum.name)
+    }
+
+    @Test
+    void ifOutputDirFunction0DefaultTake1(@Mock Function<Build, OutputDir> b1OutputDirFunction) {
+        def b0 = Build.get(outputDirFunction: OutputDirFunctions.DEFAULT)
+        def b1 = Build.get(outputDirFunction: b1OutputDirFunction)
+        def sum = b0 + b1
+        assertEquals(b1OutputDirFunction, sum.outputDirFunction)
+    }
+
+    @Test
+    void siteSpecsAdded() {
+        def b0 = Build.get(
+                siteSpec: new SiteSpec('test', '')
+        )
+        def b1 = Build.get(
+                siteSpec: new SiteSpec('', 'test')
+        )
+        def sum = b0 + b1
+        assertEquals(new SiteSpec('test', 'test'), sum.siteSpec)
+    }
+
+    @Test
+    void globalsAdded() {
+        def b0 = Build.get(globals: [a: 0])
+        def b1 = Build.get(globals: [b: 1])
+        def sum = b0 + b1
+        assertEquals([a: 0, b: 1], sum.globals)
+    }
+
+    @Test
+    void taskFactorySpecsAdded(@Mock Supplier<TaskFactory> taskFactorySupplier) {
+        def spec0 = new TaskFactorySpec<>(taskFactorySupplier, [])
+        def spec1 = new TaskFactorySpec<>(taskFactorySupplier, [])
+        def b0 = Build.get(taskFactorySpecs: [spec0])
+        def b1 = Build.get(taskFactorySpecs: [spec1])
+        def sum = b0 + b1
+        assertEquals([spec0, spec1], sum.taskFactorySpecs)
+    }
+
+}