diff --git a/runtime/src/main/java/org/capnproto/AnyPointer.java b/runtime/src/main/java/org/capnproto/AnyPointer.java
index a101cde..c7ba1ec 100644
--- a/runtime/src/main/java/org/capnproto/AnyPointer.java
+++ b/runtime/src/main/java/org/capnproto/AnyPointer.java
@@ -9,9 +9,21 @@ public final class AnyPointer {
             this.reader = reader;
         }
 
-        public <T> T getAsStruct(FromStructReader<T> factory) {
+        public final <T> T getAsStruct(FromStructReader<T> factory) {
             return factory.fromStructReader(this.reader.getStruct());
         }
     }
 
+    public static final class Builder {
+        public final PointerBuilder builder;
+
+        public Builder(PointerBuilder builder) {
+            this.builder = builder;
+        }
+
+        public final <T> T initAsStruct(FromStructBuilder<T> factory) {
+            return factory.fromStructBuilder(this.builder.initStruct(factory.structSize()));
+        }
+    }
+
 }
diff --git a/runtime/src/main/java/org/capnproto/Arena.java b/runtime/src/main/java/org/capnproto/Arena.java
new file mode 100644
index 0000000..01dce98
--- /dev/null
+++ b/runtime/src/main/java/org/capnproto/Arena.java
@@ -0,0 +1,5 @@
+package org.capnproto;
+
+public interface Arena {
+    public SegmentReader tryGetSegment(int id);
+}
diff --git a/runtime/src/main/java/org/capnproto/BuilderArena.java b/runtime/src/main/java/org/capnproto/BuilderArena.java
new file mode 100644
index 0000000..294462a
--- /dev/null
+++ b/runtime/src/main/java/org/capnproto/BuilderArena.java
@@ -0,0 +1,36 @@
+package org.capnproto;
+
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+import java.util.Vector;
+
+public final class BuilderArena implements Arena {
+    public final Vector<SegmentBuilder> segments;
+
+    public BuilderArena() {
+        this.segments = new Vector<SegmentBuilder>();
+        SegmentBuilder segment0 = new SegmentBuilder(ByteBuffer.allocate(1024 * 8));
+        segment0.buffer.mark();
+        this.segments.add(segment0);
+    }
+
+    public SegmentReader tryGetSegment(int id) {
+        throw new Error("unimplemented");
+    }
+    public SegmentBuilder getSegment(int id) {
+        throw new Error("unimplemented");
+    }
+
+    public static class AllocateResult {
+        public final SegmentBuilder segment;
+        public final int offset;
+        public AllocateResult(SegmentBuilder segment, int offset) {
+            this.segment = segment;
+            this.offset = offset;
+        }
+    }
+
+    public AllocateResult allocate(int amount) {
+        throw new Error("unimplemented");
+    }
+}
diff --git a/runtime/src/main/java/org/capnproto/MessageBuilder.java b/runtime/src/main/java/org/capnproto/MessageBuilder.java
index d54e9b8..6d8a4cb 100644
--- a/runtime/src/main/java/org/capnproto/MessageBuilder.java
+++ b/runtime/src/main/java/org/capnproto/MessageBuilder.java
@@ -1,11 +1,25 @@
 package org.capnproto;
 
 public final class MessageBuilder {
+
+    private final BuilderArena arena;
+
+    public MessageBuilder() {
+        this.arena = new BuilderArena();
+    }
+
     public <T> T getRoot(FromStructBuilder<T> factory) {
         throw new Error("unimplemented");
     }
 
     public <T> T initRoot(FromStructBuilder<T> factory) {
-        throw new Error("unimplemented");
+        SegmentBuilder rootSegment = this.arena.segments.get(0);
+        int location = rootSegment.allocate(1);
+        if (location == SegmentBuilder.FAILED_ALLOCATION) {
+            throw new Error("could not allocate root pointer");
+        }
+
+        AnyPointer.Builder ptr = new AnyPointer.Builder(PointerBuilder.getRoot(rootSegment, location));
+        return ptr.initAsStruct(factory);
     }
 }
diff --git a/runtime/src/main/java/org/capnproto/PointerBuilder.java b/runtime/src/main/java/org/capnproto/PointerBuilder.java
index 5f583c2..4f40d24 100644
--- a/runtime/src/main/java/org/capnproto/PointerBuilder.java
+++ b/runtime/src/main/java/org/capnproto/PointerBuilder.java
@@ -9,10 +9,18 @@ public final class PointerBuilder {
         this.pointer = pointer;
     }
 
+    public static PointerBuilder getRoot(SegmentBuilder segment, int location) {
+        return new PointerBuilder(segment, location);
+    }
+
     public final boolean isNull() {
         return this.segment.buffer.getLong(this.pointer) == 0;
     }
 
+    public final StructBuilder initStruct(StructSize size) {
+        return WireHelpers.initStructPointer(this.pointer, this.segment, size);
+    }
+
     public final ListBuilder initStructList(int elementCount, StructSize elementSize) {
         return WireHelpers.initStructListPointer(this.pointer, this.segment, elementCount, elementSize);
     }
diff --git a/runtime/src/main/java/org/capnproto/ReaderArena.java b/runtime/src/main/java/org/capnproto/ReaderArena.java
new file mode 100644
index 0000000..a90d740
--- /dev/null
+++ b/runtime/src/main/java/org/capnproto/ReaderArena.java
@@ -0,0 +1,7 @@
+package org.capnproto;
+
+public final class ReaderArena implements Arena {
+    public SegmentReader tryGetSegment(int id) {
+        throw new Error("unimplemented");
+    }
+}
diff --git a/runtime/src/main/java/org/capnproto/SegmentBuilder.java b/runtime/src/main/java/org/capnproto/SegmentBuilder.java
index 5641099..3d19d6d 100644
--- a/runtime/src/main/java/org/capnproto/SegmentBuilder.java
+++ b/runtime/src/main/java/org/capnproto/SegmentBuilder.java
@@ -22,6 +22,9 @@ public final class SegmentBuilder extends SegmentReader {
         return this.pos;
     }
 
+    /**
+       Allocate `amount` words.
+     */
     public final int allocate(int amount) {
         if (amount > this.capacity() - this.currentSize()) {
             return FAILED_ALLOCATION; // no space left;
diff --git a/runtime/src/main/java/org/capnproto/WireHelpers.java b/runtime/src/main/java/org/capnproto/WireHelpers.java
index 1d0d8cc..93c133d 100644
--- a/runtime/src/main/java/org/capnproto/WireHelpers.java
+++ b/runtime/src/main/java/org/capnproto/WireHelpers.java
@@ -25,6 +25,15 @@ final class WireHelpers {
         }
     }
 
+    public static StructBuilder initStructPointer(int refOffset,
+                                                  SegmentBuilder segment,
+                                                  StructSize size) {
+        int ptrOffset = allocate(refOffset, segment, size.total(), WirePointer.STRUCT);
+        StructPointer.setFromStructSize(segment.buffer, refOffset, size);
+        return new StructBuilder(segment, ptrOffset * 8, ptrOffset + size.data,
+                                 size.data * 64, size.pointers, (byte)0);
+    }
+
     public static ListBuilder initListPointer(int refOffset,
                                               SegmentBuilder segment,
                                               int elementCount,
@@ -56,7 +65,7 @@ final class WireHelpers {
 
         ptrOffset += 1;
 
-        return new ListBuilder(segment, ptrOffset, elementCount, wordsPerElement * 64,
+        return new ListBuilder(segment, ptrOffset * 8, elementCount, wordsPerElement * 64,
                                elementSize.data * 64, elementSize.pointers);
     }