diff --git a/lib/models/room.js b/lib/models/room.js
index a1cf29629..f448cceb1 100644
--- a/lib/models/room.js
+++ b/lib/models/room.js
@@ -10,6 +10,23 @@ var MatrixEvent = require("./event").MatrixEvent;
 var utils = require("../utils");
 var ContentRepo = require("../content-repo");
 
+function synthesizeReceipt(userId, event, receiptType) {
+    // This is really ugly because JS has no way to express an object literal
+    // where the name of a key comes from an expression
+    var fakeReceipt = {
+        content: {},
+        type: "m.receipt",
+        room_id: event.getRoomId()
+    };
+    fakeReceipt.content[event.getId()] = {};
+    fakeReceipt.content[event.getId()][receiptType] = {};
+    fakeReceipt.content[event.getId()][receiptType][userId] = {
+        ts: event
+    };
+    return new MatrixEvent(fakeReceipt);
+}
+
+
 /**
  * Construct a new Room.
  * @constructor
@@ -207,16 +224,10 @@ Room.prototype.addEventsToTimeline = function(events, toStartOfTimeline) {
 
         // This is really ugly because JS has no way to express an object literal
         // where the name of a key comes from an expression
-        if (events[i].sender) {
-            var fakeReceipt = {content: {}};
-            fakeReceipt.content[events[i].getId()] = {
-                'm.read': {
-                }
-            };
-            fakeReceipt.content[events[i].getId()]['m.read'][events[i].sender.userId] = {
-                ts: events[i].getTs()
-            };
-            this.addReceipt(new MatrixEvent(fakeReceipt));
+        if (!toStartOfTimeline && events[i].sender) {
+            this.addReceipt(new MatrixEvent(synthesizeReceipt(
+                events[i].sender.userId, events[i], "m.read"
+            )));
         }
 
         this.emit("Room.timeline", events[i], this, Boolean(toStartOfTimeline), false);
@@ -356,6 +367,45 @@ Room.prototype.recalculate = function(userId) {
     if (oldName !== this.name) {
         this.emit("Room.name", this);
     }
+
+
+
+    // recalculate read receipts, adding implicit ones where necessary
+    // NB. This is a duplication of logic for injecting implicit receipts,
+    // it would be technically possible to only ever generate these
+    // receipts in addEventsToTimeline but doing so means correctly
+    // choosing whether to keep or replace the existing receipt which
+    // is complex and slow. This is faster and more understandable.
+
+    var usersFound = {};
+    for (var i = this.timeline.length - 1; i >= 0; --i) {
+        // loop through the timeline backwards looking for either an
+        // event sent by each user or a real receipt from them.
+        // Replace the read receipt for that user with whichever
+        // occurs later in the timeline (ie. first because we're going
+        // backwards).
+        var e = this.timeline[i];
+
+        var readReceiptsForEvent = this.getReceiptsForEvent(e);
+
+        for (var receiptIt = 0; receiptIt < readReceiptsForEvent.length; ++receiptIt) {
+            var receipt = readReceiptsForEvent[receiptIt];
+            if (receipt.type !== "m.read") { continue; }
+
+            if (usersFound[receipt.userId]) { continue; }
+
+            // Then this is the receipt we keep for this user
+            usersFound[receipt.userId] = 1;
+        }
+
+        if (e.sender && usersFound[e.sender.userId] === undefined) {
+            // no receipt yet for this sender, so we synthesize one.
+
+            this.addReceipt(synthesizeReceipt(e.sender.userId, e, "m.read"));
+
+            usersFound[e.sender.userId] = 1;
+        }
+    }
 };