diff --git a/common/events.go b/common/events.go
index e49c49fe4..7900341f3 100644
--- a/common/events.go
+++ b/common/events.go
@@ -42,12 +42,25 @@ func BuildEvent(
 	builder *gomatrixserverlib.EventBuilder, cfg *config.Dendrite, evTime time.Time,
 	queryAPI api.RoomserverQueryAPI, queryRes *api.QueryLatestEventsAndStateResponse,
 ) (*gomatrixserverlib.Event, error) {
+	if queryRes == nil {
+		queryRes = &api.QueryLatestEventsAndStateResponse{}
+	}
+
 	err := AddPrevEventsToEvent(ctx, builder, queryAPI, queryRes)
 	if err != nil {
 		return nil, err
 	}
 
-	eventID := fmt.Sprintf("$%s:%s", util.RandomString(16), cfg.Matrix.ServerName)
+	var eventID string
+	var format gomatrixserverlib.EventIDFormat
+	format, err = queryRes.RoomVersion.EventIDFormat()
+	if err != nil {
+		return nil, err
+	}
+	if format == gomatrixserverlib.EventIDFormatV1 {
+		eventID = fmt.Sprintf("$%s:%s", util.RandomString(16), cfg.Matrix.ServerName)
+	}
+
 	event, err := builder.Build(
 		eventID, evTime, cfg.Matrix.ServerName, cfg.Matrix.KeyID,
 		cfg.Matrix.PrivateKey, queryRes.RoomVersion,
@@ -75,9 +88,6 @@ func AddPrevEventsToEvent(
 		RoomID:       builder.RoomID,
 		StateToFetch: eventsNeeded.Tuples(),
 	}
-	if queryRes == nil {
-		queryRes = &api.QueryLatestEventsAndStateResponse{}
-	}
 	if err = queryAPI.QueryLatestEventsAndState(ctx, &queryReq, queryRes); err != nil {
 		return err
 	}
diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index 9003a95a3..3cb1b8a7b 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -261,6 +261,16 @@ type QueryRoomVersionCapabilitiesResponse struct {
 	AvailableRoomVersions map[gomatrixserverlib.RoomVersion]string `json:"available"`
 }
 
+// QueryRoomVersionForRoom asks for the room version for a given room.
+type QueryRoomVersionForRoomRequest struct {
+	RoomID string `json:"room_id"`
+}
+
+// QueryRoomVersionCapabilitiesResponse is a response to QueryServersInRoomAtEventResponse
+type QueryRoomVersionForRoomResponse struct {
+	RoomVersion gomatrixserverlib.RoomVersion `json:"room_version"`
+}
+
 // RoomserverQueryAPI is used to query information from the room server.
 type RoomserverQueryAPI interface {
 	// Query the latest events and state for a room from the room server.
@@ -347,6 +357,13 @@ type RoomserverQueryAPI interface {
 		request *QueryRoomVersionCapabilitiesRequest,
 		response *QueryRoomVersionCapabilitiesResponse,
 	) error
+
+	// Asks for the room version for a given room.
+	QueryRoomVersionForRoom(
+		ctx context.Context,
+		request *QueryRoomVersionForRoomRequest,
+		response *QueryRoomVersionForRoomResponse,
+	) error
 }
 
 // RoomserverQueryLatestEventsAndStatePath is the HTTP path for the QueryLatestEventsAndState API.
@@ -385,6 +402,9 @@ const RoomserverQueryServersInRoomAtEventPath = "/api/roomserver/queryServersInR
 // RoomserverQueryRoomVersionCapabilitiesPath is the HTTP path for the QueryRoomVersionCapabilities API
 const RoomserverQueryRoomVersionCapabilitiesPath = "/api/roomserver/queryRoomVersionCapabilities"
 
+// RoomserverQueryRoomVersionCapabilitiesPath is the HTTP path for the QueryRoomVersionCapabilities API
+const RoomserverQueryRoomVersionForRoomPath = "/api/roomserver/queryRoomVersionForRoom"
+
 // NewRoomserverQueryAPIHTTP creates a RoomserverQueryAPI implemented by talking to a HTTP POST API.
 // If httpClient is nil then it uses the http.DefaultClient
 func NewRoomserverQueryAPIHTTP(roomserverURL string, httpClient *http.Client) RoomserverQueryAPI {
@@ -554,3 +574,16 @@ func (h *httpRoomserverQueryAPI) QueryRoomVersionCapabilities(
 	apiURL := h.roomserverURL + RoomserverQueryRoomVersionCapabilitiesPath
 	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
 }
+
+// QueryRoomVersionForRoom implements RoomServerQueryAPI
+func (h *httpRoomserverQueryAPI) QueryRoomVersionForRoom(
+	ctx context.Context,
+	request *QueryRoomVersionForRoomRequest,
+	response *QueryRoomVersionForRoomResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "QueryRoomVersionForRoom")
+	defer span.Finish()
+
+	apiURL := h.roomserverURL + RoomserverQueryRoomVersionForRoomPath
+	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}
diff --git a/roomserver/query/query.go b/roomserver/query/query.go
index f2e9e6155..01bbadb5b 100644
--- a/roomserver/query/query.go
+++ b/roomserver/query/query.go
@@ -804,6 +804,20 @@ func (r *RoomserverQueryAPI) QueryRoomVersionCapabilities(
 	return nil
 }
 
+// QueryRoomVersionCapabilities implements api.RoomserverQueryAPI
+func (r *RoomserverQueryAPI) QueryRoomVersionForRoom(
+	ctx context.Context,
+	request *api.QueryRoomVersionForRoomRequest,
+	response *api.QueryRoomVersionForRoomResponse,
+) error {
+	roomVersion, err := r.DB.GetRoomVersionForRoom(ctx, request.RoomID)
+	if err != nil {
+		return err
+	}
+	response.RoomVersion = roomVersion
+	return nil
+}
+
 // SetupHTTP adds the RoomserverQueryAPI handlers to the http.ServeMux.
 // nolint: gocyclo
 func (r *RoomserverQueryAPI) SetupHTTP(servMux *http.ServeMux) {
@@ -975,4 +989,18 @@ func (r *RoomserverQueryAPI) SetupHTTP(servMux *http.ServeMux) {
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
 		}),
 	)
+	servMux.Handle(
+		api.RoomserverQueryRoomVersionForRoomPath,
+		common.MakeInternalAPI("QueryRoomVersionForRoom", func(req *http.Request) util.JSONResponse {
+			var request api.QueryRoomVersionForRoomRequest
+			var response api.QueryRoomVersionForRoomResponse
+			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+				return util.ErrorResponse(err)
+			}
+			if err := r.QueryRoomVersionForRoom(req.Context(), &request, &response); err != nil {
+				return util.ErrorResponse(err)
+			}
+			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
+		}),
+	)
 }