In this article, we consider in improving the throughput of existing multicast routing protocols by using multiple multicast trees in ad hoc wireless networks. To achieve our goal, we propose virtual layer, a multi-layer container for multicast trees, that provides an scalable routing structure with transmission gain in multicast routing. Without modification of multicast routing protocols, we enable a layer concept into the protocols so that multiple multicast trees can be constructed in a multicast group. Although virtual layer improves throughput, excessive data traffic will be introduced into a network that may cause network congestion. To resolve this situation, we consider a random coding scheme, a practical network coding implementation that encodes data packets for routing in disjoint-path tree while avoiding congestion. We observed in our simulations that a transmission gain trades off the excessive traffic that causes network congestion.