Maintaining high system utilisation is a key factor for data centres. However, strictly partitioning the datacentre resources to fully isolate the concurrent applications (contention freedom) leads to poor system utilisation because of fragmentation. We present an allocation algorithm for fat-trees (which are commonly found in large-scale data centres) capable of increasing system utilisation while maintaining application isolation. Results show at least a 10% increase in system utilization compared to regular contention free allocation mechanisms, at the cost of a slight reduction in network performance or application isolation.