We characterised four recycled materials that have been manufactured into useful substrates for use on extensive green roofs. These were a crushed red brick (the U.K. industry standard substrate base and therefore used as a control) and three alternative pellets made from: clay and sewage sludge (waste clay from excavations, fly ash and sewage sludge), paper ash (from recycled newspapers) and carbonated limestone (from quarry fines). Investigations into optimal organic content – conifer-bark compost for plant nutrients – and characterisations such as pH, particle size distribution, loose bulk density, particle density, XRF and leachate analyses were performed. Greenhouse experiments showed significant interactions between the four aggregates and the amount of added organic material, meaning that organic addition did not have the same effect on plant growth in each aggregate. The addition of organics also significantly reduced the pH of the recycled aggregates, making growing conditions for plants more favourable in these substrates. Particle density and loose bulk density results have shown all substrates to be classed as lightweight aggregates and leaching analysis has confirmed that all substrates perform within legal leachate limits for drinking water. As all the aggregates are commercially available at similar costs to the crushed red brick control, we believe that the alternative substrates have great potential in the green roof market and as they can be locally sourced we would also suggest that they are as good, if not better, than the industry standard, both economically and environmentally.