Your article on green roofs (30 June page 70) states that “roof systems based on sedum blankets no more than 30 mm thick are the most common specified in the UK”.

Whereas this includes sedum blankets laid on a growing medium there is an increasing trend, in the interest of economy, to dispense with the growing medium and install the sedum blanket directly onto the waterproofing membrane. I wish to warn the industry that such systems do little to enhance the environment; they are not biodiverse, do little to retain water run-off and suffer from plant die-back within a very short period. These systems are common in Germany where the objective is to recover the subsidies that are available for so-called green roofs with the smallest outlay.

If designers and building owners truly wish to attract biodiversity to enhance city life they need to incorporate a suitable growing medium beneath the sedum mat.

John E Medlock, managing director, Axter