optimal drainage although more complex in nature the growth substrate 14 may have a lower total average density than the growth substrate 2 fig4 shows a growth substrate 16 according to the invention which comprises a similar construction as growth substrate 2 however in this case the internal surface of layer 13 is further increased by the inclusion of internal surface increasing particles 17 such as clay particles or lignite it may be possible that these particles are not homogeneously distributed over the total height of the layer 13 but may be present in the upper part of layer 13 and in the alternative in the lower part of layer 13 accordingly the internal surface is increased further thereby improving water spreadability reducing the total overall density and providing a cheaper product there will be an extra effect in the form of an improved yield improved growth rate and when included in layer 13 a slower release of incorporated fertilizer fig5 shows a growth substrate 18 according to the invention in which a layer 19 of transversally extending mineral wool fibres is interposed between two surface layers 20 and 21 the layers 20 and 21 have a higher internal surface than layer 19 this is an optimal growth substrate for roof gardens because layer 21 may have a higher density than layer 20 such that it provides walking properties layer 20 may provide water retaining buffer properties and furthermore properties for fixation to the roof by mechanical or physical means fig6 shows a growth substrate 22 according to the invention comprising a layer of short transversally extending mineral wool fibres 23 and further as surface layers in the form of curing oven profiles 24 and 25.