| SPECIFICATION FOR GRASS & WILDFLOWER ROOF Roof Build Up: for a flat (ish) roof: - assume a ventilation layer over the main roof structure to building regs requirements
- 18mm strandboard base layer
- protection layer: this might consist of building paper, baiscally something to protect the underneath of the membrane
- waterproof layer: currently recommending a butyl liner which would be prefabricated off site to suit the building
- second protection layer: again to protect the waterproof layer from damage, mainly during building operations, but also during maintenance: it might be old carpets if available, corrugated cardboard, or hessian, or thin fibreboard
- drainage layer: ideally use 25mm of chalk in stones 10 - 40mm: alternatively use Enkadrain ST from Akzo Nobel. which incorporates the filter fabric layer: the chalk will retain water whereas the Enkadrain is fairly free flowing
- filter fabric: Terram or similar with adjacent sheets lapped 100mm
- soil layer: depending on what you have available, but you are aiming for a fairly poor quality (in terms of humus content) soil: I have laid a layer of leaf mould first over the filter fabric, followed by soil: where the soil has been quite clayey, I have added sharp sand, but I have also added chalk for an alkaline soil: depth generally 50mm minimum, and 75mm ideal
- you might want to include a watering system within the depth of the soil, using porous pipe, generally available from garden centres
- you might want areas which are not soil, for access, so omit the soil over a suitable width and substitute river washed pebbels, or 10-20mm gravel (cheaper), although eventually this will get organic matter within it and things will start to grow anyway
- you will need drainage, and this could consist of a channel at the lower end of the roof, within the ventilation layer thickness, ie base lower than the main part of the roof, with the depth up to the filter fabric level filled with gravel and also containing a 50 - 75mm diam pipe bored to allow it to take in water from around, filter fabric laid over, and gravel over the lot up to top of soil
for a pitched roof - as the pitch increases, you need to start introducing measures to stop the covering slipping down the slope, and where the slope is near maximum, which I reckon is 20 degrees, I think the drainage layer is pretty redundant
- soil stabilisation: construct trellis work of timber battens within the depth of the soil to retain both moisture and soil, say 25 x 50 battens at 600 c/s laid across the slope, and 25 x 50 battens over laid up the slope at similar or greater centres, all screwed together (mind you don’t pierce the membrane): for greater stability, add in a layer of garden netting stapled to the trellis work, and also stapled at the upper end to any perimeter timberwork: the roots will grow into this and consolidate it all together
roof covering - seeds to be obtained from Suffolk Herbs, Monks Farm, Pantlings Lane, Kelvedon, Essex CO5 9PG tel 01376 572456
- seeds to be type F6 Sandy Soil Wildflower mixture, and G6 Sandy Soil Grass Mixture, using 20gm wildflowers per 100gms grass
preparation and seeding - sowing to take place in March/April or in September
- lightly cultivate surface of soil: leave for three weeks and hoe off or fork up any weed seedlings which have appeared
- prepare soil for seeding with fine toothed rake to a fine tilth, and moisten seedbed thoroughly with a fine spray
- broadcast grass seed evenly at 4 gm/sq m and rake into surface
- sow wildlflower seed randomly but very thinly at 8gm/10 sq m and lightly firm or tread into surface
- cover with netting to deter birds until germination is complete
first season maintenance - cut grass once it reaches 75mm in height and keep grass cut to 50 - 75mm: normally this will mean cutting at least twice, once in late July and again in late autumn for spring sowings, with ad additional cut in early March for autumn sowings: cut with shortbladed Turk scythe by hand: remove any perennial weeds such as docks and thistles by hand: rake off cuttings
- keep soil moist throughout dry periods and until grass cover is established by spraying with a hosepipe from ground level or from above as appropriate: use a fine spray and spray until soil is saturated and excess water begins to drain away
second and subsequent season maintenance - cut once only in late July/early August with shortbladed scythe as above and rake off cuttings
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