Green up your roof!

10 steps to install a GREEN ROOF on your shed, or even your home

A green roof...why would you install such a thing? Your shed looks perfectly 'shed-like' with its roofing felt. In fact it's just a shed and you barely give it a second glance. ‍ A garden shed. How boring is that? But does it have to be like this? Why not make your shed a thing of beauty? And give it some 'green' credentials too. ‍

A garden shed. Nothing boring here! Its green roof is a sight to behold   Let's look at how to do it: Ideally your shed roof will have a slope of less than 45° but more than 2° to prevent slippage, excessive drying or puddling. ‍ A gently sloping roof is ideal   It should enjoy a sunny position so the plants will thrive. If it's shady you will need to choose your plants carefully. Decide whether you want a simpler 'Extensive' green roof which is generally easy to maintain - or a deeper rooting 'Intensive' green roof which could even support small trees and shrubs.   ‍

An intensive green roof needs a deeper substrate and is a lot heavier. It also requires more maintenance. Next, and probably the most important aspect: ensure the WEIGHT BEARING capability of your intended site is adequate. How do you do this? If  in doubt, consult a structural engineer. Bear in mind that the base for your green roof will be a waterproof construction with appropriate load bearing capacity. The load bearing has to take account not only the substrate and planting but also the weight of water when the roof is saturated. Also a potential 'snow load' in winter. A simple green roof can be expected to weigh between 60-150 kg per square metre so it's clear to see that most shed roofs will need more supports put in place. ‍

Installing a green roof on a grander scale! Root damage needs to be considered. Particularly with Intensive systems. Some roots have the ability to penetrate the waterproofing and roof construction, thereby allowing ingress of water which will eventually rot the structure. If the waterproofing is not root resistant, an additional root barrier should be installed.  In addition to the roof surface, the upstands, perimeters, joints and roof edges also have to be protected against root penetration. ‍

A green roof construction consists of several layers including waterproof membrane, root protection, drainage and substrate.   Then you will need an 'upstand' or perimeter structure around the edges of your roof. Ideally this should be at least 10cm tall. You will put your waterproofing, root barrier, drainage layer and substrate within this. All these things can be purchased from specialist sites.   ‍

The upstand on a shed will contain the elements of a green roof. Gravel around the edge can be included for drainage. The ideal substrate varies according to your planting scheme and there are lightweight elements which reduce weight load.You'll typically need a depth of between 5-20cm for an extensive green roof. These need contain only a small amount of soil component,  typically between 5 – 20 % organic matter, plus lightweight aggregates that make up the bulk of the substrate. The depth of the substrate should be between 20-150cm for an intensive system. ‍

Substrate for an extensive green roof comprises lightweight material mixed with only a little compost or soil. When you have prepared your roof you can do the exciting bit - the planting! This is REWARDING and it's your payback for all the research and preparation! You could opt purely for Sedum and other succulent plants which are reliable and perfectly adequate. ‍

A sedum green roof mix is available from Perfect Plants and can be sown at the rate of 1g per square metre ‍ Perfect Plants also sells a mixture of seed suitable for green roofs. This one includes succulents, wildflowers and herbs. It should be sown at the rate of 2-5g per square metre.   Or you might like a mix of different species including other alpines, wildflowers and herbs. Drought-tolerant plants are a wise choice and there are green roof turfs and 'mats'  or 'blanket' systems which allow you to simply roll your roof into place. You can also seed your roof in order to save money, or buy tiny plug plants for you to place within the substrate. ‍

Laying especially grown green roof turf is a quick way to achieve instant impact   Suitable species: there many plants that will survive and even thrive. Here's a tiny selection: Antennaria dioica, Armeria maritima, Campanula rotundifolia, Cymbalaria muralis, Dianthus arenarius, arthusianorum, deltoides, Gypsophila repens, Hieracium pilosella, Petrorrhagia saxifraga, Saponaria oxymoides, Saxifraga granulata and encrusted types, Sedum acre, album, ellacombianum, hispanicum, hybridum, pulchellum, reflexum, sexangulare, spurium, telephium, Thymus pulegoides, serphyllum and other species. Water your roof during the first year after planting. Then it should become self-sufficient. You'll need to weed out unwanted species delivered by birds or parachute over the years. Good luck - and enjoy your green roof!