Green roofs add considerable weight when watered, which may strain garden shed roofs. To make sure that they can withstand this additional weight, consult with a structural engineer.
An effective and cost-efficient solution to greening a shed roof is to use ready-to-roll carpets of hardy sedums that tolerate drought, need only occasional trimming, and can even be grown from cuttings or seeds sown directly on its surface.
Choose a Substrate
Substrates are essential components of green roofs, playing an essential role in plant growth, water retention and drainage, and nutrient delivery. Selecting the most appropriate substrate will determine its success or otherwise for any given project.
Beginning your green roof soil mix project is easy with lightweight soil mixes from garden centers or landscapers, often comprising organic potting soil combined with an aggregate such as crushed slate, shale or expanded sand that helps retain moisture while draining well and avoiding compaction. Homeowners may also create their own green roof soil mixture by mixing expanded shale or slate with organic potting soil.
Consider your maintenance requirements when selecting the ideal soil for your shed green roof. For an easy option with minimal upkeep requirements, consider sedum matting; it consists of living carpets of sedum plants grown on mats that can easily be installed onto any rooftop surface. Sedums are hardy plants with minimal rainfall requirements that only need an annual trim to remain looking their best!
When planning an intensive green roof installation, it can be helpful to add an insulation layer first in order to reduce energy costs while creating a solid base for plant life. This step can also provide more stable support from which your vegetation can flourish.
Add a layer of growing medium, whether that be traditional topsoil, lightweight compost, or both. A “sedum mat” may also be an option that contains pre-grown sedum clumps to provide even greater support and insulation value.
Once your growing medium has been installed, install a waterproof membrane to safeguard against water damage and leaks in your shed. Also add a tanalised wooden frame for holding down substrate and vegetation – this helps to ensure the structure doesn’t slip when the roof becomes wet.
Frame dimensions should be at least 100mm (4in), with its inner edge located 200mm (8in). This distance will prevent water logging and pressure on plants.
Construct a Frame
Installing a green roof to your shed isn’t only a great horticultural exercise; it can also maximize space usage. A green roof typically comprises several layers – from waterproof/root barrier layers that protect the structure to drainage layers, water retention mats, growing medium and vegetation – that combine to weigh 100kg per square metre (20lbs per square foot when saturated). Before proceeding further with this project it’s important that your shed can support additional weight – otherwise if necessary you will need to create a frame to sit atop of it supported by sturdy corner posts.
Before beginning work on your roof itself, decide what you hope to accomplish with it. Do you want it as a place to grow flowers and vegetables, or maybe as an aesthetically pleasing outdoor retreat where people can lounge in the sunshine? Additionally, think about whether or not you require extra storage space for bins and bikes that might need accommodating within its design.
Build the frame first using 2m (6ft 6in) lengths of wood to form a rectangle on your roof, using L-brackets to secure each side length with L-brackets before adding more lengths at each corner. Be as accurate as possible at this step – any miscalculation could cost more later!
As soon as your frame is assembled, add insulation using thin foam fan board insulated specifically to withstand damp soil conditions. Next comes drainage mat with capillary spaces to prevent soil clogging up your system; finally finish by covering with filter sheet to filter out larger bits of plant matter.
Now, add the substrate – a mixture of aggregate and organic material spread evenly and up to the level required in your design. After spreading out this layer of aggregate and organic material evenly, set out your plants into their trays and water them until the soil settles; optional sedum mats/blankets can also be placed here at this time. Water your green roof regularly during dry spells; feed it twice each year using an affordable plant fertiliser in granular form such as watered down comfrey tea solution works great.
Install the Membrane
Imagine yourself as the owner of an average garden: you may think a green roof is beyond DIY capabilities for most gardeners, yet you’d be amazed how straightforward the process of turning a shed into a home for sedum plants can be! Break the process down into smaller steps – one day should suffice in creating stunning new features in your back garden!
Step one in creating your green shed roof requires creating a frame out of tanalised timber that fits over your existing shed roof and measures 3 cm deep, featuring an edge lip to allow water drainage from it without harming growing medium underneath it.
On the top of your frame, lay down a waterproof membrane such as EPDM or butyl pond liner to stop rainwater penetrating the growing medium and causing structural damage to your shed. A good quality membrane will also reduce heat loss from your building while offering extra frost protection.
On top of this foundation, you’ll add a thin layer of growing medium. This may consist of compost, perlite or LECA (lightweight expanded clay aggregate), which should be lightweight enough to avoid adding undue weight to your green roof while being water retaining and providing essential nutrients to the plants you plan to introduce into it.
Once your growing medium has been laid, a layer of gravel must be added on top to enhance drainage and prevent any substrate from clogging up the pores in your drainage membrane. A filter layer may also be installed; this step is not mandatory.
At this stage, it’s wise to assess the structural load capacity of your shed roof in order to make sure that it can support the weight of a green roof when completely saturated with water. If necessary, add extra corner posts or reinforce the roof itself to accommodate for this added burden.
Install the Plants
Green roofs are becoming an increasingly popular way of adding plants and increasing biodiversity in our garden spaces. They can be added to new buildings or existing structures like sheds. Installing one is relatively straightforward project that can be completed over a weekend if using pre-made kits available for assembly; alternatively constructing your own green roof from scratch may prove more rewarding and customise option can be fun and rewarding!
Installing a green roof on your shed requires the structure to support both its weight as well as any extra moisture or drainage issues that arise due to having vegetation on top of a green roof. Furthermore, its location should ideally be flat with gentle inclinations towards its back for water flow to minimise structural damage to its frame.
As an extra measure to protect the building against leakage, it is advisable to install a waterproof membrane before undertaking green roof construction. This will stop rainwater running down walls into sheds as well as plant roots from potentially damaging roof structure.
Once the membrane has been installed, build a tanalised timber frame for your shed roof with a 3cm lip to accommodate growing substrate and sedum matting – this will act as a water retention barrier and prevent soil from seeping into the shed wall below.
After spreading out sedum matting and gutter guards with growing media, spread 2cm layer of growing media over them both. I used a lightweight low nutrient mix composed of inert crushed rock, sand, and grit to provide ample drainage of excess moisture; this serves as an excellent alternative to compost and is suitable for most species of sedums.
Once again, cut a piece of sedum matting to the correct dimensions and lay it over your growing medium. Make sure it is tightly butted together, watered after installation, and secured using galvanised screws – though pre-drilling these is essential in order to avoid splitting wood frames!
