Making a green roof is actually not as difficult as it might sound. A lot of DIY-ers have successfully done this project and make their house greener and naturally friendlier.
During the entire process, the most challenging step might be searching for a guide. But if you follow this article “Green Roof Construction - How To Make A Green Roof”, I’m quite sure that the remainder of this project is a lot easier.
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Prepare The Roof
Check The Roof Strength
Before starting out this project, you should know if the chosen roof is heavy-duty enough to support the weight of plants and the engineered growing medium.
Hire a structural engineer to help you not only check the roof but also give advice on what it needs to support a green roof’s weight.
Each plant tray in a green roof system averagely adds 30 pounds of weight (per square foot) when saturated, which is a bit stressful for traditional roofs (which can only support 20-25 pounds).
A popular way to upgrade these roofs is by adding more joists.
Cover The Roof With A Waterproofing Membrane
This step is a must to protect your roof from damages caused by water absorbing into the roof material – which directly affects your safety, wallet, and roof’s durability. It could be some form of liquid waterproofing, asphalt, or a bitumen coating or membrane.
Rubber roofing membrane is one of the most popular options for asphalt, wood, and concrete.
Add Roof Barrier
This extra layer is located above the waterproofing membrane and joined with tape to reduce damages on the roof construction caused by plant root growth.
It is commonly thin polyethylene plastic membranes
Add Some Pitch To The Roof
It is essential to add pitch to the chosen roof to aid excess rain and stormwater to flow toward gutters, avoiding waterlogging, roof damages, and root damages.
The minimum highly recommended pitch for a green roof is ¼ inches/foot of run.
Filter Fabric Layer
This layer plays a huge role in stopping soil, debris, and substrates from filling the drainage membranes’ cavities, therefore, preventing water logging more effectively.
Add Plants & Substrates
Now, what to plant?
The choice is up to your purposes, the climate and weather conditions in your living area, and the green roof area and depth.
If you want to attract wildlife, like bees and other potential insects, choose local wildflowers.
For extensive green roofs, sedum, mosses, Sempervivum, and grasses are a good-to-go. Therein, sedum is the most favorite because:
- Low upfront cost
- Usually sold as pre-grown sedum mats
- Low-growing and low-maintenance
- Good at tolerating harsh weather conditions
- Fit any roof sizes
Tolerant plants with great wind and sun tolerance properties suit best for intensive green roofs.
Meanwhile, ornamental grasses and dry habitat perennials are perfect for semi-extensive green roofs. For example, Stipa tenuissima, helictotrichon sempervirens, dianthus, Armeria, potentilla, achillea, and rudbeckia.
For dry shady conditions, Asplenium Trichomanes, polypodium vulgare, and other types of ferns are a good choice.
Notes: All the recommended plants above are according to RHS (the Royal Horticultural Society).
Growing Medium Or Substrate
While potting soil is the choice that most gardeners think of, it is not the best option for green roofs. Instead, many experts advise choosing substrates or growing mediums.
Substrates are usually sold in bags of various sizes – from dozens of kilograms for DIY, small projects to m³ for larger projects.
There are a lot of suppliers available on the market for you to look for and speak to.
Substrates also come in various types, mostly are custom-blended between organic and aggregate materials based on the buyer.
But they all share one thing in common – which consists of several materials mixed together to create the required characteristics for different vegetation growth needs.
In most cases, the mix should bring a fine balance between water absorption capabilities to support the plant growth and drainage. It should also be lightweight.
Those materials are usually medium clay soils, compost, bricks and aggregates, and even volcanic rocks like pumice and lava.
Notes: Sedum blankets and sedum mats don’t require an extra growing medium. Instead, just lay them on the filter layer.
Optional: Use Planted Trays
For flexibility and convenience, consider growing plants on planted trays – which are commonly 1’ x 2’ black polypropylene trays. They are stackable, easy to buy, and frequently sold in pre-grown vegetation trays at local nurseries.
They are also easy to install and can be cut to fit in specific roof shapes.
You should install decorative L-shaped aluminum edging before installing these trays for extra security.
Planted trays will be placed in rows on the green roof, starting from a corner and from right to left. Once finished, remove the soil elevators.
After adding plants, you should water them thoroughly to let the soil or substrates settle down. You can use a self-coiling garden hose to do this task.
Maintain The Green Roof
There is actually nothing much on green roof maintenance. You don’t need to water it frequently unless during a month's spell of high heat with under 1” of rainfall.
Trim it yearly and fertilize it occasionally based on the soil test.
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