How to make a Green Roof

Every morning I will go and look at the garden from the window. Looking down at it instead of being immersed within the space provides an interesting perspective. I often find myself judging the space, continuously looking at how I can improve the design, utility and most importantly, the wildlife provision. From this elevation one thing was abundantly clear and that was of the unused space on my potting shed roof.

My potting shed roof is 6x4ft and I saw this as untapped potential and a green roof was ideal for the space. Firstly, my potting shed has a Pent Roof, meaning it contains a single slope, and as such rainwater naturally drains away. Those with an Apex Roof, meaning it contains two slopes meeting at a high point/peak, will also benefit from rainwater naturally draining away. This slope will prevent water logging onto the roof which would rot root systems.

Secondly, because of the slope of the roof draining the rainwater away I can divert the rainwater to a water storage unit for increased sustainability. I then use this stored rainwater to water the garden during the summer months.

Another sustainable aspect to a green roof is from its thermal protective properties. The green roof prevents heat leaving the potting shed through the roof, maintaining a warmer temperature. This increased heat retention is particularly helpful for when I am starting seedlings off in spring.

Green roofs create biodiversity especially in hostile environments, and in particular, urban settings and new build areas. Providing more for pollinators as well as an increased habitat space, they will also aid air quality, absorb airborne pollutants, and reduce carbon dioxide.

In general, green roofs are very resilient and need little maintenance for most of the year, mostly weeding and feeding. However, extreme changes in weather can become an issue for green roofs but I have some top tips to help you along the way.

I decided to install a sedum green roof from a specialist company and the sedum blanket was farmed in remarkably similar conditions to that of where I am in Scotland, including weather patterns. The first year was incredible and the roof was visited by bees all season long but due to large fluctuations in weather, I unfortunately experienced excess growth which then died back across the roof. This meant I then had to reseed the entire roof. However, every project has its lessons and the sedum farmers I purchased my sedum carpet from gave me a fantastic piece of advice on what to do if I experienced leggy growth again. This advice would generally prevent the roof needing to be reseeded and I will share it with you in my Top Tips at the end.

Here I will give you three tutorials for installing a green roof. They will be Preparation, Sedum Blanket and Sedum Reseed.

Preparation

Creating a watertight space is essential and trims along the edges need to be in place from the beginning to support the green roof and its substrates. Also note that rainwater will need to escape the roof via the trim at the lowest point.

Step 1 – Install trims around the roof with the lowest trim having holes included for water drainage. These can be bought and are often aluminum but to save money I used larch timber to create the trims. The lowest trim had holes drilled into it which I then covered with a permeable textile membrane so that no substrate could pass through and block the holes.

Step 2 – Place a waterproof membrane over the entire roof making sure the membrane goes all the way to the top of the inside of the trim. I stapled the membrane in place. Cut away any excess material and cut away the waterproof membrane in front of the lowest trim to allow rainwater to flow out.

Step 3 – Check all your work. Trims should be secured in place. The waterproof membrane should cover the entire roof and up the inside of the trims, and the lowest trim should have holes in it and not be covered by the waterproof membrane.

Step 4 – Place pebbles in front of the lowest trim. This will act as a division between the substrate and the drainage holes.

Sedum Blanket

Installing the sedum blanket is simple. The kit I purchased came with a woolen sheet, substrate, and sedum in the form of a blanket.

Step 1 – Lay the woolen sheet over the surface area and cut it to size.

Step 2 – Empty the substrate over the woolen sheet and level.

Step 3 – Install the sedum blanket and fill in any remaining space with off cuts.

Step 4 – Water lightly.

Sedum Reseed

Reseeding a sedum green roof is a quick and easy task. The hardest part will be clearing away the dead material and as such, you will want to carry out this task on a dry day.

Step 1 – Remove all dead material from the roof whilst leaving as much substrate in place as possible.

Step 2 – Once cleared, gently scuff the top surface of the substrate. This will aid aeration and will be a better surface for the new seed mix to root to.

Step 3 – The seed kit should come in a large bag and will consist of substrate, slow-release fertilizer, and small cuttings of different sedum varieties. Empty the bag onto the previous substrate.

Step 4 – Level out the seed mix. To do this, I find using the pole from a broom an excellent solution in creating a level surface and reaching far to reach areas.

Step 5 – Water lightly.

Top Tips:

If you experience weather fluctuating between sun exposure, high heat, and rain, it is quite common for sedum to put on excess growth which then struggles to survive. If this happens, and I have this on good authority, you should trim the sedum down and leave the sedum cut-offs in place where they will root and become a new plant.

Do not over feed.