Set your own green roof!

This little guide will help you to understand the various considerations, materials, and step-by-step process you have to take into account to install your very own green roof. Commercially, plenty of different "green roof" systems exist nowadays, but... how about setting your own vegetated area within your balcony or create a lovely spot to plant and take care of your vegetables.

At this point, this sounds great, right? You will be able to save a lot of money by disregarding the services of several contractors and doing, basically, most of the tasks by your own hand.

Calhoun School Green Roof

Layers and Materials

First of all, to be able to create a beautiful landscape for us to enjoy when we get up in the morning and look out the window or simply develop our own vegetable patch for self consumption, we will have to get our material checklist done.

We will need the following materials for our green roof system and these will be assorted into six layers:

1. Bottom layer or First layer, also called the "waterproofing layer":

If you are considering a permanent "green area" within a terrace, application of one or two layers of rubber membranes is necessary to avoid system water leakage vulnerability (modified bitumen ones are a great option). Installation of waterproofing layers is the only step I believe necessary to be done by an expert, because of the complexity of the task and the special equipment that is used.

A simpler approach is to use plant pots or a raised bed to cover up the planned area and just verify that water drains correctly and rapidly.

2. Second layer, defined as the "root barrier layer":

Sometimes it is incorporated to the waterproofing system as a anti root solution, so be sure to check if the selected rubber membrane has this added value.

If not, you can check for commercial garden root barrier sheets that are easily found in gardening departments of large retail stores.

3. Third layer, the "drainage layer":

Vital to the functionality of our project, the drainage may be composed of small gravel between 1/4'' and 3/4'' diameter. This will allow the quick drainage of excess water outside our system. The layer thickness is variable to the total thickness and total area of installed green roof or plant pot. Use of light weight elements like volcanic rocks and expanded clay aggregates are strongly recommended to diminish the overall weight of the arrangement.

As an alternative for this option, there are several drainage systems designed specially for gardens and green roofs, usually these products are costly. For the purposes of this article, we will decide to go with the cost saving choice. You can see from the image that these systems come in form of trays, giving ability to drain excessive water and store some for humidity retention inside their integrated cups.

4. Fourth layer, the landscape fabric:

Used to separate the drainage stratum from the soil bed, commercial landscape polyester fabric can be purchased from large retail stores in the form of rolls. Most of them have weed control and UV resistance.

5. Fifth layer, the decisive soil layer:

Basic to the growth of plants, optimal retention of water, and availability of nutrients, the soil tier needs to be picked out carefully. The characteristics must address good water retention, proper nutrient composition, and it is advised to add in special components to enrich the drainage capabilities. This latter can be perlite or vermiculite aggregates blended in within the soil.

Good news come when knowing that prepared garden soil mixes are easily acquired at gardening stores.

6. Finally, the Sixth layer, the creative layer:

Choose from a wide variety of plants, be creative and choose whatever goes along with your personality or intentions! You can create a small grass garden for your children to play on top of or, perhaps, you just want to have your own gardening leisure spot.

Remember! The type of plants you can set are directly connected to the available soil thickness of the system. So be careful to take this into account. For a more detailed description of this and other convenient installation tips, I invite you to check the next section!

Vegetated Roof Components

Step-by-step practical example

Now that we have reviewed the concepts, we will check how the installation process works by suggesting the installation of a small grass area 10' x 10' planned for a terrace roof.

Step 1. Drainage: Check for drainpipes within the projected surface, there must be at least 1 drain pipe per 270 square feet.

Our example yields 100 square feet, so exit to 1 drain pipe says we are good to go!

Step 2. Waterproofing: We are intending to waterproof the selected area with a modified bituminous membrane with integrated weed control solution aiming to not worry about the succeeding root barrier layer. Be sure to ask for these characteristics when selecting the product.

CAUTION! This is the only task advised to be done by a professional because torch-down application is needed. The advantages of this waterproofing system is its durability and its reliability. The good news is that the area is pretty small so it will not take long to place the modified bitumen layers (less than an hour) and labour and materials are not going to cost much (a standard roll 8' x 32' costs from $70 to $90, maybe you can resolve with the waterproofing contractor to pay only for the amount of roll used for this project, this should lower the costs a bit).

To prove that the waterproofing installation works, seal the drain pipe temporarily and fill the area with water until a small pond is made. Let this pond stay like thar for a week and check for dampness on the ceiling bellow. If no dampness shows up, then the waterproofing has been done correctly. If not, corrections to the system must be done (this tasks are included within the contractor's warranty).

Step 3. Aggregate and soil calculations: Okay, now that the tricky part has been done, let's begin with the fun part! The material volume calculations, to plan for this we will need to answer the following questions:

a. How deep should my drainage bed should be? An overall 3 in. lightweight aggregate bed will work fine for this project.

Volume of aggregate = Overall thickness of aggregate needed x Surface = 0.25 feet x 100 square feet = 25 cubic feet of aggregate!

b. How deep should the soil bed must be for proper grass growth? Grass only needs a 4 in. soil bed to develop well. 

Volume of garden soil = Overall thickness of soil needed x Surface = 0.33 feet x 100 square feet = 33 cubic feet of soil!

Step 4. Placing aggregate layer: Now that we have calculated what we need, we will be placing the aggregate layer carefully over the waterproofing system, keeping an eye on maintaining an homogeneous 3 in. thick bed.

A nice tip to avoid wearing out of waterproofing membranes is to place a sheet of polyester landscape fabric as a base for the above aggregate bed.

Step 5. Landscape fabric sheet: To avoid mixing aggregates with soil, we place another sheet of polyester landscape fabric after the aggregate layer and beneath the soil preparation stratum.

Step 6. Soil layer: Doing the same as in Step 4, just be careful at placing soil evenly, trying to maintain the 4 in. bed thickness and, in the end, level the terrain with a piece of timber.

Step 7. Final Step! Turfgrass rolls!: Rolls of  turfgrass are sold in dimensions of 10 square feet (3.3' x 3.3') so we only need 10 rolls to cover up our green area! And that is it! To place the turfgrass just dampen the soil layer and place the rolls gently and closely tight between the adjacent pieces so grass roots can intertwine.

Grass should adapt quickly, but it is necessary to water on a daily basis for the first week and thrice a week for the following two weeks.