A roof garden, also known as a rooftop garden, is built on top of a building’s roof. Roof gardens are becoming increasingly popular in urban areas as they allow city dwellers to have green spaces and grow plants, fruits, vegetables, and herbs, even if they don’t have access to a backyard.
Roof gardens provide many benefits: They help insulate your building, reducing energy costs. They absorb rainwater and provide stormwater management. They filter pollutants and dust from the air. And they allow you to grow your food!
But before you start building your oasis in the sky, consider some critical factors when planning and constructing a roof garden. This complete guide will walk you through everything you need to know, from assessing whether your roof is suitable to choosing the right plants to installing roof garden layers and irrigation systems.
Assessing if Your Roof is Suitable for a Garden
The first step is to evaluate if your roof structure can support a roof garden’s weight. A simple roof garden with a few pots and containers can weigh 100-150 pounds per square foot when fully saturated with water. More extensive roof garden systems with deeper soil beds can weigh 150-200 psf when wet.
Consider hiring a structural engineer to inspect your roof and ensure it can handle the load without reinforcement. Generally, commercial roofs need to support at least 20 psf for a basic roof garden, while residential roofs need at least ten psf capacity.
Find out the age of your roof – newer roofs can better handle the weight load. Older roofs may need updating before installing a garden. The roof slope and surface material also impact the feasibility and weight capacity. The optimal slope for drainage is between 1:12 and 5:12. Steeper pitches make installing soil and plants more difficult.
Choosing the Right Plants
Selecting plants suited to roof conditions is critical to creating a thriving garden. Consider the following factors when picking plants:
Climate
Choose plants that withstand local temperatures, rainfall, wind, and sun exposure. Drought-tolerant succulents and sedums do well in dry, hot environments, and wind-tolerant shrubs are needed for high-rise buildings.
Sun Exposure
Determine how much sun your roof area gets. Full sun? Partial shade? Morning sun and afternoon shade? Pick plants that align with the sunlight patterns.
Wind Conditions
High winds can damage plants on an exposed roof. Opt for low-growing wind-resistant plants like sedums, aloe veras, lavenders, oregano, thyme, alliums, and ornamental grasses. Avoid large trees and shrubs that act as wind sails.
Weight
Lightweight plants reduce stress on your roof. Succulents, perennials, herbs, vegetables, and grasses are lighter than trees, shrubs, and dense groundcovers. Shallow-rooted plants also minimize weight.
Water Needs
To minimize maintenance, choose drought-resistant plants suited to your climate. Succulents, aloe vera, sedums, oregano, thyme, sage, and lavender grow well in low water conditions.
Maintenance
Consider the upkeep your plant selections will require. Hardy herbs and succulents are low-maintenance compared to plants that need frequent watering, pruning, and fertilizing.
Growth Habit
Choose plants that grow low to the ground and won’t overgrow the depth of your growing medium. Trailing succulents and groundcover plants are great options.
Steps to Make a Flat Roof Garden
Here are the key steps to make a flat roof garden:
- Evaluate if the roof structure can support the weight of a garden. Hire an engineer to inspect if needed.
- Make any necessary roof repairs and install additional drains or gutters for drainage.
- Install a protective root barrier membrane over the entire roof surface.
- Add a drainage layer of gravel, expanded shale, or recycled plastic panels.
- Install 10-15 inches of a lightweight growing medium engineered for rooftop gardens.
- Cover the growing medium with a permeable filter fabric that is overlapped and secured.
- Incorporate hardscape elements like paths, planters, and terraces if desired.
- Install a drip irrigation system with timers and intelligent controls.
- Select plants suited to rooftop conditions, like succulents, grasses, herbs, and perennials.
- Plant your roof garden based on a planned layout for spacing and sun exposure.
- Mulch around new plantings to retain moisture and suppress weeds.
- Care for the garden by weeding, watering, fertilizing, and making any needed repairs.
Following these steps allows you to successfully convert a flat roof into a beautiful garden oasis! Let your design creativity flourish.
Structural Components of a Roof Garden
In addition to live plants, a roof garden system requires several layers of structural components to waterproof and protect the roof, support growth, and drain properly.
Drainage
Adequate drainage is crucial to prevent damage from water ponding on the roof. The roof surface needs a slope of at least 1:50 to allow drainage. Install drains, scuppers, and gutters around the roof perimeter and direct water safely off the roof.
Protection Layers
Install a root barrier above the waterproofing membrane to prevent roots from penetrating and damaging the roof. Use a thick plastic sheeting or specialized root barrier product.
Add a drainage layer above the root barrier using gravel, lightweight expanded shale, clay, or recycled polyethylene panels with egg crate-like plastic cups. This creates an air space to hold moisture while allowing excess water to drain.
Growing Medium
The growing medium provides nutrients, hydration, and support for plant growth. Lightweight soil mixes are best to reduce the weight load on the roof. Aim for a blend containing 20-30% organic matter like compost, 30-40% inorganic materials like expanded shale, pumice, or perlite, and 30% good quality topsoil. The medium should drain quickly but also retain some moisture. 10-15 inches of growing medium depth is sufficient for most roof gardens.
Filter Fabric
Lay a permeable filter fabric over the growing medium to keep it from eroding while still allowing water to pass through. Choose a durable, non-biodegradable geotextile fabric.
Wind Protection
If the area is exposed to high winds, install a windbreak around the roof perimeter. Use a solid barrier or trellis system planted with dense vines. This protects plants in the interior from harsh winds.
Installation Process
Once you’ve assessed your roof structure, chosen appropriate plants, and gathered structural supplies, it’s time to start building your roof garden! Follow these key steps:
Deck Preparation
Sweep the roof deck thoroughly to remove debris. Cover any drains and outlets to prevent clogging during construction. Inspect the waterproofing membrane and make any necessary repairs to seal the surface.
Install Root Barrier
Roll out the root barrier membrane, fully covering the roof surface. Seal the edges and seams with manufacturer-recommended tape. The barrier should run up over the roof parapets.
Install Drainage Layer
Spread the gravel or other free-draining material over the root barrier to create the drainage layer. Maintain a 2-3 inch depth evenly across the area.
Install Growing Medium
Spread your growing medium mix over the drainage layer, raking it to an even depth of 10-15 inches. To prevent compaction, avoid walking directly on the medium.
Install Filter Fabric
Roll the permeable filter fabric over the growing medium. Overlap the edges by 6-12 inches and secure the fabric with landscape staples, galvanized nails, or glue.
Create Landscape Design
You can now bring materials to create paths, terraces, planters, garden beds, and hardscapes. Install irrigation components. Shape the growing medium and create contours.
Plant Garden
Start planting! Follow your roof garden design, ensuring plants are adequately spaced for mature size. Water thoroughly after planting.
Effective Irrigation Systems
Since carrying watering cans up to a roof is impractical, installing an irrigation system tailored to your roof garden is highly recommended. Drip irrigation, which delivers water directly to plant root zones, is the most efficient option.
Water Source
Determine how you will get water up to the roof. Possibilities include using an existing plumbing line, adding a new supply line, or using rainwater harvested in cisterns.
Pipes and Tubing
Use UV-resistant tubing suited for exterior conditions. Install a main feeder line around the roof perimeter with lateral lines branching off to reach all plant areas.
Emitters
To deliver water, add drip emitters, micro-spray jets, or lawn sprinkler heads to the tubing. Use emitter types suited to your plant types.
Timers and Controls
Automate the system with programmable timers and intelligent controls. This saves time and ensures consistent irrigation scheduling.
Filters
Install filters to remove debris from harvested rainwater or greywater used in a roof garden system. Keeping water clean prevents clogs.
Backflow Preventers
Use a backflow prevention device if connecting to building plumbing to avoid contamination.
Caring for Your Established Roof Garden
Once installed, a bit of regular care and maintenance will keep your roof garden looking great:
- Inspect the garden after storms and repair any damage immediately.
- Weed every week when plants are young. Mature plantings discourage weeds.
- Water thoroughly when the soil appears dry. Adjust irrigation schedules seasonally.
- Fertilize organically 2-3 times per growing season. Slow-release fertilizers are ideal.
- Prune back overgrown plants and remove dead or diseased plant material.
- Check soil pH annually and amend if needed to keep the appropriate range.
- Combat pests naturally with beneficial insects, neem oil, and insecticidal soap, or remove them by hand.
- Repair any leaks or malfunctioning irrigation components.
- Remove leaf debris and clear drains and scuppers to encourage drainage.
- Overwinter any non-hardy plants in a greenhouse or indoor space.
Following a good maintenance regimen keeps your roof garden looking fabulous!
Conclusion
Creating a beautiful, eco-friendly roof garden gives you additional green space while benefitting the environment. Proper planning, structural components, plant selection, and routine care allow you to install and maintain a thriving oasis in the sky on your building’s roof. Assess your roof structure, choose appropriate plants, and establish adequate irrigation and drainage systems. Follow the steps and tips outlined in this guide to successfully grow a gorgeous roof garden tailored to your site and climate. Get ready to relax among your plants, flowers, and fresh herbs overlooking the world below!
FAQs
Q1. What is the best type of roof for a roof garden?
A1. Flat or gently sloping roofs work best to install a roof garden. Pitched roofs steeper than 10 degrees can make installing soil, plants, and an irrigation system challenging. Concrete, PVC, TPO, EPDM, and built-up roofs are common roof types suitable for roof gardens.
Q2. How much does a roof garden cost?
A2. The cost of a roof garden varies greatly based on size, complexity, structural considerations, and types of plants and materials used. Basic extensive roof gardens can cost $15-$40 per square foot. More complex intensive designs with elaborate hardscaping, furnishings, and irrigation systems can cost $100 per square foot or more.
Q3. What kind of soil should I use in my roof garden?
A3. Lightweight engineered soil mixes are best for roof gardens, weighing 80-100 pounds per cubic foot when saturated. Aim for 20-30% organic matter, 30-40% porous inorganic material like expanded shale or perlite, and 30% quality topsoil. The soil should drain freely but also retain moisture effectively.
Q4. How much weight can a roof hold for a garden?
A4. Residential roofs should support at least ten psf dead load capacity, while commercial roofs need 20 psf capacity or higher. Extensive roof gardens with 3-6 inches of lightweight soil weigh 10-25 psf when saturated. Intensive designs with beds over 12 inches deep can weigh up to 100 psf when wet.
Q5. How much maintenance does a roof garden require?
A5. During the establishment period, roof gardens need weekly watering and weeding. Once plants are established, roof gardens require only occasional watering, fertilizing, pruning, and pest management. Overall maintenance is less than a regular ground-level garden since roof gardens don’t deal with compacted soils, pervasive weeds, or some pest issues common at ground level.
