Just last month, we interviewed the landscape architecture firm Arterra LLP on our sister blog, The Architect’s Take. Kate Stickley and Vera Gates were so much fun that I thought I’d ask them about Living Roofs – not exactly energy compliance, but a “green building” topic nonetheless. Turns out they’ve done several, and as landscape architects, they bring an artistry and a focus on creating a meaningful sense of place… it’s not just a functional piece of “turf” for a corporate building, where no one ever actually goes up there to enjoy it.
What’s the definition of a “living roof” exactly?
A living roof consists of a thin strata of vegetation and soil, contained within the structure of a building roof and integral to it. Similar to a natural ledge, a select variety of plant types can be grown in the strata, creating additional garden space for use or viewing. Industry definitions characterize different types of Living Roofs according to the depth of the soil strata:
Intensive: This is what we think of as roof gardens, where we have access and usable space. Soil is at least 6” deep. These gardens are intended to be viewed and used as gathering spaces and may include hardscape. They are like gardens planted at the ground level. When we do living roofs, this is more our focus than the thin-strata type as mentioned below.
Extensive: Where there is no use or access and the stratum is very thin. Soil is 1”-6” in depth, planting is treated in mass (as in sheets of succulents or grasses). These are not intended for human use. We don’t have much experience with the latter. For now, just note that the reasons for putting a living roof on a private residence might be different, more personal, than the rationale for installing one on a large corporate or public building.
There is a very good book available on living roofs called “Planting Green Roofs and Living Walls“, by Nigel Dunnett and Noel Kingsbury. Hydrotech is the living roof system we typically use, and they used to have a pretty good web site.
What are some good residential examples – your favorites?
One project we did, we’ll call it the “Sky Garden” for the sake of our client’s privacy, involved very extensive roof gardens and some vertical living walls as well. Unfortunately we don’t have photos available at this time, but we can describe it a bit. The house is on a steep hillside, leaving very little usable outdoor space. There are several roof areas, all fully usable, including a fire pit, a patio, and a vegetable garden. Part of the roof is heavily shaded, and the other part gets a lot of sun; there are different plantings in each portion to take best advantage of the sun conditions.
Under the planted area, the roof itself has several drainage areas similar to what you might have in an indoor shower. A layer of foam in each drain area acts as a leveler to maintain an even 6″ soil layer throughout. Rainwater runs underneath the foam to the drainage openings. Roof vents and other mechanical features are concealed inside decorative shrouds that relate to the building materials and guardrail.
[See drawing details further down in this article.]
The most interesting part in a way is the water management, something we mentioned in our last discussion with you as well. Our “Sky Garden” project actually uses four sources of water, in a hierarchy that prioritizes water sources: first, grey water. Next, rainwater. Third, well water. Finally, if and only if all other water sources are dry, say during a drought, the roof garden can be irrigated using city water.
Here’s another recent project, a GreenPoint Rated house in Palo Alto, done in collaboration with architect Cathy Schwabe.
Although we tend to work on projects where the roof garden is fully usable, it’s also possible to create small rooftop gardens just to add visual interest in an urban setting, especially if the view out the window would otherwise be unappealing – bare rooftops or concrete walls, for example.
What are the benefits of having a living roof? Why would you want one?
The greatest benefit of having a living roof is expanded livable and/or visual garden space. For residential design, this is typically why you would want one. There are many additional benefits that may or may not be a factor, depending on the project, program and climate.
Additional benefits can include:
- Great insulation quality for the building. Living roofs help cool the building in summer and retain warmth in winter.
- Increases longevity of the roof system, by protecting the membrane from ultra-violet light and extremes of temperature.
- Decreases heat sink effect on hot sites-most apparent on a neighborhood level, when many roofs are planted.
- Retains and slows water runoff during and after rain storms, releasing it slowly, over a prolonged period of time. This reduces risk of flash flooding in urban areas during heavy rains.
- Increases opportunity for plant life, habitat & wildlife.
Is there any hard data on these claims for energy savings, heat sink reductions, or comfort improvements?
Yes, but I don’t know that off the top of my head. A lot of research in Europe and Canada has been done. [In densely built-up urban settings, a certain percentage of the rooftops might need to be planted in order to achieve a noticeable change in the overall "urban heat island" effect. Even simply making more roofs light-colored instead of black tar can help.]
Can you have a living wall, too? (e.g. plain old invasive-species ivy)
Yes, there are several, naturally clinging vines. There are also structural green wall systems you can attach to a wall, which provide an armature of support for any vines to grow through. They are separate systems and structures that have different plant types, irrigation and details. There are a number of pre-made systems out there: Fyto Wall and Tournesol VGM Modular Living Wall to name a few, plus the custom hydroponic system pioneered by Patrick Blanc. We have used both Fyto Wall and the Tournesol systems.
One thing to point out is that living walls are in many ways a completely different animal from a living roof. We could probably write an entire article just about living walls. Different structures are used to hold the plants in place for a living wall than for a roof. The physics of verticality vs. horizontality affects how weight is supported, how water flows down and out, and of course plants grow upwards so they’ll look different growing on a wall than on a flat surface.
Being vertical, living walls can’t capture as much rainfall, and thus require more irrigation. Sometimes, a lot more. You wouldn’t create a living wall to save water! And they don’t help much with slowing down rain runoff, either. However, they can be a great asset in urban areas as a break from all the concrete, where space is at a premium, and you can’t expand horizontally.
Sounds like a living wall would be impossible in a desert, if they need that much watering.
It’s possible to do a living roof with succulents, but yes – a living wall, even an all-succulent one, would need too much water to be a good choice for a desert area.
When are living roofs NOT a good idea?
- On remodels where the existing structural system and waterproofing wasn’t originally designed to support the added weight of a living roof, where the structure has not been designed to take that load, and there is no budget or scope for reinforcing it. You also need a higher level of waterproofing because living roofs have a rim and might need to tolerate standing water if the drains get clogged up.
- If the roof is sloped more than 2:12 pitch. Sloping roofs are more complicated but they can be done, up to that 2:12 pitch.
- If the living roof area is only a small portion of the entire roofing area. Even for new construction, there is an economy of scale. The smaller the living roof, the less cost benefit it provides.
How much does it cost, as compared to a regular roof?
The range is broad. Simple seeded systems might drop down as low as $15 per SF. However something like our Marin project would be closer to $40 per SF. That includes everything except the roof membrane, which is an additional $8-$12 per SF. It can be hard to come up with a hard number for a project, because if the scope of work included extensive groundscaping and the roof garden is only one portion of that, there are economies of scale that can make it difficult to price that same roof garden as a separate entity.
What are the top things an architect should consider when adding a living roof for a home project?
- The living roof should be well integrated, correctly sized, and serve multiple purposes.
- Waterproofing is key and cannot be fudged.
- The living roof should be an element incorporated early in conceptual design. Curb and parapet detailing, structural engineering, and waterproofing are all integral to the design and the look of the overall building.
What might an architect have to do differently on a project with a living roof?
Decide sooner, rather than later. Adding a living roof later in the design process incurs a great additional cost. However, if a living roof is a possibility, it opens up so much opportunity to expand living space out and into the garden, on multiple levels throughout the home. It also provides an opportunity for an interesting visual feature, as seen from key living areas of the home that might otherwise not have an interesting view.
What sort of special consultants would you need to create a roof garden, aside from a landscape designer? What about installers?
There are manufactured systems readily available on the market today that provide all the information an installer would need. Typically a waterproofing specialist is involved, as with most roof installations today.
A landscape contractor typically installs the living roof system. They need to coordinate closely with the general contractor, the roofing subcontractor, and the waterproofing consultant, as the waterproof membrane must be covered as soon as possible after it is tested.
The division between the general contractor and landscape contractors’s scopes of work needs to be definite and clean. It’s best if the general contractor is responsible for the structural construction of the roof and the waterproofing. Then a water test can be done. When the membrane is deemed waterproof, the landscape contractor can begin installation of the layers of material, soil and plants.
The Neumann Sloat project is a good example of a “non-use” application, where you’re not using it as a patio, but you’re making a visual feature out of something that would otherwise just be an ordinary-looking garage roof.
How are living roofs put together? What are the major materials, components and systems?
They are basically a big bathtub, integrally built into the roof and parapet. First, the roof structure is monolithically waterproofed and heavily tested to ensure the waterproof membrane is intact. Then the pre-manufactured living roof soil system is installed. These vary slightly but they basically consist of a root barrier to keep plant roots from penetrating the roof membrane, a soft insulation layer (which protects the membrane), a drainage layer (like an egg crate), and a layer of filter fabric.
Layering of components from bottom to top are as follows:
- Roof deck (wood frame or concrete)
- Protection Board
- Drain Mat
- Geotextile/filter fabric
- Irrigation piping
Along the perimeter of the building at the curb or parapet you will need:
- A curb, which must be at designed to be above the top of the soil
- Minimum 6” width of light-weight gravel between curb and soil area, separated by stainless steel edge restraint (Green Roof Solutions is one supplier)
- The roof deck must be sloped to drain underneath the soil layer. The roof design should include at least 2 drains that are waterproofed and hard piped out to the stormwater or rainwater catchment systems.
- The sloping of roof deck affects thickness of the soil profile above, and must be cross-checked with the project’s structural engineer.
- Lightweight foam board can make up depth difference if needed, to control weight.
- Inspection Chambers (SS, Green Roof Solutions) will be needed at drains and irrigation valves.
A specially formulated soil mixture is then brought in, the thickness of which varies. We prefer to work with a minimum of 9″ of soil, going as deep as 18″ in some situations. The medium tends to be very low in organic matter and have crushed lava or small gravel, which gives it enough heft that it won’t blow away.
Plants go in as they would in any garden although they are usually fairly small container stock. The whole thing is then mulched to secure the soil and insulate the plant roots. If hardscape elements, such as paving or decks, are required to be built over the green roof, that can be done in ways similar to how they would be installed onsite for ground-level landscaping. Hardscape features may be specially designed to minimize weight.
Hydrotech supplies most of these layers. You will pay a premium, because the pre-manufactured costs are usually higher than purchasing the same materials elsewhere. For instance, root barriers are used in ground-level landscaping and are a readily available material. However, Hydrotech requires you use all of their component layers in order to warranty their system. Some clients are willing to pay for this just for the peace of mind. However, a sound green roof system is entirely possible to achieve with products other than theirs.
How do you decide what to plant on a living roof?
The living roof is really like a ledge planting. It’s more exposed. You have only a thin stratum of soil, with higher winds and fuller sun exposure than you might find on the ground level. Plants native to ledges and shallow soils often work well. Most grasses work very well. We design each project uniquely, according to site conditions, depth of planting soil, exposure, etc. There are a lot of resources available now to help in identifying plants that have done well on living roofs.
Are living roofs heavy? Do you need structural reinforcement?
Yes, absolutely – especially if you’re going to be walking and sitting out there. They are engineered to assume that the drainage may fail and the whole “bath tub” could then fill up with water. The engineering design must be integrated into the overall design for the structure to take this potential condition into account.
What about retrofits or remodels?
It’s possible, depending on engineering. If the retrofit/remodel is being re-engineered anyway, it is a good possibility.
Do living roof projects ever run up against permitting issues or local building codes? There’s no special credit for them in the California energy-efficiency code, I know that, although green building codes such as CALGreen and BuildItGreen’s GreenPoint Rated system do recognize green roofs to some extent.
Building codes always apply, but we have not run up against anything unusual. If the living roof is accessible, it will need to meet codes for guardrails, etc. There are some LEED and BuildItGreen credits for living roofs, depending on multiple factors.
There are some fire departments that have a say on the plant materials used on living roofs. They review the planting as they would a ground-level landscape. Certain fire-prone plants are prohibited. If local fire codes specify highly fire-resistant roofing materials, they will probably be concerned that plant material should be fireproof as well. On one of our projects, the fire department also required that there be a walkable perimeter around the roof in case firefighters needed roof access during an actual emergency.
What about waterproofing?
The waterproofing makes or breaks these systems, and there is simply no cheating this one. The waterproofing must be a monolithic membrane or there will be problems. That said, the technologies available now for waterproof systems are incredible. Hydrotech seems to be an industry leader. They provide several types of membranes as well and the component layers (including soil). Again, you have to use all of their components in order to get their warranty.
What about maintenance? Do owners need to hire a special service?
The garden needs to be irrigated, fertilized and maintained, as with any garden. Drains need to be serviced as with any roof drain. It is a man-made environment, just like any other landscape. But, roof gardens are more sensitive than gardens created on the ground in full-depth soil. While they may not take more man-hours to maintain than ground-level plantings, living roofs do require more frequent monitoring. Getting the irrigation dialed in correctly takes a bit of attention while the plants are growing in. Over time, they will require less time and be on a par with maintenance for ground-based plantings.
How do you keep a living roof healthy?
If you have planted the right plants and set the right irrigation, they will only need periodic fertilization and regular garden maintenance to stay healthy.
What about weather damage?
This can be a factor, of course. All conditions are more severe on a roof, and damage can occur. Wind is probably the greatest concern and affects the plant choices made.
How easy is it to add greywater irrigation?
Technically, it is easy. Building code sees it differently, though, and at this point I doubt you could get this approved in California. If you did, there would be some concern with a buildup of salts in the soil, so at best it would have to be a dual system, to ensure that you could periodically flush out the salts.
Are grey water and rainwater systems the same thing?
No! Grey water and rainwater catchment are two completely different things. Grey water is from household use: laundry, showers, and bathroom sinks. You can’t really store it long-term in a big cistern because it’s not as clean as, say, well water. Too many organisms can “bloom” in grey water. The way to think about grey water is that it’s continually replenished. You can use it right away, or same day, for landscape irrigation. If you have a household with a lot of kids, and a lot of laundry, you might generate a significant amount of grey water to make this worthwhile.
Rainwater catchment systems are an attractive idea but they can be expensive. You need a lot of storage tanks. Rainwater storage might cost around $1.50-$2.00 per gallon. Depending on your garden’s water needs, you could be spending tens of thousands of dollars just for the storage tanks.
Tell me more about water budgets. I assume you’d start by factoring in the average annual rainfall.
Yes, that is where you start, but then you have to identify supplemental water sources in case of a shortfall. It’s also dependent on the plantings, the area, and how shaded the roof area is. We do detailed water budgets as part of all our design projects.
Is a living roof just like having another lawn?
It can be, if you plant grass and mow it. All the accompanying maintenance of having a lawn would apply: heavy irrigation, weekly mowing, heavy fertilization and weeding. We don’t recommend it. Most of our plantings are not lawns. If we do incorporate grasses, they are no-mow natives or ornamental grasses.
Can you put a living roof in any climate? What would you do different in Tahoe than in Livermore?
Yes, you can have one in any climate. Living roofs have been used extensively and for many years throughout Europe and Canada, so winter climates are fine. We don’t have much experience designing systems for northern climates, so we don’t know how they handle the snow load. We’d guess that it is mainly an engineering question. The main difference would be the plant choices, as those climates are so different.
Are there other design features that could interfere with a living roof? Certain types of roofing assemblies, tilted roofs, skylights, solar panels, vents, mechanical on the roof, etc.
Yes – all of these can interfere with or modify the design of a living roof. All of these factors should be considered as a whole in the early phases of design. Roof vents can be concealed, of course – solar panels would have to fight for space. And, local fire departments may require a walkable perimeter on a roof, which would also take away space from either gardens or solar arrays.
Some of the DIY living roofs look very untidy. I can’t imagine putting them on a Modern house.
Yes, they can be. That’s the old “hippie” look. We can design water-wise gardens that have a very sophisticated look, but don’t require a lot of water. Each living roof is designed to be in character with the rest of the project.
How do surrounding shade trees affect living roof design?
If you are fortunate enough to have mature trees high enough to shade the roof, it is a blessing. Shade is great. Typically, there is more of an effect from surrounding buildings casting shade or creating erratic wind patterns. Our “Sky Garden” project had shade from redwoods on once side and bright sun on the rest. It was designed like any other landscape: shade-tolerant plants on the side shaded by the trees, and drought-tolerant sun plants in the open areas.
Are there any cautionary tales worth mentioning?
Our experiences have been very positive. This is a wonderful way to expand livable and visually attractive space in tight urban settings and on properties with limited buildable space on the ground plane. Steep hillside sites are always looking for more area to create flat outdoor spaces, regardless of of the size of the lot.
About the author
Rebecca Firestone has been working in the Bay Area since 1998 as a technical writer, business content developer, architectural filing lady, marketing director, and sorcerer’s apprentice.