Inverted Roof Systems: A Complete Guide to Design, Installation, Costs & Benefits

by Jun 5, 2025

An inverted roof is a low slope roofing system where the insulation layer is placed above the waterproof membrane instead of below it. The insulation is then covered with another membrane and potentially covered with soil, plants, and/or stones. This simple reversal protects the membrane from weathering and temperature extremes.

Inverted roofs often take advantage of the natural UV and water drainage properties of soil and plants to create a roof that provides excellent R-value and durability while remaining eco-friendly.

Inverted roofs are particularly common on terraces, passive construction homes, and other specialty applications, like apartment building roofs in densely populated urban areas. Inverted roofs can essentially appropriate the soil into ballast which both holds the membrane in place and increases the R-value of the structure. The rooftop can be multipurposed by adding usable space that could otherwise become an eyesore.

Here we will discuss in detail what an inverted roof design entails, and why they are used on low slope roof structures. We will discuss the pros and cons of installing an inverted roof, and how to decide if the option is a good choice for your project.

What Is an Inverted Roof?

On conventional low slope roofing systems, the insulation is installed directly to the decking. With a cover board installed atop it, both layers comprise a low slope roof’s substrate, which supports the roof’s waterproof membrane. Because inverted roofs flip this process, they’re also known as “protected membrane systems” or “upside-down roofs.”

An inverted roof uses the same membrane as a conventional system, but instead of protecting the insulation, the membrane itself is protected. It’s installed directly on the roof deck and covered with a separation layer, typically a geotextile fabric, which further protects the membrane. The insulation is laid on top of the separation layer, followed by ballast or pavers to secure the system.

How to Build an Inverted Roof: 5 Key Phases & Material Choices

To recap, here’s the typical order of material installation starting from the roof deck upwards:

  1. Waterproof Membrane: Typically, the system is waterproofed using a layer of modified bitumen or a single-ply membrane, such as TPO or PVC. This waterproof layer acts as the roof’s water shield and vapor barrier. It is important in this type of roof system to use a 60 or 80-mil thick TPO or PVC due to its durability and resistance to puncture.
  2. Separation Layer: Usually a geotextile fabric that protects the waterproof membrane and prevents mechanical damage or chemical incompatibility with the membrane.
  3. Insulation Layer: Rigid thermal insulation boards are placed over the waterproofing layer in a staggered pattern to prevent seam overlap. Typically, three layers of XPS (extruded polystyrene) are used due to its thermal performance, excellent water resistance, low water absorption rate, ability to withstand freeze-thaw cycles, UV radiation, and mechanical stress from ballast and foot traffic.
  4. Filter Medium: This layer is designed to reduce the rate at which rainwater flows through the insulation layer. It improves the overall thermal efficiency and sustainability of the roof by preventing excessive moisture accumulation within the insulation.
  5. Ballast Layer: The insulation is weighted down with a layer of river rock, gravel, soil, or other materials to protect against wind uplift and flotation. This ballast holds down and protects the water flow layer and the insulation layers below it.

Step 1: Waterproof Membranes

Most popular types of roof membranes, such as TPO, PVC, and EPDM, can accommodate an inverted roof. Each membrane offers unique characteristics for inverted roofs.

TPO

Thermoplastic polyolefin, or TPO, is the most common membrane for inverted roof systems. TPO is durable, repairable, and provides enough flexibility to reduce tears and seam separation when the ballast is overlaid. The potential shearing of TPO’s field is reduced by using TPO with appropriate thickness, usually 60 mil or greater.

PVC

Polyvinyl chloride, or PVC, is another thermoplastic membrane option used in inverted roof systems. Widely used in construction, from plumbing to siding, PVC is valued in roofing for its resistance to UV exposure and chemical contaminants, making it a strong choice for facilities like restaurants or manufacturing buildings.

EPDM

Ethylene propylene diene monomer, or EPDM, is a synthetic rubber used to waterproof roofs, foundations, and even custom shower pans due to its flexibility and puncture resistance. EPDM is usually the least expensive of the three membranes included here, but it often requires additional labor during installation because of its shorter roll lengths and additional seams.

Step 2: Separation Layer

The separation layer between the waterproof membrane and the insulation serves more than one function. The geotextile fabric typically used is made from polypropylene, or sometimes polyester strands, and allows the membrane and insulation to expand and contract independently.

Separating the layers separates their movement, which reduces friction between the two surfaces. This friction would otherwise cause the materials to tug against one another and create stress points. These stresses can cause the courses to separate over time, so the separation layer helps extend the service life of both components.

Step 3: Insulation Layer

The insulation layer is composed of sheet insulation board panels laid side-by-side to form a continuous thermal barrier. The project may use uniform boards stacked together, or interlocking panels designed to both insulate and establish the proper roof slope toward drainage points.

An example of insulation used to create positive drainage on a low slope roof.

Standard flat boards typically require more labor to seal. This includes insulative joint tape or spray foam at panel seams. Tapered insulation systems are custom engineered for the roof and arrive with labeled panels that are assembled like a jigsaw puzzle on site.

XPS, or extruded polystyrene, is the most common insulative layer. XPS is available in sheets of various thicknesses, which makes adding R-value a cinch. XPS allows for stacking, so the installer just adds the number of inches required for the desired R-value.

To be clear, other forms of sheet insulation can also be used with an inverted roof, such as EPS, aka expanded polystyrene, and rigid foam boards made from polyisocyanurate, and VIPs, also known as vacuum insulation panels.

XPS

XPS is hydrophobic, meaning it does not readily absorb moisture. XPS is an ideal choice for an inverted roof, as XPS provides plenty of support for the ballast above it. XPS is not easily compressed and holds its shape well over time. While rigidity is important for conventional low slope roofs, it’s especially important for insulation on inverted roof systems due to its direct exposure to load and the environment.

EPS

EPS is a lightweight polystyrene material made from expanded polystyrene beads that are fused together. The bead-like texture remains visible in the finished product. Unlike extruded options, EPS is less dense and less compressed, but it remains effective in many roofing applications.

EPS is not as resistant to pressure as XPS, and it’s liable to absorb more water over time than XPS. In an inverted roof design, the insulation is protected on both sides, so it tends to receive less wear and tear than insulation panels on a standard low slope roof, despite being closer to the roof’s surface. However, XPS is preferred for long-term performance.

VIPs

Vacuum insulation panels, or VIPs, are often made from a durable base material like aerogel, silica, fiberglass strands, or perlite. The panel is formed under pressure in a vacuum, creating a dense, thin insulation panel.

VIPs provide excellent R-value per inch at R25 to R30, making them ideal for low slope roof systems with limited space. This is especially useful when a roof already has another membrane installed.

VIPs are ideal for directing runoff to an internal drain, gutter, or scupper drainage system because a thin insulation board is much easier to manipulate than a thick one. If the slope only needs a minor adjustment, VIPs allow for more control without sacrificing R-value.

Step 4: Filtering Layer

The filtering layer is often a non-woven geo-textile fabric designed to contain solids but allow water to pass through. This protects the underlying layers from erosion and prepares the surface for ballast or a vegetative system.

Because the membrane is beneath insulation, ballast, and potentially vegetation, it is protected from UV exposure, foot traffic, and extreme temperature fluctuations. All the filtering layer needs to do is contain soil and manage water flow.

Step 5: Adding the Ballast and Plants

An inverted roof design effectively creates a lawn on the roof by adding soil and plants on top of the membrane and insulation. Whether an inverted roof is vegetative or not, it requires sufficient ballast to protect against wind and membrane displacement.

The final ballast of soil, stone, or both, is added after the filtering layer to weigh down the assembly and prepare the surface for rooftop use. Raised planting beds may be installed to create deeper soil zones for larger plants and better control root growth and drainage. Many inverted rooftops also have a lightweight greenhouse, potting station, and other gardening features.

Inverted roof design experts are always aware of the typical weight of the materials they add, and where those materials reside on the roof. Designers avoid placing heavy features over drainage valleys without structural reinforcement and proper water flow accommodations.

Just like conventional roofs, inverted roofs require adequate protection around protrusions. In inverted systems, protrusion detailing must ensure that ballast doesn’t interfere with drainage or compromise seals.

What Are the Advantages of Inverted Roofs?

  • Membrane Protection: The insulation protects the waterproofing membrane from temperature fluctuations, UV radiation, and physical damage, potentially extending its lifespan.
  • Durability and Longevity: The protected membrane experiences less expansion and contraction, reducing the risk of tears or bursting. Inverted roofs can last up to 40 years, compared to 25-30 years for well-maintained conventional roofs.
  • Lower Maintenance Costs: The protected membrane requires less frequent repairs and maintenance.
  • Energy Efficiency and Cost Savings: Building owners aiming to improve their building’s energy efficiency often benefit from inverted roofs because the insulation minimizes heat loss in winter and reduces heat gain in summer, leading to lower energy bills.
  • Flexibility of Roof Alterations: The insulation layer can be easily lifted and replaced or reused if the building is altered. You may also choose to add more insulation at a later point without stripping the waterproof layer. This means additional insulation can be incorporated without major alterations to the roof system.
  • Usable Rooftop Space: Inverted roofs create functional spaces. Conventional low slope roofs can suffer from incidental damage caused by foot traffic from HVAC technicians while inverted roofs can accommodate terraces and gathering spaces, creating tenant-friendly real estate.
  • Superior Warranties: Because the membrane is better protected from typical sources of damage, inverted roofs often come with 20 to 30-year manufacturer warranties, several years longer than conventional membrane applications.

What Are the Disadvantages of an Inverted Roof?

  • Hard to Find Leaks: Ballasted roofs can make leaks more difficult to spot. Inverted roofs use ballast and often accommodate structures, both of which can complicate the identification of leaks. Runoff will drain through the soil before it reaches a crack or hole in the waterproof membrane, making it more difficult to proactively identify leaks without regular, thorough inspections. It’s worth noting, however, that leaks are less likely to infiltrate a building’s interior with inverted roof systems because the waterproof membrane isn’t vulnerable to wind, UV radiation, and traffic. Additionally, the separation layer can facilitate finding a leak by channeling water to designated inspection points, which mitigates this disadvantage somewhat.
  • Higher Initial Cost: An inverted roof costs more both to install and purchase than standard low slope roof systems. Because an inverted roof has an additional layer between the soil and the insulation layer, the materials and labor cost can be 15-20% more than a standard low slope roof installation. Additionally, inverted roofs have demanding structural requirements that must be met prior to installation due to the increased load.
  • Complex Installation: The installation process for inverted roofs is generally longer and more complicated than traditional roofing methods. Inverted roofs require a great deal of skill, knowledge, and expertise. If you are in the market for an inverted roof, you’ll want to choose an experienced and trusted roofing professional
  • Limited Applications for Layovers: Unless a roof is due for a complete replacement, inverted roofs aren’t usually tenable. Weight is the limiting factor. Inverted roofs can be laid over existing roof systems if they’re in good condition and the building’s structure can support an inverted roof.
  • Potential for Dirt Build-up: Dirt can accumulate rapidly on inverted roofs, which can lead to vegetation growth if not properly maintained. This can lead to higher maintenance costs due to the need for regular weed management.

Ballast Materials & Their Roles in Inverted Roof Systems

Ballast is material used primarily for its weight. Inverted roofs allow for many types of ballast, including soil, gravel, or concrete pavers – each suited for different situations.

Soil

The soil used on an inverted roof is typically topsoil or planting soil and will usually be devoid of filler like clay. Because the soil is confined above the membrane and can’t access subsoil nutrients like it would in natural ground, periodic fertilization is required for healthy plant growth.

The moisture in topsoil often weighs more than the dry soil itself. Residual water collected on the roof that does not drain away must be allowed to evaporate. If either of these functions are impeded, the water will become trapped, adding excessive weight to the roof structure.

Water weighs about 8 pounds per gallon, so its retained weight must be considered during planning, especially when selecting insulation. If the system is expected to retain water or dry slowly, the system will require additional rigidity from its insulation to prevent compression.

Gravel

Gravel is the most common type of ballast on all ballasted roofs because it’s dense, heavy, and simple to distribute evenly. Smooth stones like pea gravel are preferred because they’re less abrasive to both insulation and membranes.

Gravel is often combined with other ballasts for added weight to prevent wind uplift, especially at perimeter edges.

Ballast surrounds a skylight on a low slope roof. Paving stones allow for foot traffic without interfering with the membrane beneath the ballast.

A ballasted modified bitumen roof with pavers.
On conventional low slope membrane roofs, pavers allow for foot traffic without interfering with the membrane below.
On inverted roofs and other ballasted roof systems, pavers assist in securement. 

Concrete Pavers

Concrete pavers are an efficient and economical way builders ballast an inverted roof. These provide the walking path needed to access and care for plant life and are often easier to transport to the roof than gravel.

Concrete pavers are often used in conjunction with stones and soil to meet ballast requirements. This allows for more creativity, as a low slope roof often contains obstacles that impede straight lines of crops. Concrete pavers function as oversized walking stones, typically surrounded by stones or soil to distribute weight and provide stability.

Installation Challenges of Inverted Roof Systems

Inverted roofs are more complicated to install than a standard low slope roof membrane due to the additional components required, water management, and use of ballast.

Inverted roofs typically require ballast, which eliminates many of the seams in the membrane that could lead to a leak, but the additional weight must be properly supported by the building’s framework.

The weight of an inverted roof requires additional structural support. A low slope roof built to support a standard membrane may not be structurally strong enough to support the additional weight of the ballast, which could preclude an inverted roof altogether without intervention from a structural engineer.

What Can Make Inverted Roofs More Expensive?

The cost of an inverted roof will vary widely depending on whether the installation is a retrofit or a new installation, or if the roof requires structural changes to support it. If the existing roof already has two layers of membrane, they must be removed first. Code requirements may vary from region to region, but no more than two roofing layers may be present at a time.

Necessary structural changes can inflate the price of an inverted roof dramatically. Because inverted roofs require additional labor to design a proper water management system and can accommodate multiple insulation layers, costs can add up quickly.

Generally, you can expect a 15-20% higher upfront cost than a conventional membrane roof system.

When Is an Inverted Roof the Right Choice?

With all the advantages and disadvantages of inverted roofs in mind, they are a great option for virtually any low slope roof design. Inverted roofs require regular upkeep, just like conventional low slope roofs. If not properly maintained, plants on inverted roofs are more likely to die than grow out of control, making neglect a bigger issue than overgrowth.

The most popular uses of inverted roofs are for terraces and other specialty roof configurations. Although terraces are the most common location, inverted roofs can be erected virtually anywhere a low slope roof exists.

Urban low slope rooftops are popular gardening spots when an inverted roof is in place, increasing the building’s appeal to prospective tenants. Inverted roofs often become a gathering spot for gardening enthusiasts, providing opportunities for people to gather and spend time together.

Ultimately, the decision to install an inverted roof on your building should be made after consulting with an experienced roofing contractor. While they offer many benefits, that doesn’t make them a one-size-fits-all solution for low slope commercial buildings.

Key Takeaways:

  • Insulation Above, Membrane Below: On an inverted roof, the insulation layer sits above and protects the waterproofing membrane from temperature fluctuations, the sun’s damaging UV radiation, and physical damage from hail or dropped equipment.
  • Anatomy of the Inverted Roof: Inverted roofs have multiple layers: ballast layer, water flow reducing layer, insulation layers, waterproofing membrane layer, and finally the decking.
  • Durability and Longevity: One of the key benefits of an inverted roof comes from the fact that the waterproof membrane is shielded from the sun’s harsh rays, and experiences less thermal expansion and contraction, reducing the risk of tears and blistering. Inverted roofs can last up to 20 to 30 years, compared to 20-25 years for conventional roofing systems.
  • They Have Trade-Offs: Inverted roofs have a higher upfront cost and can require structural reinforcement. Installation can be complicated by involved drainage design and unique maintenance challenges.

 

Featured image: “Highline in New York” by La Citta Vita, licensed under CC BY-SA 2.0, via Wikimedia Commons

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