Membrane out of the weather
No UV, little temperature swing, and no direct hail strikes on the waterproofing.
Flat roof system
Inverted Roof Systems in Edmonton
The membrane goes on the deck and the insulation goes on top. It is the most protected waterproofing you can put on a concrete roof.
Overview
An inverted roof in Edmonton flips the usual order of layers. Instead of insulation under the membrane, the waterproofing goes directly on the deck, and rigid insulation and ballast sit on top of it. The industry also calls this a protected membrane roof (PMR) or IRMA roof. The membrane stays close to indoor temperature year-round, out of the sun and away from hail and foot traffic.
That protection comes at a cost. Inverted roofs are heavy, they usually need a concrete deck, and they cost more to build. For the right building, they are among the longest-lasting roofs available. Rain Roofing installs inverted roofs with hot rubberized asphalt and SBS membranes.
Built for use
Pavers on pedestals turn a roof into a plaza, terrace or green roof without exposing the membrane.
Long membrane life
Protected membranes regularly outlast exposed systems by many years.
How an inverted roof works
In a conventional roof, the membrane sits on top and takes everything the weather throws at it. In an inverted roof, the membrane is the lowest layer, bonded to the deck, and everything above it protects it. The insulation is extruded polystyrene because it tolerates being wet; water drains at the membrane level below it.
- Concrete deck.
- Fully bonded membrane (hot rubberized asphalt or SBS).
- Protection sheet.
- Extruded polystyrene insulation.
- Filter fabric.
- Stone ballast or pavers.
Why an inverted roof suits Edmonton
Freeze-thaw is the main thing that wears out a membrane here, and an inverted roof takes it nearly out of the picture. Under insulation, the membrane avoids the daily swings from sun to frost that work exposed seams loose. Hail lands on stone or pavers, not waterproofing, which matters in a region that has seen storms like the August 2019 hail.
Designers allow for one effect: cold meltwater running under the insulation carries away some heat, so inverted roofs are usually specified with a little extra insulation.
Where it's the right choice, and where it isn't
Inverted roofs suit concrete-deck buildings: hospitals, schools, government and institutional buildings, office podiums, parkades, and any roof that will be used as a plaza, terrace or green roof.
They are rarely a fit for light steel-deck warehouses, which cannot carry the ballast weight. Leak location is also harder, because the insulation and ballast must be lifted to reach the membrane. For a typical industrial building, 2-ply SBS or single-ply is the practical choice.
Inverted roofs are best decided at design. Converting an existing conventional roof means checking the structure for the added ballast, raising curbs and flashings to clear the new insulation, and confirming drains can work at membrane level. It can be done, but it is rarely the cheapest re-roof.
Maintaining an inverted roof
Inverted roofs need less membrane maintenance, but drains need more attention, because the water runs at membrane level under the insulation. Spring and fall inspections should check drains and scuppers, ballast that has shifted in wind, paver levelling, perimeter flashings, and any vegetation on green roofs. Flashings above the ballast are still exposed and age like any other flashing.
Warranty options for inverted roofs
Warranties depend on the membrane. Hot rubberized asphalt and SBS manufacturers offer long-term system warranties on qualifying inverted assemblies, often 20 years or more. Because the membrane is bonded directly to the deck, a well-built inverted roof can often have its insulation and ballast reused at the end of a cycle. Rain Roofing backs the install with a 10-year workmanship warranty.
Repair or replace: signs to watch
Most inverted roof problems are at flashings, drains and edges, which are visible and repairable. Membrane leaks in the field are less common but take more work to locate.
- Recurring leaks below drains or perimeter flashings.
- Ballast scoured away, exposing insulation to UV.
- Insulation boards broken, floating or displaced.
- Pavers settling or rocking on pedestals.
Inside the assembly
What goes under the Inverted roofs surface
Every layer has a job. Skip one, or install it carelessly, and the roof fails early. This is the assembly we build and warranty.
- 07Cap sheet or membraneThe weathering surface: granulated SBS cap sheet, or a heat-welded TPO, PVC or EPDM membrane.
- 06Base sheetThe first waterproofing ply, fully bonded and lapped at every seam.
- 05Cover boardA hard, flat surface that protects the insulation from foot traffic and hail.
- 04Tapered insulationBuilt-in slope that moves water to the drains so it never sits and ponds.
- 03InsulationPolyiso boards, staggered so no joint lines up and heat cannot short-circuit.
- 02Vapour barrierStops warm, moist indoor air from condensing inside the insulation in January.
- 01Steel deckThe structure. We check fastening, rust and deflection before anything goes on top.
Process
How we install it
Structural and deck review
We confirm the structure can carry the ballast and that the concrete deck is cured, dry and sloped to drains.
Membrane on the deck
The deck is primed and the waterproofing (often hot rubberized asphalt with reinforcing fabric) is applied directly to it, including all flashings.
Protection and flood test
A protection sheet goes over the membrane, and the roof can be flood tested before anything covers it.
Insulation
Extruded polystyrene boards, which tolerate water, are laid loose over the membrane to the required thickness.
Filter fabric
A filter fabric over the insulation keeps fine material from washing down to the membrane and drains.
Ballast or pavers
Stone ballast or concrete pavers on pedestals hold the insulation down and create a walkable surface where needed.
Warranty
Three layers of cover on every roof
Our workmanship, an independent ARCA certificate, and the manufacturer's system warranty, each backed by a different party.
Spring and fall inspections every year keep all three warranties valid. Included in every Rain maintenance program.
Questions
What facility managers ask us
Why put the insulation on top of the membrane?
It protects the waterproofing from UV, hail, foot traffic and freeze-thaw. The membrane stays near indoor temperature year-round, so it ages far more slowly than an exposed one.
Can an inverted roof go on a steel-deck building?
Rarely. The ballast is heavy and the system works best bonded to concrete. An engineer would need to confirm the structure. For most steel-deck buildings, a conventional system is the better fit.
How do you find a leak on an inverted roof?
We start with flashings and drains, where most leaks are. If the field membrane is suspect, we lift ballast and insulation in sections, sometimes guided by electronic leak detection, until we find it.
Can we add a rooftop patio or green roof?
An inverted roof is the right base for one. Pavers on pedestals or a green roof assembly sit above the insulation, provided the structure can carry the extra load.
Does an inverted roof need maintenance?
Yes, mainly drains, ballast and flashings. Twice-yearly inspections keep drains open and catch shifted ballast before the insulation is exposed.
Is an inverted roof worth the extra cost?
On a concrete building you plan to hold for decades, often yes. The membrane lasts longer, hail rarely touches it, and the insulation and ballast can usually be reused. On a light steel-deck building, the structural cost usually outweighs the benefit.
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