Building envelope · Roof Assemblies
Inverted (Protected Membrane) Roof Assembly
A reference account of the inverted roof as a deliberate reversal of the normal layer order, covering how water travels through the build-up to a concealed membrane, why the stack has to be weighed down, and what an owner should ask about a roof they will never see the membrane of again.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What inverted roof is
An inverted roof puts the waterproofing directly on the structural deck and the insulation on top of it. Rain therefore lands on a surface that is not the waterproofing, passes down through or around the thermal layer, and reaches the membrane from above but from beneath the insulation. The membrane spends its life at a temperature close to that of the building, out of sunlight and out of reach of anything dropped or dragged across the roof.
The test against its nearest sibling, the warm deck flat roof, is whether you can see the membrane. Stand on a completed warm deck and the weathering layer is the surface underfoot. Stand on a completed inverted roof and the surface underfoot is ballast, paving or planting, and the layer actually resisting water is somewhere below, unreachable without taking the roof apart in that area.
That reversal changes the duty of the thermal layer. Insulation in this position is repeatedly wetted and is expected to keep functioning while wet, so the choice of material is constrained in a way it is not elsewhere in a building. A layer above the insulation influences how much water travels through the thermal layer and how much runs across its surface, which is why an inverted build-up has a component that a warm deck has no equivalent of.
Finally, the stack has to be held down by weight. Insulation floating on water is a real condition rather than a theoretical one, and the ballast or paving that resists it is a structural part of the assembly rather than a finish laid over it. Removing surfacing from a completed inverted roof, for any reason, removes the restraint from everything under it.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- protected membrane roof
- PMR
- upside-down roof
- insulation-above-membrane roof
- ballasted inverted roof
- inverted roof terrace
- protected membrane terrace
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Keep the waterproofing at a stable temperature and out of ultraviolet light by burying it beneath the thermal layer.
- Protect the membrane from foot traffic, dropped tools and the fixings of anything later placed on the roof.
- Allow rainwater to reach the membrane and travel across it to outlets set at that level.
- Restrain the whole build-up against wind and against the buoyancy of insulation surrounded by water.
- Let the roof surface be finished as ballast, paving or planting without that finish becoming the water-resisting layer.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 7. Order is meaningful.Primary supportStructural deck
The plate the waterproofing is applied to and the element carrying the weight of insulation, ballast, retained water and anything placed on the roof. Because the restraint here is weight rather than fixings, what the deck is asked to carry is central rather than incidental.
- Layer 2 of 7. Order is meaningful.Control layerWaterproofing laid on the deck
The layer that actually resists water, applied directly to the deck and then covered. Once the build-up above it is complete it cannot be seen, tested by eye or repaired without removing everything over the affected area, which shapes how it is specified and verified.
- Layer 3 of 7. Order is meaningful.Drainage planeWater path at membrane level
The surface of the waterproofing itself, which is where rainwater travels once it has passed the insulation. Falls, ponding and the route to each outlet all happen at this concealed level rather than at the surface people can see.
- Layer 4 of 7. Order is meaningful.Thermal layerMoisture-tolerant insulation above the membrane
The boards laid over the waterproofing, laid loose and separated by joints that water moves through. They are expected to keep working while wetted and drying repeatedly, which narrows the families that are found in this position.
- Layer 5 of 7. Order is meaningful.ProtectionWater-control and filter layer
The layer placed over the insulation which influences how much rainwater passes through the thermal layer rather than running off its surface, and which keeps fine material from the surfacing above from washing down into the joints between boards.
- Layer 6 of 7. Order is meaningful.AttachmentBallast or paving providing restraint
The weight holding the stack down against wind and against the tendency of insulation to lift in standing water. It is doing a mechanical job, so it cannot be reduced, thinned or partly removed on aesthetic grounds without changing the roof.
- Layer 7 of 7. Order is meaningful.Edge and terminationOutlets, upstands and perimeter restraint
Outlets have to collect water at membrane level while remaining reachable through the layers above, and the perimeter has to hold the loose stack in place. Both are junctions where the concealed and visible parts of the roof have to be reconciled.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
Rainwater is the medium through which the layers talk to each other here. It passes the surfacing, is divided by the filter layer between what soaks through the joints and what runs across the top, and arrives at a membrane that, although it sits on the warm side of the insulation, is cooled by the water passing over it. Change the layer above the insulation and you have changed how much water reaches the layer below, and with it how much heat leaves the roof that way.
Buoyancy links the ballast to the membrane in a way nothing else in roofing does. Water standing on the waterproofing surrounds the underside of loose-laid boards, and boards surrounded by water want to rise. The weight above is therefore resisting wind from one direction and flotation from the other, and stripping surfacing from part of a roof to carry out unrelated work leaves the layers beneath it restrained by nothing.
Outlets pull the concealed and visible levels together. If an outlet collects only at the surface, water that has already passed into the build-up never leaves it; if it collects only at membrane level, the surface has to be able to shed into it. The arrangement of the outlet, the depth of the insulation and the finished level of the surfacing therefore have to be settled as a single detail.
Concealment reshapes every later decision. A penetration made after completion crosses ballast, filter, insulation and membrane, and each has to be reinstated in an order nobody can check afterwards. Damage traced by dampness inside the building cannot be located by walking the roof, because the leak is at a level that is not the surface and water can travel along it before appearing.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Deck families of this kind are commonly encountered under inverted build-ups. Whether a deck can carry the weight this arrangement relies on is a structural determination for an engineer, taken for the particular roof rather than from the material family.
Control layer
These waterproofing families are commonly encountered directly on the deck in this arrangement. Because the layer will be concealed, what may be used and how it is verified before covering are governed by the manufacturer's documentation and the designer.
Thermal layer
This is the insulation family most commonly encountered above the waterproofing, because the position demands a material that continues to work while repeatedly wetted. Whether a given product belongs in an inverted build-up is a documentation and design question.
Protection
Surfacing and ballast of these kinds are commonly encountered as the restraining layer of an inverted roof. What weight is required, and where it has to be increased near edges and corners, is a design determination and not a property of the surfacing.
Edge and termination
Perimeter trims, restraint kerbs and copings are commonly formed in these families. How each meets the concealed waterproofing, and how it holds a loose-laid stack at the roof edge, is settled in the detail rather than by the material chosen.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Outlets collecting at membrane level
The outlet has to receive water from the level of the waterproofing while being reachable through insulation and surfacing above. That usually means an assembly rather than a fitting, and it means the point of highest maintenance need is the point hardest to get to.
Read about Roof Rainwater Drainage →Perimeter upstands and restraint
The upstand has to be measured from the concealed membrane rather than from the visible surface, so a build-up of insulation and paving raises the finished level relative to a termination that stays where it was. The perimeter also has to stop the loose stack from migrating.
Read about Parapet System →Roof used as a terrace
Where the surfacing is walked on, everything above the waterproofing becomes a usable deck with its own thresholds, access and guarding. The layer order underneath is unchanged, but the demands placed on the ballast layer are no longer only mechanical.
Read about Trafficked Roof Deck →Paving carried on adjustable supports
Pedestals concentrate the restraining weight into points rather than spreading it, and they stand on the layer above the insulation rather than on the membrane. How load passes down through a loose thermal layer to the deck is a design question raised early.
Read about Pedestal Terrace Paving →Anything passing through the stack
A penetration here crosses more layers than in an exposed roof and ends at a membrane nobody will see again. Grouping penetrations, and forming them before the roof is covered rather than after, is materially easier than working back down through a completed build-up.
Read about Penetration Sealing →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- Water is expected within this build-up rather than excluded from it, so the question is where it goes rather than whether it arrives. How that behaviour affects the assembly in a given climate and use is assessed by a qualified professional, not inferred from the layer order.
- Thermal
- Rainwater passing across a membrane beneath the insulation carries heat with it, which is a behaviour peculiar to this arrangement. Quantifying it is a calculation for the designer, and no thermal outcome is stated here.
- Durability
- The membrane is sheltered from ultraviolet light, temperature swing and impact, which is the argument for the arrangement. What that means for any specific product remains a matter for its documentation rather than a general expectation.
- Maintenance and access
- Outlets, upstands and the membrane itself sit under a layer that has to be lifted to reach them. Planning which areas can be opened, and how surfacing is returned afterwards without losing restraint, belongs in the design rather than in later improvisation.
- Interfaces
- The perimeter has to reconcile a concealed water level with a visible finished level, and both are moving relative to each other as the build-up depth changes. Most of the difficult decisions in this assembly are found at that reconciliation.
- Documentation
- Because nothing below the surfacing can be inspected once complete, the record of what was installed, and of any verification carried out before covering, becomes the only evidence available to a later owner or surveyor.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Has the deck been assessed for the weight this arrangement relies on, including any surfacing intended later?
- Where do the outlets collect water, and how is the surface above them able to shed into that level?
- What is the layer above the insulation intended to do, and who has selected it in relation to the membrane below?
- How will the waterproofing be verified before it is covered, and is that record being kept?
- Which areas of the roof are intended to be openable for maintenance, and how is restraint restored afterwards?
- How does the perimeter hold the loose-laid stack in place, particularly at corners and along exposed edges?
- Is the roof going to be used as a terrace, which would bring guarding, thresholds and access into the same design?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- The surfacing is not the waterproofing, and a roof that looks sound underfoot tells an observer nothing about the layer that is actually keeping water out.
- Ballast is not decoration or a finish choice; it is the restraint holding the assembly down, and reducing it changes how the roof behaves in wind.
- The insulation is not being kept dry by the layers around it; it is expected to be wetted, which is why only some families are found in this position.
- A leak in this build-up rarely appears inside the building beneath the point where the membrane is damaged, because water travels along a concealed plane first.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What restraint has been assumed for this build-up, and how does it change towards the edges and corners of the roof?
- How is the outlet detail arranged so that it collects at membrane level and can still be maintained?
- What insulation family has been selected for a position where the material is repeatedly wetted, and on what basis?
- How will the concealed waterproofing be checked before the roof is covered, and what record will exist afterwards?
- If dampness appears inside, what is the process for locating a fault beneath the surfacing?
- How does the finished surface level relate to any door threshold or upstand at the perimeter?
What this page does not do
- This entry describes the arrangement of the assembly. It does not state that an inverted build-up is appropriate for any particular roof, which is a design determination.
- Watertightness is a property of the completed, correctly detailed and correctly installed assembly, and no layer described here achieves it on its own.
- Removing ballast or paving from an inverted roof, even temporarily and even in a small area, removes wind restraint from the layers beneath and should not be done without advice.
- No weights, depths, falls or drainage capacities are given here; all of them are engineering determinations for the specific roof.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Roof Assembly Systems
Roofs answering two questions at once: where the insulation sits relative to the deck, and how the water-shedding layer achieves continuity across the whole surface.
Browse all roof assemblies entries →